Recently in Accessibility Category

We released Access Across America a little over a month ago. The following summarizes some discussion of the report.

Price Roads | Accessibility map:

"Excellent accessibility ranking by Dr. David Levinson (the Transportationist) and the Nexus Research Group at University of Minnesota:

California has the #1 and #2 most accessible cities, and they provide an interesting contrast in two ways to get accessibility. Accessibility can come from density (everything is close together so it doesn’t take long to go from one place to another) or mobility (everything is far apart but there are huge highways so you can traverse long distances).

I’m excited to see academics using visual media to put across point about public policy."

Reihan Salam When Thinking About Infrastructure, Focus on Accessibility

David Levinson, a transportation economist at the University of Minnesota, has emphasized that one of the key issues in infrastructure investment is improving accessibility, or the ease of reaching valued destinations. One way to improve accessibility is make it easier to traverse long distances, so you can reach a larger number of jobs and consumption opportunities, etc., in a given amount of travel time from home. Another way to improve accessibility is to bunch up jobs and consumption opportunities and homes, i.e., by increasing density. Levinson finds that while accessibility has deteriorated relative to 1990, it has improved relative to 2000. My sense is that the best way to increase accessibility is to focus on implementing peak road-user fees and using the resulting revenue stream to carefully add capacity at bottlenecks, and also to ease local land use regulations that have proven a barrier to increased density in high-productivity regions. These strategies ought to be pursued in tandem. One crude way of putting this is that while we tend to fixate on the “hardware” layer of infrastructure, we should devote more attention to the “software” layer, i.e., the systems governing the allocation of infrastructure resources. Focusing on accessibility rather than infrastructure spending levels as such will get us much closer to tackling the frustrations that plague commuters.

Robert Poole, Surface Transportation Newsletter #115: New Study Ranks Access to Jobs via Auto Commuting

New Study Ranks Access to Jobs via Auto Commuting

Transportation is not an end in itself; it's a means to other ends, such as getting to and from work. Taking this point to heart, a growing number of researchers in recent years have promoted the concept of "access" as being more important than speed or travel time, per se. One of the leaders in this field, David Levinson of the University of Minnesota, defines accessibility as "the number of destinations reachable within a given travel time" by a particular mode of transportation. He is the author of a new study called "Access Across America," released last month by U of M's Center for Transportation Studies.

In this study, Levinson estimated the accessibility to jobs by car for the 51 largest U.S. metro areas. His data are for 1990, 2000, and 2010, so in addition to providing a snapshot of conditions as of 2010, the data also allow him to document trends over the past two decades. The results may surprise many of those concerned about traffic congestion in the largest metro areas, because Levinson finds that the 10 metro areas that provide the greatest accessibility to jobs via auto commuting are, in order: Los Angeles, San Francisco, New York, Chicago, Minneapolis, San Jose, Washington, Dallas, Boston, and Houston. And over the past two decades, the places with the largest increases in accessibility by car are Las Vegas, Jacksonville, Austin, Orlando, and Phoenix. Those with the largest decreases are Cleveland, Detroit, Honolulu, and Los Angeles.

What accounts for these findings? Although Levinson doesn't really get into the details, I think one of the most important factors is the ongoing suburbanization of jobs. Remember, Levinson's data are for entire metro areas, and there has been a huge dispersion of jobs throughout these metros over the past 50 years. A good summary of the data was provided last month by Wendell Cox in "Job Dispersion in Major US Metropolitan Areas, 1960-2010." (www.newgeography.com/content/003663-job-dispersion-major-us-metropolitan-areas-1960-2010) For example, in 1960 54% of employment in 35 major metro areas was in the historical core municipalities—but by 2010, that figure had dropped to 30%, with 70% in suburban and exurban areas. The suburbanization of jobs has made huge numbers of workplaces more accessible by car than before, leading to shorter average work-trip travel times than in Canada or Europe.

Levinson's data show that in 31of the 51 metro areas, all the jobs can be reached by car in 30 minutes or less; upping the limit to 40 minutes brings the total to 39 of the 51, and at 60 minutes, almost everyone can reach nearly every job in every one of the 51 metro areas. That's pretty outstanding performance by the highway system, despite the existence of serious congestion.

It's instructive to contrast Levinson's auto accessibility figures with the findings of a Brookings Institution study from 2011 on accessibility to jobs via transit ("Missed Opportunity: Transit and Jobs in Metropolitan America"). Using a 45-minute transit commute time, that study found that only 7% of jobs could be reached, in the 100 largest metro areas. Even at 60 minutes, transit could get people to only 13% of the area's jobs. To reach 30% of the jobs, you need an average travel time of 90 minutes, which is more than three times the duration of the average U.S. auto commute.

Knowing this, some advocates of Smart Growth therefore disparage the suburbanization of employment as "jobs sprawl" and seek to promote public policies that would reverse it, so that transit could do a better job. But that confuses means with ends. If the purpose of an urban transportation system is accessibility, we should work to make the system serve that goal, not engage in a utopian quest to massively reshape the urban landscape. And, as I have written in previous issues of this newsletter, the implication for transit is to develop more flexible systems that can link more people cost-effectively to jobs. That argues for grid-based bus systems as opposed to radial bus and rail systems focused on what used to be the "central business district."

Angie Schmitt: A Better Way to Grade City Transportation Systems

A study recently released by the University of Minnesota presents an interesting alternative to the TTI’s metrics. UMN Transportation Engineering Professor David Levinson recently analyzed metropolitan commuting according to a very different criterion: accessibility, or “the ease of reaching desired destinations.”

Levinson attempted to improve on the TTI report by tracking the time it takes for people in the 51 largest U.S. metro areas to reach jobs. His findings stand in stark contrast to the TTI’s report. Large metros like Los Angeles, San Francisco, New York and Chicago offered the greatest number of jobs within a 10-minute car commute, Levinson found.

While TTI’s methodology penalizes cities for locating homes and businesses close together, because that increases congestion, in Levinson’s analysis, higher concentrations of destinations are rewarded for helping to reduce travel times.

“There are two ways for cities to improve accessibility—by making transportation faster and more direct or increasing the density of activities, such as locating jobs closer together and closer to workers,” Levinson writes.

“Accessibility is not a new idea,” he adds. But his is the first study that uses it to systematically attempts to measure how different metro areas compare. The report focuses only on auto access, but the same concepts could be applied to walking, biking, or transit access, he says.

To measure accessibility, Levinson factored in average job density, the average speed of car traffic in the transportation network (from the TTI analysis), and the circuity of trips (how indirect they are). The analysis also looked at the number of destinations within 10-, 20-, 30-, and 40-minute “donuts” around the city.

