Two summers ago in Paris, I was astounded at the volume of traffic that somehow managed to negotiate the traffic circle at the Arc de Triomphe without incident. Here’s the (poor quality) video I made to document traffic flow there:
I learned to drive in my 20s in New York City. Like Paris, New York has a traffic rhythm all its own, where lane markings are mere suggestions. In New York, parking is tolerated nearly anywhere, as long as traffic isn’t unduly impeded. I’ve seen people double-parked, triple-parked, parked on corners, on sidewalks, you name it.
Driving into Manhattan several times a week also helped me developed what I liked to call “highway justice” — the sense that if someone did something “unjust” as a driver, they would deserve whatever punishment the others meted out to them in traffic.
The biggest injustices in New York always seemed to revolve around highway merges. If traffic was merging onto a busy highway, justice dictated that they should be allowed in every-other-car. If mergers tried to double up, or if highway drivers didn’t allow cars in, they were violators who deserved to be punished (although I was smart enough not to attempt to mete out this justice myself).
But it was completely acceptable to wait until the last second to merge (if, for example, a lane was closing ahead due to construction). Highway space was at a premium, so why not use it all? Once I moved to North Carolina, however, I soon learned there was a very different sense of justice. Late merging and horn-honking were considered rude, and the worst injustice was driving too slowly in the left lane. So which justice system is better — North Carolina’s, or New York’s (or Paris’)? Surely science has something to say about all this.
Of course it does, and Tom Vanderbilt does a fantastic job negotiating through the vast array of studies about traffic and driver behavior in his book Traffic: Why We Drive the Way We Do.
The problem of early- versus late-merging is one of the first examples Vanderbilt uses in the book. As with nearly every problem relating to traffic, it turns out the answer is extremely complicated, and much depends on the context of the merge. Is the merge so disruptive that traffic is slowed considerably? Then late-merging is probably preferable, since this better-utilizes the capacity of the highway. Why back people up for miles when an extra lane is available? But if the highway is relatively clear and people are whizzing along at 70 miles per hour, waiting until the last minute can be downright dangerous. Perhaps the best solution is to install digital signs that can change instructions to mergers based on traffic levels. But even this is problematic — once regular drivers accustom themselves to one style of merging, it’s difficult to get them to change.
This, perhaps, is the central lesson of the book. Traffic and highway management isn’t like other engineering problems, because driver behavior adapts quickly to new situations, which can create completely different traffic problems. Consider a narrow road with a speed limit of 30 miles per hour, lined on both sides with imposing oak trees. Each year there are several crashes involving vehicles hitting the trees or cars emerging from driveways between the trees, so engineers decide to cut down the trees and widen the road, creating ample shoulders and improved sightlines. The expected result is a safer road. What actually happens is that drivers increase their speed on the wide (if less scenic) road. There are just as many crashes as before, but because cars are traveling faster, there are more fatalities. The “safer” road ends up being more dangerous.
There are similar issues when engineers decide to add lanes to roads. You might think that adding an extra lane to a busy two-lane road would reduce traffic, but in many cases, the opposite occurs. More people start to use the road because it’s available, and capacity doesn’t actually increase as much as you’d think. Adding an extra lane in each direction to a two-lane road only increases capacity at intersections from 625 to 1100 vehicles per hour, not the 1250 you’d expect. This is because it takes longer to cross a wider intersection. Adding a third lane adds only an extra 385 vehicles per hour.
Another counter-intuitive rule-of-thumb for vehicles is named after my college Econ 101 professor, Sam Peltzman. The Peltzman Effect says that safety improvements for vehicles such as seatbelts and airbags will never reduce the impact of traffic accidents. As people feel safer in their vehicles, they increase risky driving behaviors: talking on the phone, speeding, or aggressive passing and merging. Early research suggested that the introduction of a third brake light in the center of cars would reduce rear-end collisions by 50 percent. In fact, as people adapted to the new technology, their net benefit was found to be 4.3 percent. As the technology became more widespread, people seemed to reason that if it’s easier to see when someone is stopping, why not follow a little closer?
So how is it that people manage to navigate the unbelievably crazy traffic circle at the Arc de Triomphe? Vanderbilt says it’s precisely because people know it’s so crazy. They’re more alert, and they drive slower and more carefully than they would in other “safer” situations. Sometimes it’s precisely these “dangerous” places that are actually some of the safest places to drive, because people slow down and pay attention.
There’s lots of other great stuff in Vanderbilt’s book — it’d make an excellent late Christmas gift. Just make sure the person you buy it for isn’t the sensitive type — they might take it as a subtle hint that you don’t think they’re a very good driver.
Tom Vanderbilt also has an excellent blog with frequent updates and discussions of the psychology and engineering behind traffic and the way we drive.