The Antiplanner’s Library: Traffic
posted in Book reviews |The Antiplanner finally picked up a copy of Traffic: Why We Drive the Way We Do (and What It Says About Us), a popular book by journalist Tom Vanderbilt (whose web site, How We Drive, seems to be down at the moment). Vanderbilt interviewed numerous people (including several of the Antiplanner’s friends) to learn about the psychology and physics of driving.
Vanderbilt explains that the word “traffic” doesn’t mean “congestion” but simply “movement.” Appropriately, the book focuses at least as much on safety as on congestion. Vanderbilt is an entertaining writer who manages to present a balanced view of many controversial issues.
For example, one chapter is titled “Why More Roads Lead to More Traffic.” He explains that, in many urban areas, congestion has led to a “latent demand” for more travel: that is, people would drive more if the areas were less congested. So, when new roads are built, they soon fill up. That doesn’t mean that new roads would get congested if they were built in an area that didn’t suffer from so much congestion that there was a significant amount of latent demand.
“Latent demand is often described by another phrase, ‘induced travel,’ which is really just a twist on the same thing,” Vanderbilt adds. “There is a huge and enervating literature about this, which I heartily do not recommend” (pp. 154-155). The Antiplanner would add that induced travel is a spin term designed to make us think that such travel is frivolous or (even more irrationally) that new roads really do increase travel demand (just like new cell towers lead to more phone calls and new hotels lead to more adultery). But Vanderbilt, perhaps wisely, avoids this issue.
I have a few minor quibbles with Vanderbilt’s book; perhaps the biggest one has to do with congestion pricing. Vanderbilt makes the common mistake — one sometimes made even by advocates of congestion pricing — of suggesting that the goal of congestion pricing is to reduce auto driving. “Congestion pricing can help reverse a long-standing vicious cycle of traffic,” says Vanderbilt, “one that removes the incentives to take public transportation” (p. 167). In other words, congestion pricing can get more people to ride transit.
In fact, the real goal of congestion pricing is to increase traffic flows and allow more people, not less, to drive during rush hours. As Vanderbilt himself notes, when freeway speeds are 55 mph, a single lane can move as many as 2,400 cars per hour. But at 35 miles per hour, the same lane can move only 1,700 cars per hour (p. 121). If there are 2,000 cars per hour trying to get through the lane and one of the slows to 35, even for an instant, suddenly the lane’s capacity is below the demand and speeds will remain at or below 35 until the demand falls below the new capacity. (Traffic engineers call this “level of service F.”)
The purpose of congestion pricing is to make sure that volumes remain low enough that “incidents” (such as one car slowing down) do not cause capacities to fall below demand. A well-designed congestion pricing system can therefore maintain throughput at, say, 2,000 cars per hour instead of the much lower throughputs that would result if speeds fell to 20 or 30 mph. Potentially, then, congestion pricing can effectively double rush-hour highway capacities.
Maybe if we did a better job of explaining how congestion pricing will increase, not decrease, people’s mobility, there would be less opposition to the idea. Unfortunately, some people do want to use congestion pricing to reduce driving (not necessarily including Vanderbilt), which means they would charge much higher rates than needed to keep traffic flowing so that they can divert the funds to expensive rail transit projects.
Congestion pricing isn’t the only tool that can maintain traffic flows. Another is adaptive cruise control, available on on the top-of-the-line Prius as well as many more expensive cars. This technology combines lasers or radar to detect a car in front of your car with electronic acceleration and braking to keep your car a fixed distance from the car ahead.
Vanderbilt explains that a lot of congestion takes place because cars run in “platoons.” If the lead car in a platoon slows, all the cars behind it must slow a little more because of people’s slow reaction times. Eventually, if the platoon is long enough, one car and all the cars behind it grind to a halt, leading to a stop-and-go traffic situation even though no one who has to drive through it ever knows what caused the slowdown in the first place.
Adaptive cruise control can prevent this from happening. As Vanderbilt notes, simulations have found that if “as few as two in ten drivers” use adaptive cruise control, many traffic jams “could be avoided altogether” (Vanderbilt’s emphasis, p. 129). So one alternative to congestion pricing might be to accelerate the introduction of adaptive cruise control on more affordable cars.
Still, Vanderbilt’s book is less about solving congestion than helping people understand their personal driving habits. It is very good at doing so, but those who want a book that focuses on reducing congestion will have to look elsewhere.
posted on September 21st, 2009 at 2:32 am
posted on September 21st, 2009 at 9:30 am
posted on September 21st, 2009 at 1:48 pm