Levinson found that his measure of “accessibility” is linked to a number of positive economic indicators. For example, he found that home prices in a metro area increase 0.23 percent with every 1 percent increase in accessibility. He also found that doubling accessibility leads to a 6.5 percent increase in real average wages.

There are environmental and quality-of-life connections, as well. Levinson found that a 1 percent increase in accessibility is linked to a 0.06 percent reduction in the share of commuters who drive. He also found that accessibility tends to be linked to shorter overall commute times. A 1 percent increase in “accessibility,” he found, is correlated to a 90-second reduction in average commute time each way.

All of this suggests that prioritizing “accessibility” in transportation investment — rather than alleviating congestion – might be more economically beneficial for metro regions.

Levinson found that accessibility, or the ease of reaching important destinations, has declined in the United States over the past two decades. Image: University of Minnesota
Levinson also measured how accessibility has changed in metro areas over time, finding that it has worsened in American regions overall, both since 1990 and since 2000.

Some additional blog discussion below

Atlanta: JunctionATL Opening Our Eyes to Accessibility

Charlotte: PlanCharlotte: Easy access to work? Charlotte’s not atop list

Los Angeles: CurbedLA Real Study Found That LA is Best in the US For Car Commuting - The Commute

Los Angeles: Crikey Is Los Angeles the best US city for commuting?

NRDC Switchboard: 'Accessibility' trumps traffic:

Some other briefer mentions below

Los Angeles Transportation Headlines : LAX Transit Link, ONT Airport, LA2050, Access Across America, LA Bike Wars, LA Light Syncing, Port Railyard,

The Direct Transfer: Accessibility Ranked in 51 Metro Areas

reprinted at Sustainable Cities Collective

UMNews: New U of M study measures accessibility to jobs in top U.S. cities

ITS Library Access Across America: How accessible are the jobs?

Minnesota Public Radio Study ranks Minneapolis-St. Paul #5 in accessibility to jobs by car in 2010

Drew Kerr Five for Five

TRB E-News

I was interviewed a few weeks ago by Dale Connelly for KFAI Community Radio. The edited interview (aired April 22, 2013) is below as an MP3 (5:51)

Every so often we get a discouraging report on traffic congestion in metropolitan areas. The latest rating from Texas A & M’s Transportation Institute gave Washington DC the worst rating for congestion, followed by Los Angeles, San Francisco, New York and Boston. No big surprise there.

But one University of Minnesota professor says counting stationary cars is only part of the story. David Levinson is a professor of civil engineering and author of the Access Across America Study. He told KFAI’s Dale Connelly there’s more to consider when looking at the problem of traffic congestion.

David Levinson holds the Richard P. Braun/Center for Transportation Studies Chair in Transportation at the University of Minnesota. His study, Access Across America, says some of the cities regularly identified as most congested actually have transportation networks that provide good access to jobs. You can see the study online at http://cts.umn.edu.

0422_Levinson.mp3

CTS Catalyst summarizes our Access Across America report:

Moving beyond mobility: measuring accessibility in U.S. cities

Every year, Americans face a steady stream of discouraging news. We’re spending more time stuck in traffic. Congestion in our metro areas is on the rise. Yet these reports focus almost exclusively on traffic mobility—how quickly travelers can move between any two points via automobile or transit. But according to a new University of Minnesota study, there’s much more to the story.

“Focusing solely on mobility and traffic delay doesn’t provide a complete picture of how the traffic system is functioning,” says Professor David Levinson, the R.P. Braun/CTS Chair in Transportation Engineering. “Travelers in many of these cities have the ability to reach their desired destinations, such as shopping, jobs, and recreation, in a reasonable amount of time despite congestion and slower travel because these cities have greater density of activities. In short, these travelers enjoy better access to destinations.”

A new study, Access Across America, goes beyond congestion rankings to focus on accessibility: a measure that examines both land use and the transportation system. The study is the first systematic comparison of trends in accessibility to jobs by car within the U.S. By comparing accessibility to jobs by automobile during the morning peak period for 51 metropolitan areas, the study shows which cities are performing well in terms of accessibility and which have seen the greatest change.

To generate the rankings for this study, Levinson created a weighted average of accessibility, giving a higher weight to closer jobs. Jobs reachable within 10 minutes are weighted most heavily, and jobs are given decreasing weight as travel time increases up to 60 minutes. Based on this measure, the 10 metro areas that provide the greatest average accessibility to jobs are Los Angeles, San Francisco, New York, Chicago, Minneapolis, San Jose, Washington, Dallas, Boston, and Houston.

“It can be surprising to see that some of the cities often ranked as the most congested also have the highest levels of job accessibility,” Levinson says. “This is due to the density of jobs those urban areas offer.”

Levinson also found that job accessibility has changed over time. In the past two decades, Las Vegas, Jacksonville, Austin, Orlando, and Phoenix have seen the largest percentage gains in job accessibility while Cleveland, Detroit, Honolulu, and Los Angeles have seen the largest percentage drops.

According to Levinson, this research offers an important takeaway for metro areas interested in increasing accessibility. “There are two ways for cities to improve accessibility—by making transportation faster and more direct or by increasing the density of activities, such as locating jobs closer together and closer to workers. While neither of these things can easily be shifted overnight, they can make a significant impact over the long term.”

This report extends the Access to Destinations study, an interdisciplinary research and outreach effort coordinated by CTS with support from multiple sponsors.

 

Related Links

• Research project page
• Access to Destinations Study

Reihan Salam talks about infrastructure, and cites our Access Across America work.

When Thinking About Infrastructure, Focus on Accessibility :

"… David Levinson, a transportation economist at the University of Minnesota, has emphasized that one of the key issues in infrastructure investment is improving accessibility, or the ease of reaching valued destinations. One way to improve accessibility is make it easier to traverse long distances, so you can reach a larger number of jobs and consumption opportunities, etc., in a given amount of travel time from home. Another way to improve accessibility is to bunch up jobs and consumption opportunities and homes, i.e., by increasing density. Levinson finds that while accessibility has deteriorated relative to 1990, it has improved relative to 2000. My sense is that the best way to increase accessibility is to focus on implementing peak road-user fees and using the resulting revenue stream to carefully add capacity at bottlenecks, and also to ease local land use regulations that have proven a barrier to increased density in high-productivity regions. These strategies ought to be pursued in tandem. One crude way of putting this is that while we tend to fixate on the ‘hardware’ layer of infrastructure, we should devote more attention to the ‘software’ layer, i.e., the systems governing the allocation of infrastructure resources. Focusing on accessibility rather than infrastructure spending levels as such will get us much closer to tackling the frustrations that plague commuters. "

Today we released a new report describing the level and change of accessibility across the 51 metropolitan areas in the United States: Access Across America. The website includes an interactive map, the report can be downloaded directly here.

