# Volume or Flow

Wikipedia says: "Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance (solid, liquid, gas, or plasma) or shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic metre."

Wikipedia defines traffic flow: "Flow (q) is the number of vehicles passing a reference point per unit of time, and is measured in vehicles per hour. "

But In telecommunication networks "traffic volume is a measure of the total work done by a resource or facility, normally over 24 hours, and is measured in units of erlang-hours. It is defined as the product of the average traffic intensity (in erlang) and the period of study (in hours)."

In the some of the transportation literature, both flow and volume are terms that mean the same thing. This is confusing. Flow is clearly used as a rate per unit time, while a volume is a quantity, more analogous to a total count. Of course that count occurs over time, so they can be equivalent. But we have "volume to capacity ratios", where the volume and capacity are both in units of vehicles per unit time, which is to say, in units of flow. This is even worse when we consider that the next sentence in the description of volume says: "The volume of a container is generally understood to be the capacity of the container." Which implies all V/C ratios are 1. Clearly capacity is not understood as a flow either.

If I were a student, I might easily confuse "volume" and "density", though of course they are different. Density is the instantaneous number of vehicles per unit space, and is thus more intuitively aligned with the basic geometric notion of "volume".

I propose we excise the term "volume" from the transportation literature when we mean flow. I will purge it from my vocabulary. Flow is a count per unit time. Most of the time when we say "traffic volume", we could simply say "traffic" or "traffic count" or "traffic flow", depending on the context. Wikipedia says "Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel. "

I also sometimes hear this technical term "Flux", which is in most sciences defined as flow per unit area. In traffic it seems to be essentially synonymous with the word flow (someone please educate me on the difference, if there is one).(They both come from the same Indo-European root, though by different routes).

This page contains a single entry by David Levinson published on December 7, 2011 8:19 AM.

In defense of zoning was the previous entry in this blog.

Deconstructing Busytown is the next entry in this blog.

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