Our first report in the Access to Destinations Series: Development of Accessibility Measures
has finally been released.
The most interesting finding (which still awaits corroboration) is that despite the rising congestion of the past decade, accessibility in the Twin Cities region (measured as the number of things (jobs, workers, etc.) that you can get to in a fixed period of time) has been improving. Clearly this would be because there are more things per unit time, not because you can cover more distance per unit time. Increasing density increases accessibility, this is why cities form in the first place, it is nice to see it in the data. More in the final report. Thanks to my colleague Ahmed El-Geneidy who did most of the number crunching.
Executive Summary: (adapted from report)
Transportation systems are designed to help people participate in activities distributed over space and time. Accessibility indicates the collective performance of land use and transportation systems and determines how well that complex system serves its residents.
The word ‚Äúaccessibility‚Ä? has been around in the transportation planning field for more than 40 years, yet one often sees the term misused, so clarity in definition is important. Accessibility measures the ease of reaching valued destinations. Several cities use congestion levels and annual mobility reports to evaluate the performance of the transportation system, yet this misleads by looking only at the costs of travel while ignoring the benefits. This research demonstrates how accessibility can be used as a tool for evaluating the land use and transportation system in the Twin Cities region.
Individuals interpret accessibility based on their individual priorities. Different types of accessibility that can be considered (to jobs, to work, to shop, to school) by mode. People rank the cells in this table (purpose vs. mode) based on individual priorities and their preferred mode(s) of transportation. For a public agency (department of transportation), the target may be increasing accessibility in all the cells. More columns can be added to the right-hand side of the matrix to represent other important opportunities. More rows can be added to consider other modes (e.g., freight). More pages can be added to indicate different points in time.
Observing the accessibility matrix, it is clear that it includes many of the factors affecting residential location. The matrix also includes many variables that affect land value, so any increase in accessibility can be translated to a dollar value or a premium. The focus of this research is to demonstrate what kind of accessibility measures can be used to fill in each cell in the above matrix. The research team focuses on accessibility to jobs and residents (or labor) using the automobile mode as an example to demonstrate the various measures of accessibility.
This research project comprises three main tasks. The first task reviews the literature on accessibility and its performance measures with an emphasis on measures that planners and decision makers can understand and replicate. The second task identifies the appropriate measures of accessibility, where accessibility measures are evaluated in terms of ease of understanding, accuracy and complexity. The third task illustrate these accessibility measures. During this process, a new accessibility measure named ‚ÄúPlace Rank‚Ä? is introduced as an accurate measure of accessibility that can take advantage of the vast amount of origin and destination information that is now available for land use and transportation planners. It is a measure that can be implemented and adopted in other regions without knowing point-to-point travel time.
In the place rank measure, the level of accessibility in a zone is determined based on the number of people coming into this zone to reach an opportunity. Place rank accounts for the number of opportunities that an individual passes over in a zone to reach an opportunity in another zone. As a result, a destination zone has a higher ranking if it is able to attract more workers from zones with high numbers of jobs.
In addition, several previously-defined accessibility measures are reviewed and demonstrated in this report. Cumulative opportunity and gravity-based measures tend to be similar when travel time is less than or equal to 30 minutes. The gravity-based measure is widely used in the literature, yet cumulative opportunity tends to be easier to
understand and interpret by planners and higher level administration. A major contribution of this research is the comparison of accessibility measures over time and among various modes. Various accessibility measures are used to generate a longitudinal analysis measuring the changes in accessibility levels in the region. The report shows accessibility over time using a cumulative opportunity measure for the years 1990 and 2000, while the consecutive figures show the difference between accessibility to jobs and accessibility to residents measured in 1990 and 2000 using 15 minutes of travel time as the base and auto as the mode of transportation. The travel time estimation is obtained from the Metropolitan Council transportation planning model, while the land use data comes from the Bureau of the Census.
All of the studied measures of accessibility possess similarities, which are observed using both visual and statistical methods. Effects of accessibility on home sales are also tested to generate a better understanding of the value of accessibility to individual homebuyers. All tested accessibility measures to jobs are found to have a positive and statistically significant effect on home sales, while keeping all other variables affecting home sales at their mean values. On the other hand, all tested measures of accessibility to resident workers (the labor force) show a negative and
statistically significant effect on home sales, while keeping all other variables affecting home sales at their mean values. Homebuyers pay a premium to live near jobs and away from competing workers.
Accessibility promises to be a useful tool for monitoring the land use and transportation system, and assessing and valuing the benefits of proposed changes to either land use or networks. This report proposes to use it in a way that engineers, planners, administrators, decision-makers, and the public can easily understand. Finally the report includes a discussion regarding how accessibility over time can be used to generate a land use and transportation performance measure to help in evaluating these systems both within a metropolitan area and between cities.
Accessibility illustrates clearly the benefits that transportation provides, connecting people with destinations given the travel times on the network, rather than simply focusing on costs (the congestion that people experience when moving along roads).