Executive Summary

Accessibility is the ease of reaching valued destinations. It can be measured across different times of day (accessibility in the morning rush might be lower than the less-congested midday period). It can be measured for each mode (accessibility by walking is usually lower than accessibility by transit, which is usually lower than accessibility by car). There are a variety of ways to define accessibility, but the number of destinations reachable within a given travel time is the most comprehensible and transparent as well as the most directly comparable across cities. This report focuses on accessibility to jobs by car. Jobs are the most significant non-home destination, but it is also possible to measure accessibility to other types of destinations. The automobile remains the most widely used mode for commuting trips in the United States.

This study estimates the accessibility in the 51 largest metropolitan areas in the United States for 2010, and compares results with 2000 and 1990.

Rankings are determined by a weighted average of accessibility, giving a higher weight to closer jobs. Jobs reachable within ten minutes are weighted most heavily, and jobs are given decreasing weight as travel time increases up to 60 minutes. Based on this measure, the ten metro areas that provide the greatest average accessibility to jobs are Los Angeles, San Francisco, New York, Chicago, Minneapolis, San Jose, Washington, Dallas, Boston, and Houston.

Job accessibility has changed over time. In the past two decades, Las Vegas, Jacksonville, Austin, Orlando and Phoenix have seen the largest percentage gains in job accessibility while Cleveland, Detroit, Honolulu and Los Angeles have seen the largest percentage drops.

Key findings

1. In 2010, the average American living in the top-51 metro areas could reach slightly fewer jobs by automobile than in 1990, but more jobs than in 2000.
2. Automobile speeds were faster in 2010 than in 2000 (and about where they were in 1990).
3. Overall job losses in these 51 areas have limited accessibility gains associated with faster networks.
4. The average American city is slightly more circuitous in 2010 than in 1990 because roads in newer areas (suburban growth) are not as well connected as those in older areas of the metropolitan region.
5. The overall most accessible metropolitan areas in 2010 were (in order): Los Angeles, San Francisco, New York, Chicago, Minneapolis, San Jose, Washington, Boston, Dallas, and Houston.
6. There have been significant changes among accessibility leaders since 1990, when New York, Philadelphia, Chicago, Miami, Los Angeles, Boston, Cleveland, Detroit, Washington, and Dallas made up the top 10.
7. People living in many smaller metropolitan areas can reach as many jobs by car as people living in much larger areas within both the 10- and 20-minute time frames. For instance New Orleans, Salt Lake City, and Jacksonville are all among the top 10 for number of jobs that can be reached within 10 minutes. Jacksonville, Milwaukee, and Las Vegas are among the top 10 for number of jobs that can be reached within 20 minutes.

There are two ways for cities to improve accessibility—by making transportation faster and more direct or increasing the density of activities, such as locating jobs closer together and closer to workers. While neither of these things can easily be shifted overnight, they can make a significant impact over the long term.

Andrew Owen will represent the Nexus group at the FOSS4G (Free and Open Source Software
for Geospatial - North America 2013) conference happening in Minneapolis, May 22-24.

FOSS Experiences in Transportation and Land Use Research

Andrew Owen, University of Minnesota ­­ Nexus Research Group

The Nexus Research Group at the University of Minnesota focuses on understanding the intersections of transportation and land use. In this presentation, we will examine case studies of how open ­source geospatial software has fit into specific research projects. We will discuss why and how open­ source software was chosen, how it strengthened our research, what areas we see as most important for development, and offer suggestions for increasing the use of open­ source geospatial software in transportation and land use research. Over the past two years, we have begun incorporating open­ source geospatial data and analysis tools into a research workflow that had been dominated by commercial packages. Most significantly, we implemented an instance of OpenTripPlanner Analyst for calculation of transit travel time matrices, and deployed QGIS and PostGIS for data manipulation and analysis. The project achieved a completely open research workflow, though this brought both benefits and challenges. Strengths of open ­source software in this research context include cutting ­edge transit analysis tools, efficient parallel processing of large data sets, and default creation of open data formats. We hope that our experience will encourage research users to adopt open­ source geospatial research tools, and inspire developers to target enhancements that can specifically benefit research users.

Now at streets.mn: Just-in-time consumption: Does the `pint of milk test’ hold water?:

"As with stores, houses too are getting larger over the long run. New suburban homes have more space to store goods in-house. While urban residents export storage to common stores, suburban residents more likely to have second freezers, have more space to store stuff."

This is an update from a post first published in 2007.

At streets.mn I note that the Twin Cities is Fastest Metropolitan Area in the United States (outside of California)

The Webcast for Accessibility Now and in the Future is now available. (Andrew Owen presented it at CTS yesterday). The presentation is also available for download.

The full research reports are:

• Annual Accessibility Measure for the Twin Cities Metropolitan Region, and
• Using Twin Cities Destinations and their Accessibility as a Multimodal Planning Tool

Brendon Slotterback responds to Andrew Owen's post yesterday at Street.MN at Net Density on a Department of Accessibility :

"What if instead of a Department of Transportation we had a Department of Accessibility and it’s mission was to improve accessibility while meeting environmental standards, building resilient systems, and being economically viable? I bet it would look at lot different than our current DOTs (hint: it would do a lot more with land use)."

I think this compares favorably against Richard Florida's proposal for a Department of Cities.

The Department of Accessibility is conceived of as at the state (or metropolitan) level, where the decision making for transportation does (and should) occur and where decision making for land use might occur (now it is mostly local). In contrast, the Department of Cities is a federal initiative, without a clear reason as to why Cities (or any specific places) are a federal responsibility (since the benefits of improving any given city are almost all local).

Andrew Owen writes at Streets.MN : I Love Accessibility :

"That, in a nutshell, is accessibility. It combines the value that we get from travel with the cost of making the trip. I love accessibility because it neatly encapsulates what I consider to be the fundamental purpose of transportation systems: providing ways for people to get to places they want to reach, at costs they are willing to pay."

Accessibility Now and in the Future:

"Tuesday, February 19 2:45 – 3:45 p.m. CST

Room 1130, Mechanical Engineering Building
University of Minnesota
Minneapolis, MN

Presented in conjunction with a joint meeting of the Transportation Planning and the Environment Council and the Transportation and the Economy Council.

About the Event

This presentation will discuss the techniques developed for a 2010 accessibility evaluation of the Twin Cities metropolitan region as part of the interdisciplinary Access to Destinations project. The 2010 accessibility evaluation sought to generate an accurate representation of accessibility in 2010 and to identify data sources, methods, and metrics that can be used in future evaluations.

