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Physics of traffic jams explained

Marc Lefkowitz  |  08/31/16 @ 3:45pm  |  Posted in Transportation choices

It’s rush hour and the street is about to fill up with traffic again, frustrating all the people who expect to make it home on time. You think, if not for all these cars, this commute could take half the time. (Not to mention save Americans 3.1 billion gallons of fuel and 400 hours of personal time that we could put to other uses).

Transportation space explained<br />Three modes of transportation and the space they require on the same stretch of road. 17,600 pedestrians or 14,400 cyclists versus just 4,400 motorists can move through the same space in one hour. Image: Eric Sanderson, Terra Nova.

But, how do we untangle the knot of traffic? The answer may be found with physics.

Imagine that a street is a pipe. Each vehicle, person, dog, shopping cart and bicycle is the material we want to flow through. Our goal is to move a lot of people to the end of the pipe. Vehicles are like friction; in a limited space they cause people to slow down. How can we use the street, then, to maximize flow of people home?

Thankfully, some very smart folks have already worked out this problem—using the physics model of “fluid dynamics.” It is illustrated really well in Eric Sanderson’s book Terra Nova (shown above). The author, a conservation ecologist, writes:

“We have choices about how we use the roads. During rush hour, cars turn out to be the least efficient use of the public domain, moving the fewest people in an hour.

Walking and biking move three to four times more folks.

Shared forms of transportation (like a bus) move twelve to fifteen times more passengers.”

This phenomenon can be explained by measuring the space taken up by people on the sidewalk (4 square feet) and comparing it to the space a car needs to operate and park (20 times that).

Traffic engineers attempt to fix the problem by widening the road. The road still clogs. So they widened it again, and again.

It turns out, the road doesn’t make a very good “pipeline” in the sense that widening it doesn’t unclog it. Roads continue to fill up. Wider roads are a less hospitable environment for alternatives to driving. Building trades have to be paid, and the cost is usually passed on to everyone, even those not operating a vehicle. Rents go up. So do the costs of goods and services because businesses have to pay the rent.

How can the dynamic change? For a start, cities can take a closer look at their zoning codes for subsidies and market distortions to driving only. A classic example is the parking minimum requirements that force the hand of the private market to add more parking than is necessary (think big box parking lot). Another is preventing high-density, mixed-use places to be built.

Reforming these two areas would bring destinations closer together and create more walk-friendly places. Cities would be able to free up more street space dedicated to moving people. Road diets, for example, would be an easier sell as a low-cost way to carve out transportation space for people to bike and walk safely and comfortably.

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