Archive for the ‘Congestion’ Category

Space on our streets

August 13, 2009

I’ve read several interesting posts in the past few days on how we allocate space on our streets, and the implications of those decisions for public space, transportation, and congestion.  This hearkens back to discussions of older urban designs, where streets were laid out as public spaces, not delineated into auto lanes, sidewalks, medians, and the like.

Matt Yglesias: Why compartmentalize street space in the first place?

Whenever people start complaining about urban cyclists not following traffic rules, the typical response is to say that cyclists need more dedicated space on the road rather than awkwardly being shoved into street traffic.

But when I think about this, I’m always reminded of the fact that arguably we need fewer traffic rules. The basic idea of traffic rules—separated uses, painted lane markers, giant signs, etc.—is to make it safe for the drivers of cars to drive their cars very quickly. That’s an okay design principle for a highway, but its nearly-universal adoption as a design principle for urban roadways is arguably very misguided. If it were up to me, more city streets would follow Hans Monderman’s shared space principles and just be undifferentiated stretch on which cars, bikes, mopeds, pedestrians, etc. are all free to travel. The over-arching “rule” would be “don’t collide with anyone.”

Ryan Avent: Make better use of the road.

Because, you know, there were roads before there were cars. In Washington, for instance, Pierre L’Enfant set out an entire street system for the capital, despite the fact that there would be no cars at all using those streets for over a century. None! And New York and Boston, and London and Paris also had tons of streets, as far as I can tell, before a motor vehicle ever puttered into those cities.Meanwhile, I’m not suggesting that pedestrians and cyclists “unite against cars.” I’m suggesting merely that the way we currently use city streets gives too much deference to drivers. There’s nothing antagonistic here; I’m simply suggesting that current policy doesn’t actually use street space all that well.

Amongst all of that road space dedicated to cars, a couple of posts (including a throwback) about the nature of congestion within that space.

Freakonomics: More road choice slows everyone down.

Two physicists and a computer scientist used Google maps to study traffic in Boston, London, and New York, and found that when people use real-time driving maps to try to pick the fastest routes, traffic slows down.

And, the throwback – the physics of gridlock.

World Streets: What causes traffic jams?  The answer may be nothing at all.

But the physicists added some terms to the equations to take the differences into account, and the overall description of traffic as a flowing gas has proved to be a very good one. The moving-gas model of traffic reproduces many phenomena seen in real-world traffic. When a flowing gas encounters a bottleneck, for example, it becomes compressed as the molecules suddenly crowd together — and that compression travels back through the stream of oncoming gas as a shock wave. That is precisely analogous to the well-known slowing and queuing of cars behind a traffic bottleneck: as cars slow at the obstruction, cars behind them slow too, which causes a wave of stop-and-go movement to be transmitted “upstream” along the highway.

The eeriest thing that came out of these equations, however, was the implication that traffic congestion can arise completely spontaneously under certain circumstances. No bottlenecks or other external causes are necessary. Traffic can be flowing freely along, at a density still well below what the road can handle, and then suddenly gel into a slow-moving ooze. Under the right conditions a small, brief, and local fluctuation in the speed or spacing of cars — the sort of fluctuation that happens all the time just by chance on a busy highway — is all it takes to trigger a system-wide breakdown that persists for hours after the blip that triggered it is gone. In fact, the Germans’ analysis suggested, such spontaneous breakdowns in traffic flow probably occur quite frequently on highways.

Basically, we’re talking about Chaos Theory applied to traffic movements.  It all stems from the fact that you’re dealing with thousands of individual actors (drivers) within a complex system.

If breakdowns in flow can result from such small and random fluctuations, then the world is a very different place from the one that most traffic engineers are accustomed to. The very notion of maximum capacity for a highway is called into question, because even at traffic densities well below what a highway is designed to handle, jams can spontaneously arise. “If this flow breakdown can take place just anywhere,” says James Banks, a professor of civil and environmental engineering at San Deigo State University, “then we’re in trouble, because there’s a lot more potential for congested traffic than we thought was the case. And it makes a control strategy much more difficult.”

Some of us would argue that the world is already a different place from the one the traffic engineers like to envision (wink wink).  It also highlights the capacity advantages of mass transit in urban areas.  Looking back at the hypothetical Frumination post on NYC without the subway (mentioned here), the point becomes more clear.  Under the assumption that each train is carrying over 1,000 people (and assuming, for the sake of argument, that each one of those would otherwise be driving a single occupant vehicle), the advantages of the subway become clear.  Not only is it more spatially efficient, but you’re dealing with one actor and one vehicle (the subway train and the operator) rather than 1,000.

Furthermore, getting into the earlier discussion of traffic rules, individual drivers tend to emphasize different points of the law.  Each person has a different driving style in how they accelerate, brake, when they change lanes, etc.  Subway trains do not have that freedom, the details of their operations are far more controlled, and thus far more predictable.

The freedom that many people (like Gary Imhoff) associate with the automobile is also the basis for massive congestion.  The car giveth, and the car taketh away.  This fundamental reality often escapes folks arguing for more auto-based infrastructure.  In many ways, it’s a tragedy of the commons – each additional car in traffic makes sense for that self-interested driver, but the overall impact is bad for everyone.