The seminar will include a discussion of the methodology that can be used to implement future evaluations of accessibility, including a discussion of the development and use of software tools created for this evaluation. Study recommendations for the future will also be shared, such as the importance of standardizing data sources and parameters to ensure comparability between multiple evaluations over time. Other highlights include the need for data sources and methodology that provide a good representation of actual conditions, are based on measurements rather than models, and are usable with a minimum of manual processing and technical expertise.

Speakers

David Levinson is a professor of civil engineering at the University of Minnesota and co-leader of the Access to Destinations study. His current research focuses on understanding the process of network growth, evaluating transportation technology and policy, and modeling travel behavior.

Andrew Owen is a graduate student at the University of Minnesota, where he is pursuing master's degrees in civil engineering and urban and regional planning. His research interests include network structure, travel behavior, transit systems, and multimodal accessibility. 

Webcast

The seminar will also be broadcast live on the web and available for later viewing. The live broadcast access link will be available here on the day of the event."

Recently Published:

• Huang, Arthur and David Levinson (2013) The structure and evolution of a skyway network The European Physical Journal: Special Topics January 2013, Volume 215, Issue 1, pp 123-134 [download]

We study the structure and evolution of the downtown Minneapolis, Minnesota skyway network. Developed by private building-owners, the network evolved from tree-like to grid-like over the course of 50 years. We find that decentralized forces with the goal of maximizing individual buildings’ profitability shaped the network. Our analysis shows that a building with greater office size, a sign of greater accessibility, was more likely to be connected earlier. The distribution of existing skyway segments is found to follow a power-law function of the average degree, closeness, and eigenvector centralities of the vertices. We further explain and model the evolutionary process using an agent-based model. The simulation results suggest that the model replicates the network structure and its evolutionary process.

Recently published:

• Owen, Andrew and Levinson, David (2012) Access to Destinations: Annual Accessibility Measure for the Twin Cities Metropolitan Region. Minnesota Department of Transportation

"This report summarizes previous phases of the Access to Destinations project and applies the techniques developed over the course of the project to conduct an evaluation of accessibility in the Twin Cities metropolitan region for 2010. It describes a methodology that can be used to implement future evaluations of accessibility, including a discussion of the development and use of software tools created for this evaluation. The goal of the 2010 accessibility evaluation is twofold: it seeks both to generate an accurate representation of accessibility in 2010, and to identify data sources, methods, and metrics that can be used in future evaluations. The current focus on establishing replicable data sources and methodology in some cases recommends or requires changes from those used in previous Access to Destinations research. In particular, it is important to standardize data sources and parameters to ensure comparability between multiple evaluations over time. This evaluation recommends data sources and methodology that provide a good representation of actual conditions, that are based on measurements rather than models that provide a reasonable expectation of continuity in the future and that are usable with a minimum of manual processing and technical expertise."

WAMU reports @ Transportation Nation: Prediction: D.C. Area Highway and Transit Crowding Will Get Worse : "

The Washington metropolitan region faces worsening traffic congestion and transit crowding as its population and job growth expand over the next three decades, according to a forecast released on Wednesday by a regional planning group."

Another scare forecast from another Metropolitan Planning Organization. In general when reading these studies:

1. Will behavior not change in response to anything?

2. Will technology not change?

3. Will policy not change?

(On the positive side, they do use a 45 cumulative opportunity accessibility measure for transit).

Daniel Graham, Patricia Melo, and David Levinson (2012) Agglomeration, Accessibility, and Productivity: Evidence for Urbanized Areas in the US. (Working paper)

This paper undertakes an empirical analysis with the aim of improving the current understanding of the relationship between labor productivity and urban agglomeration economies across a sample of urbanized areas in the US. Agglomeration economies are represented with driving time measures of employment accessibility to establish a direct account for the link between transport and agglomeration economies. The paper investigates the presence of nonlinearities in the relationship between labor productivity and agglomeration economies, and examines the spatial decay pattern of the effects arising from this relationship. The findings indicate that there is considerable nonlinearity in the relation between productivity and transport induced agglomeration effects, implying that the estimation of country-level aggregate elasticities is likely to misrepresent the actual magnitude of any productivity gains from urban agglomeration. The results also suggest that the magnitude of the productivity-agglomeration effects decays very rapidly with time and is very strong within 20 minutes driving time. This suggests that knowledge spillover externalities are likely to be a very important Marshallian source of agglomeration economies.

JEL Classification: J31, R12, R40
Key words: agglomeration economies, network accessibility, labor productivity



Paul Anderson, Andrew Owen, and David Levinson (2012) The Time Between: Continuously-defined accessibility functions for schedule-based transportation systems. (Working paper)

Accessibility is traditionally considered to be a property of a point or region in space, and to be invariant over time (or at least over some computationally convenient time interval). How- ever, a locations accessibility can vary over time on a wide range of scales. This temporal variation is especially significant for schedule-based transportation systems. Current measures of accessibility generally reflect the accessibility only at points in time corresponding to the departures of one or more trips; accessibility between these time points remains unconsidered and undefined. Consequently, these measures are insensitive to changes in route frequency and the distribution of trip departure times. Furthermore, these approaches ignore the disutility experienced by a system user who is limited to departing or arriving at scheduled times rather than at preferred times. As a result, they systematically overestimate the accessibility experienced by users of scheduled transportation systems. We establish new methods for representing the accessibility provided by a schedule-based transportation system from a specific location as a continuously-defined accessibility function (CDAF) of desired departure time, defined for all time points. Using schedule and route information from metropolitan transit providers, we demonstrate the application of these methods to gain new insight into the accessibility provided by real-world transportation systems. Four examples are developed to represent common service types in metropolitan transit networks. The results confirm that accessibility is significantly overestimated by measuring single points and show that trip frequency is more valuable for sustained accessibility than high accessibility on individual trips.

Andrew Owen, Paul Anderson, and David Levinson (2012) Relative Accessibility and the Choice of Modes. (Working Paper).

The factors influencing commute mode choice are a subject of ongoing research and policy. Existing literature explores a wide range of factors which may influence mode choice; many of these focus on demographic factors as well as user preferences and perception, thereby highlighting the unique characteristics of each mode. This analysis hypothesizes that mode share, the aggregate expression of individuals' mode choices, is determined in large part by more fundamental properties of transportation systems. Accessibility, which measures the ease of reaching destinations, is used as a tool for comparing modes which focuses on their properties as abstract transportation systems. It explores the potential to predict the relative commute shares of non-auto and auto modes from the relative accessibility provided by each. Using public data sources and methods selected for their simplicity and ease of interpretation, a model is estimated which accounts for 41% of the variation in commute mode share at the block group level in the Minneapolis--Saint Paul, MN metropolitan area.

Adie Tomer @ Brookings: Where the Jobs Are: Employer Access to Labor by Transit:

"The typical job is accessible to only about 27 percent of its metropolitan workforce by transit in 90 minutes or less. Labor access varies considerably from a high of 64 percent in metropolitan Salt Lake City to a low of 6 percent in metropolitan Palm Bay, refl ecting differences in both transit provision, job concentration, and land use patterns. City jobs are consistently accessible to larger shares of metropolitan labor pools than suburban jobs, reinforcing cities' geographic advantage relative to transit routing."

I wrote more about Accessibility Futures : @ | streets.mn.

Recently published, and summarized in CTS Research E-News: Using accessibility to evaluate future planning scenarios:

"Understanding the interdependent relationship between transportation and land use is important for planning the future growth of cities. Recognizing how this relationship affects accessibility—the ability of people to reach the destinations that meet their needs and satisfy their wants—can help policymakers and planners make decisions that optimize a city's efficiency, livability, and economic competitiveness.

In a study funded by the Minnesota Department of Transportation, researchers from the Department of Civil Engineering compared a set of planning scenarios for the Twin Cities metropolitan area using accessibility as a performance measure. Associate Professor David Levinson, undergraduate research assistant Paul Anderson, and graduate student Pavithra Parthasarathi used the scenarios to evaluate the accessibility of various land use and transportation network combinations.

The researchers analyzed the accessibility of 60 different scenarios, including combinations of six land-use scenarios and 10 highway and transit networks. The land-use scenarios included existing 2010 conditions, projected 2030 conditions, and various combinations of centralized and decentralized population and employment conditions.

Highway networks used in the scenarios included 2010 conditions, projected 2030 conditions, an ideal freeflow network with no congestion, and a hypothetical diamond lane network that added high-occupancy toll lanes to all freeways inside the I-494/694 beltway. Transit networks ranged from 2010 conditions to projected 2030 conditions to a 'retro' network that added all 1931 rapid transit streetcar routes to the 2030 network. 

In terms of land use, results show that centralized employment and centralized population had the highest accessibility across all networks, resulting in more access to jobs and labor as well as shorter commute times. The researchers found that fully centralized growth produced about 20 to 25 percent more accessibility than the projected 2030 scenario, depending on the accompanying transportation network.

Of the transportation networks, the researchers found that the freeflow network had the highest accessibility—20 percent more than the projected 2030 network—followed by the diamond lane network.

At first, the researchers say, it would be easy to choose the land use and transportation network combination with the highest accessibility as the future planning goal. However, the scenario of centralized population and employment on a freeflow network—while ideal for accessibility—is not likely to be cost-effective or feasible under current conditions.

Instead, the researchers say, these study results could be used to help prioritize future investments and land-use strategies based on how accessibility-effective they are—how much accessibility they deliver per dollar of investment.

A final report on the project, Using Twin Cities Destinations and Their Accessibility as a Multimodal Planning Tool (MnDOT 2012-05), is available on the CTS website."

A selection process for a local university identified the factors in the attached image as most important. I don't know how the list was formulated.

#1. Vehicular Access. Near the bottom, Pedestrian access. At the bottom, airport access.

Walk Score has put out: Most Bikeable Cities

Just for the record:

Minneapolis - 79

Portland - 70

Their map is here:

I am not sure how exactly they decided this, but we do have our Bike Accessibility data online for your mapping pleasure.

Big cities have lots of congestion. Big cities have lots of accessibility. Therefore congestion causes accessibility. . Therefore accessibility causes congestion.

I don't think the last sentence is necessarily correct either. It might be more accurate to say that congestion is an indicator of economic activity (and poor resource management). However increasing the number of people per unit space (density) without increasing the transportation capacity to move them, or changing their travel behavior, will increase congestion. Similarly, increasing the number of people per unit space (density) without changing their travel costs will increase accessibility, and may increase accessibility even if you increase their travel costs.

As a reminder, accessibility is a measure of the ease of reaching valued destinations, which we often operationalize as the number of things (jobs, shops, schools, etc.) you can reach in a given time (10, 20, 30 minutes) by a particular mode (car, bus, walk, bike) at a particular time of day (7 am, noon, 5 pm, midnight). This operationalized measure is called cumulative opportunities, and is easily explained, even to politicians. Big cities have more things, and higher density, and thus usually have more accessibility, especially at longer time distances. The opposite of accessibility is isolation (or access to nothing, how much of nothing I can reach in 10, 20 or 30 minutes). We know people care about accessibility, since the cost of land in the most accessible places is much higher than the least accessible places.

Congestion occurs when a facility is overcrowded. So if a bridge can carry 3600 people an hour (its capacity), but 4000 people want to use it (its demand), we say it is congested. At the end of the hour, there will be 400 people left waiting to use the bridge (who will presumably be first in line to use it during the next hour). Congestion occurs potentially on all modes, since every mode has a capacity, and on any facility, which again have some capacity. Many modes however are operating very far from capacity, have excess, unused, or spare capacity, and thus don't see congestion with additional users. Congestion matters to users because of the delay that results, the travel time above and beyond what the user would have experienced in its absence.

So can we increase accessibility, which people value, without increasing congestion, which people dislike?

There are several possible strategies:

1. Increase capacity
   1. Expand capacity on the congested mode or facility.
   2. Expand capacity on an alternative mode or facility.
   3. Build new facility.
   4. Better manage existing capacity

2. Reduce per capita demand
   
   
   
   1. Price
   2. Ration
   3. Exhortation
   
   

There are many different ways these can be implemented.

To start, we could simply increase capacity without trying to address demand. Unfortunately, there are rising costs (and diminishing returns) to capacity investments. All the high benefit, low cost projects have already been done (the economist's proverbial $10 bill is not lying on the street is it?). So we are left with really expensive projects to get more capacity, which often involve tunneling or elevated structures (since all the good rights-of-way are already used). Perhaps there will be technological breakthroughs which reduce the costs of tunneling, or other construction. It hasn't happened yet.

We might better manage capacity. This is certainly an option. If we can get the driver out of the loop, so cars can drive themselves, we could narrow lanes (and thus get more of them per unit of pavement) and cars could follow more closely. Both of these things will increase throughput. There are simpler technologies (ramp metering, rapid incident clearance, and so on) that have already been widely deployed in many cities.

Pricing comes in numerous flavors. It might be very precise to time and location, or might be in selected areas, or on selected facilities, or at selected times of day, or everywhere. HOT Lanes that charge by time of day on selected facilities may guarantee that a particular facility is uncongested (by limiting demand on that facility). This is as much pricing for reliability as pricing to reduce congestion. The political advantage of HOT lanes is their voluntary nature, and the untold alternative. We could certainly extend this to truck-only toll lanes (TOTs) and perhaps to some other domains. The London Congestion Charge affects most traffic entering Central London, regardless of time of day or miles traveled. In Germany heavy goods vehicles pay to travel on the autobahn. Some people have even come up with the silly concept of paying people not to drive. By pricing more in the peak (and less in the off-peak) flexible travelers will switch travel times from when capacity is fully utilized to when there is spare. Of course, induced demand operates in all cases, but that can be considered when setting prices.

Rationing seems fair, and is used in many megacities throughout the world, either rationing number of vehicles, or the days they can be driven. During World War II, the US rationed rubber tires and fuel. Rationing however often devolves into a black market (and thus pricing), as people pay for rations. For instance there is a large market in license plates in cities that have day of week license plate rationing.

We often mock exhortation (well at least in my social circles), but on occasion exhortation has been used fairly effectively, often coupled with monetary incentives: smoking has gone down, and recycling up in the US in part due to exhortation. (My 5 year old daughter tells me how recycling is better than throwing in the trash, so clearly the education campaigns start young). How much of the change is due to changing mores and social preferences, and how much to cigarette taxes or trash collection discounts for recycling is hard to say. On the other hand, modern campaigns to encourage transit use and carpooling have been notable failures.

In short, to paraphrase John Lennon and Yoko Ono, Congestion is over! If you want it. We can have accessibility without (or with less) congestion. I don't think we want it badly enough. The choice is really congestion OR pricing, and the political cost of pricing has to date outweighed the political cost of congestion. We don't value the time savings of accessibility enough for politicians to do the things necessary to save time. On the other hand, voluntary tolls (HOT lanes, TOT lanes, and so on) are more politically acceptable, giving people travel time reliability for an uncertain price, which is better than nothing, but certainly not the best possible.

A large part of the problem is thus its political nature. We would not in the US tolerate periodic blackouts from an electric utility because they could not manage supply and demand. In fact this was the what in large part led to the downfall of Governor Gray Davis of California and the rise of Arnold Schwarzenegger. Why do we tolerate the transportation equivalent of queueing? Because roads are owned and operated by governments. If they were separate (and regulated) organizations, not directly responsible to the state legislature, we might have different outcomes. The notions of "free" and "already paid for" and "double-taxation" that are used to politically defeat tolling proposals would be replaced with a "fee for service" concept common in public utilities. My mental model of governance is that "institutions loosely coupled", each with specific missions, management, and revenue, would outperform a giant monolithic government that tries to do everything for everyone.

Getting from our current world to a slightly better one can be achieved calmly and rationally via white papers and deliberation, or through a real or politically generated "crisis" (the preferred mode of governance in the US). I have the feeling that so many people have played the "crisis" card that there is "crisis-fatigue", and as a rationalist would certainly rather we went at this systematically. However, politics does not seem to want to make transitions without some stress. Perhaps the rise of electrification and the collapse of gas tax revenue will be the crisis required to move to a new and different organizational regime in surface transportation. But this is a slowly building crisis that could take the rest of my career, and I am impatient.

Recently released: NCHRP 08-36, Task 102 Assessing Alternative Methods for Measuring Regional Mobility in Metropolitan Regions (pdf):

"The objective of this project is to assess methods for defining and measuring mobility in metropolitan regions. How an agency or jurisdiction defines and measures mobility greatly determines selection of strategies and ultimately investment decisions. In metropolitan areas, measuring mobility at the system level is often limited to the measure of traffic congestion and resulting delay on the freeway and signalized arterial networks. Although traffic congestion does inhibit mobility, it alone may not be a sufficient measure of system performance, particularly as transportation agencies strive to embrace a more multimodal approach to transportation planning."

The report supports the use of accessibility as a standard performance measure.

h. Accessibility as an integrated transportation-land use measure. Trip- based mobility measures are the starting point for accessibility measures, but they are blind to trip purpose or opportunity; they just measure the performance of trips within a given time window. Accessibility measures layer on the trip purpose or type of destination represented by the trip and are meant to measure the ease of reaching opportunities – goods, services, activities and other destinations. Three factors affect accessibility: congestion (or impedance), transportation system connectivity, and land use patterns. Thus, accessibility measures capture all four of these simultaneously; it is still important to understand the contribution of components, especially mobility as this is under more direct control of transportation agencies and easier to communicate to a general audience for a greater range of purposes. Note the accessibility can apply to the ease of getting to activities (such as jobs, recreation, shopping) or aspects of the transportation system itself (freeways, transit route, bike facilities).

[The lead was Cambridge Systematics, with Dowling Associates and TTI, I was on the project team]

Recently Published:

• Iacono, Michael and David Levinson (2011) Location, Regional Accessibility and Price Effects: Evidence from Home Sales in Hennepin County, Minnesota. Transportation Research Record: Journal of the Transportation Research Board #2245 pp.87-94 [doi][download preprint]

Abstract

Regional location factors exert a strong influence on urban property markets, and measures of accessibility are foremost among them. More local influences, such as proximity to urban highway links, also may positively or negatively influence the desirability of a location. This study used a cross section of home sales in Hennepin County, Minnesota, from the years 2001 through 2004, along with a set of disaggregate regional accessibility measures, to estimate the value of access to employment and resident workers. The effects, whether as amenity or disamenity, were estimated for locations near major freeway links that had recently under-gone major construction to add capacity (or were scheduled to undergo such construction) at the time of the home sales. The richness of the home sales data set allowed for control of a number of structural attributes, as well as some site characteristics. Additional neighborhood characteristics (such as income levels and local educational quality) were added from supplemental data sources. Empirical results indicated that households highly valued access to employment. Access to other resident workers (i.e., competition for jobs) was considered a disamenity. Proximity to local highway access points associated positively with sale price, whereas proximity to the highway link itself associated negatively with that price. The study concluded with some implications for research and practice of the concept and measurement of the relationship between location and land value."

Recently published: Levinson, David (2011) Network Structure and City Size. PLoS One PLoS ONE 7(1): e29721, January 12, 2012 [doi]

Network structure varies across cities. This variation may yield important knowledge about how the internal structure of the city affects its performance. This paper systematically compares a set of surface transportation network structure variables (connectivity, hierarchy, circuity, treeness, entropy, accessibility) across the 50 largest metropolitan areas in the United States. A set of scaling parameters are discovered to show how network size and structure vary with city size. These results suggest that larger cities are physically more inter-connected. Hypotheses are presented as to why this might obtain. This paper then consistently measures and ranks access to jobs across 50 US metropolitan areas. It uses that accessibility measure, along with network structure variables and city size to help explain journey-to-work time and auto mode share in those cities. A 1 percent increase in accessibility reduces average metropolitan commute times by about 90 seconds each way. A 1 percent increase in network connectivity reduces commute time by 0.1 percent. A 1 percent increase in accessibility results in a 0.0575 percent drop in auto mode share, while a 1 percent increase in treeness reduces auto mode share by 0.061 percent. Use of accessibility and network structure measures is important for planning and evaluating the performance of network investments and land use changes. Keywords: Connectivity, Network Structure, Transportation Geography, Network Science, City Size, Scaling Rules, Accessibility, Travel Behavior, Mode Share, Journey-to-Work

This paper has several features:

1. The paper includes a ranking of 50 US cities by estimated accessibility (Table 3). This estimate is macroscopic, though I think quite plausible, and shows the variation in the 10 minute vs. 20 minute ... vs. 60 minute and composite accessibilities. The composite numbers are more or less what you expect, but some small cities are quite fast, so have high 10 or 20 minute accessibilities by car. Lots of work remains to be done on this (both multiple modes and multiple points in time) but this should be a valuable metric.
2. Larger cities are better connected. They are also more productive. This research suggests a hypothesis (which further research will need to test) that variations in network structure may explain variations of economic output. More connected cities are more efficient. It is not simply how many people are in the city (the classic economy of agglomeration argument) but how they are connected that affects their productivity.

I will also comment about the publication itself. It was published in PLoS One, a first for me. PLoS ONE is a newish, open content journal across part of the Public Library of Science family that aims to represent all fields of study. I did this as an experiment as much as anything. The paper is out less than 4 months after submission, and 2 months after revision. This is *fast*, much faster than for-profit publishers offer. The journal is interdisciplinary, and does not winnow for importance (letting the field do that), instead winnowing for quality of the work and its description. Everyone in the field knows how arbitrary publication is when paper is a constraint. This seems an improvement.

Reihan Salam at NRO questions the Kotkin hypothesis, asking: "Are people choosing low-density metropolitan areas — or did rising prices in high-density metropolitan areas [like Marin County, in the Bay Area pictured -ed.] drive the population shift?"

What is cause, and what is effect, is not immediately obvious. There are trade-offs. High-density areas are naturally more expensive (due to greater demand, otherwise they would not be high-density). High-density areas are also typically formed by physical constraint, meaning less supply. They are also more regulated due to their higher density. Density naturally produces more regulation because density naturally produces more externalities [See tomorrow's post]. Even expensive metro areas have inexpensive housing in places, it just tends to be either lower quality, or in less desirable neighborhoods.

I wrote Height limits produce a positive externality recently, to which the technology/urbanist/libertarian blogger Timothy Lee tweeted (but I took more than a month to respond to, since I don't live on Twitter), and I replied (this is reformatted for presentation, but I think captures what was said in the right sequence)...:

You don't think there's a shortage of space near multi-line transit stops like Metro Center and Gallery Place/Chinatown?

The higher you build there, the shorter you build elsewhere. There is plenty of land in DC that could be denser at less than 10 stories.

why does taller buildings one place mean shorter buildings elsewhere? There is a region-wide housing shortage.

Regional demand is largely fixed. Someone who can't get in block X will be far more likely to locate in block Y than Charlotte.

you don't think real estate prices affect migration between metro areas?

not much. People have jobs before they migrate. Firms locate for lots of reasons, but a shortage of hsg in a small dtwn no.

there is plenty of moderately priced real estate in metro DC, SFO and elsewhere, how else could poor people live there?

Only if the savings on the labor costs outweigh the savings on economies of agglomeration. This indicates few e of a.

I recommend amazon.com/Gated-City-Kin…

I read Avent and Glaeser, I believe they overstate e of a. The more important point is full social costs. New dev. should pay.

Firms move to save on labor costs (which are connected to housing costs) all the time.

If everyone paid full social cost, (and compensated losers) build away. In the absence of FSC pricing, we regulate.

Clearly Twitter is not a good way to have an academic discussion. Blogs are much, much better. There are several points wrapped up in this:

• Empirical questions about Intra-urban vs. Inter-urban migration
• The rights of the property owner vs. the rights of the community
• Economic productivity (positive externalities?) vs. Pollution/Congestion etc. (negative externalities)
• Empirical questions about the scarcity of land
• Empirical questions about what constitutes good urban form
• Empirical questions about the need to be downtown or simply in the metro area [plenty of suburbs even in DC would be happy to accommodate growth]

In short, I think height limits are not the dominant issue in any US metropolitan areas. This is not to say that regulation does not matter at all, as there are lots of regulations beyond height limits, but that its effect is limited. I discussed this previously in Zoning and Externalities.

If there is value, where are the side payments from otherwise rejected developers? My perhaps cynical view, 'No side payments, no evidence of super-normal social profits, no evidence of huge value being lost'. While the developer may be losing potential profits, society is not, as those who are negatively affected are not being given compensation to offset the negative effects they would receive were the project to go forward. I realize there are transactions costs limiting the ease of implementing side payments, but surely some institution could arise to facilitate this.

I also wrote on the Limits to agglomeration, suggesting that agglomeration economies arguments are overstated, and in fact it is agglomeration externalities that create density, not (or not so much) vice versa.

DC and New York are both edge cases, being political and financial capitals respectively, both of which at least historically generated important economies of agglomeration.

One of the critical problems here, as with much research is the difference between marginal and average effects. E.g. Clearly transportation matters. If there were no transportation there would be no economic activity. However, that does not mean that a marginal increase in transportation supply will have a significant, or even positive, effect on economic activity, that depends on context. The network is mature, the marginal returns to new investment now are much lower than the historical average returns. Similarly, the marginal returns to density might be much smaller than average returns. Cities exist for a reason. That reason is economies of agglomeration in various forms. That said, where cities are continuing to grow, those economies must be valuable. Where suburbs are growing, the daily face-to-face inter-firm interactions emerging from the classical 19th and early 20th century transit-based downtown has declined relative to the need to be within auto-commuting distance of places that are to be dealt with on a short-term basis. When new cities grow, new patterns of economic activity are forming, and these may be more valuable than incremental changes to mature cities.

All of which is to say cities and their economies are dynamic, and the first order factor is the underlying market economics, while regulations (which are themselves the product of political market preferences) are second order effects.

WaPo: Maryland cuts ribbon on new toll road between Shady Grove and Laurel :

WaPo: ICC puts strain on Maryland’s transportation funds

Baltimore Sun: ICC opening to bring regions closer :

"Next month's opening of the main section of the Intercounty Connector linking Interstate 95 with Interstate 270 in Montgomery County is expected to have significant effects on Baltimore's economy as it brings the state's richest job and commercial market a half-hour closer to its largest city.

The debut of the new section Nov. 22 will close the gap between the already opened western section of the ICC and I-95 in Prince George's County. Unlike the first section, which has been mostly used for local traffic, the opening of the new stretch is expected to bring immediate benefits to many Baltimore-area drivers for whom the trip to Rockville or Gaithersburg has long been a traffic nightmare."

I drove the first section this past summer on my periodic visits to Montgomery County. It is a very nice ride, perhaps one of the more pleasant roads to be on. Compared with most long-distance roads on Montgomery County, it does not (yet) suffer congestion (and with tolls, it may almost never). It may even be faster to places like Aspen Hill from Tysons than using surface streets, despite the added distance (and cost).

As a planning question, rather than a subjective ride question, my views are more mixed. I did not work on the road plans when I was at MNCPPC, though we did model it (and tested what would happen with and without, just for fun). Clearly induced demand and induced development applies. Build it and they will come, don't and they won't. That does not of itself mean it is a bad thing, but it will shape development patterns assuming the tolls are not so high as to capture all of the benefits.

Montgomery County has lots of land use controls, so the induced development will, at first be in compliance with existing plans. But as new accessibility creates new value, the pressures to change zoning to accommodate that potential value will intensify. My own sense is the key points are the interchanges at I-95 in PG County, US 29 (itself mostly converted to freeway) and Shady Grove. Maybe not this election, maybe not the next, but at some point. And once that window is opened, facts on the ground will be essentially irreversible.

The other pressures will be to extend eastwards and westwards. I think eastwards (as once proposed to US 301) is more likely, since it is within the state. The flows on to and off of the ICC will inevitably create pressures on upstream and downstream links. Whether those pressures get relieved depends on numerous factors (roads are obviously slow to build and long to unbuild).

The Montrose Parkway, once the Rockville Facility, is a stub that was originally to connect the ICC with I-270 south of Rockville. As the right-of-way for that was turned into Matthew Henson State Park in the early 1990s, it would be difficult to revive. On the other hand, Matthew Henson State Parkway has a nice ring.

Ryan Avent in the NY Times: One Path to Better Jobs - More Density in Cities (His article is apparently based on his new Kindle Single Gated City, which I have not yet read). I have several comments.

1. I like density. This is a personal preference. Being mostly (small "l") libertarian, I feel no need to impose this on others. If lots of people like density, they should create dense places, or hire developers to do this for them. If they are more productive in dense places and earn more money, or are more consumptive and get more value for money, they are even more incented to do so. I am all for fewer regulations so long as negative externalities associated with density (congestion, pollution, parking spillover, etc.) are properly internalized. The lack of good property rights, road pricing, parking pricing, pollution pricing, etc., however, has led communities to develop a regulatory rather than pricing approach. See: Zoning and Externalities.

2. Population density of the earth, the United States, Minnesota, etc. increases over time if population increases and land area does not.

2a. Density of suburban areas is increasing over time as they are transformed from farms to not-farms.

2b. Local density may decrease as cities of fixed boundaries depopulate. Part of this problem is the fixity of boundaries, part is consumer's desires as shaped by markets and policies to consume areas with locally lower density.

3. I think the most important part of this argument is backwards. The primary causality is that cities (metropolitan areas) with growth create density as people and firms are attracted to faster growing areas (NIMBYism is certainly a cost, but there is plenty of housing in the Bay Area that goes relatively undemanded, think of Oakland, much of which begs for gentrification. Clearly housing is not so dear as to cause people to do *that*.). Take away the growth and leave the built infrastructure and even the people and you have a decaying city which slowly (or quickly) depopulates.

4. Accessibility vs. Density. Density of itself does not create growth (despite economic correlations suggesting otherwise - correlation is not causation) or lead to more productivity without a useful transportation system. This is why fast growing areas are auto-oriented in many places, cars are faster than transit under many land use pattern/road network configurations and connect to more places in less time.

Accessibility is the product of both density and speed, how much one can reach in a given time. Certainly density helps increase accessibility to a point (where the benefits of more people are outweighed by the congestion/pollution costs, etc.), but so does a faster transportation network. At the point where connecting by personal transport fails, mass transport may be appropriate, and is certainly necessary for very large conurbations like New York. The pedestrian city maxes out at about 50,000 (see City of London c. 1800 - 1850).

5. Density is quite low in many places he describes (Silicon Valley, rather than San Francisco, is the job engine of the Bay Area, Oakland has plenty of density compared with say Cupertino or Mountain View). But there is still plenty of accessibility by auto (if not by walking, biking, or transit).

6. Some industries (finance, government, media) value the connectivity provided by accessibility more than others (agriculture, large scale manufacturing), and so are more likely to cluster together. It turns out, these are also presently the faster growing industries in the economy. Unfortunately the rules that help finance be more productive (concentrate everything in Manhattan or London) does not necessarily generalize to other industries or other cities. Those cities just don't get as many financial headquarters and offices, and don't see the growth in those sectors, and don't need the accessibility as much (the premium they are willing to pay for accessibility), and aren't as dense.

Similarly, within metro areas, the old, dense, downtown retain the regional financial, legal, and government headquarters while the new suburban areas that don't value the propinquity get less, since the density is costly and the benefits for those in certain sectors is small.

7. There is also the inter-firm vs. intra-firm spillover argument. Many large firms, like universities and hospitals, build campuses to capture internally the spillovers from the random contacts that accessibility brings. This might lead to less vibrancy and turmoil in the market sector, as there are fewer independent actors competing, but it is also a natural stage of development from birth to maturity in sectors. To obtain economies of scale, firms consolidate, and fewer and fewer firms capture more and more market share. Ultimately gales of creative destruction come and the stagnancy of the few firms leads to their own demise, but that is the natural order of things. If firms could never capture their downstream profits by growing large, no one would ever invest. After all, the first few years are much less profitable than those after the monopoly/oligopoly has been established. (And capitalism is not about free markets and free trade, it is about profit (not that there is necessarily anything wrong with that, but let's be honest)).

There are thus natural cycles. Firm formation is naturally higher in some sectors, in some periods, and in some places than others. But the successes of dynamism lead to consolidation. Increasing density exogenously (or removing some regulatory constraints in a few places where they are binding) has very little to do with this.