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The Driver's Dilemma
10/02/2003

This weekend, James Bow made some observations about traffic patterns, and found a link to someone who has done a lot of personal philosphizing and experimenting on the subject.

It got me thinking, as well. Well, not my own thoughts, but it reminded me of a passage in Joseph Heath's The Efficient Society. Here's a long quote:

Anyone who drives on a major road with any frequency has undoubtedly had the experience of being scared witless by an insane or aggressive truck driver. Part of this is just because truckers can afford to drive more recklessly. If there's an accident, they won't be the first ones hurt. (Personally, I've noticed that drivers of minivans -- the ones with airbags on every square inch of the interior -- are even worse.) In any case, we are all familiar with the standard litany of complaints about these sorts of drivers.

In Holland, the government decided to get serious about it. It is common knowledge that trucks pose the greatest danger to other vehicles while overtaking and passing. This is why, in many parts of North America, trucks are prohibited from entering the third lane on a highway with more than two lanes. The Dutch decided to take it up a notch. They enacted a law that prevented trucks from entering the second lane as well, even on two-lane highways. This made it illegal for trucks to pass on more than 70 per cent of highways in the country.

The truckers, predictably, went ballistic. They claimed that the law would paralyze the economy. No more fresh vegetables! No more just-in-time manufacturing! All transactions and trade would take longer and cost more. International competitiveness would be crippled! The government ignored them.

A couple of weeks after the law came into effect, truckers began to notice something strange. They discovered that it was starting to take them less time, not more, to reach their destinations. A study was commissioned. Sure enough, the average capacity of affected roads had increased by 36 per cent. Furthermore, the average speed had increased in both lanes. Even the truckers were forced to admit that the new regulation was an improvement. By preventing the truckers from passing slower vehicles, the law had the unintended consequence of actually speeding up all the traffic, including the truckers themselves.

How is this possible?

Passing is not a problem when one car is moving very slowly, holding everyone else back. The problems arise when people try to beat the average speed of traffic on the road. If traffic on a highway is moving just over the usual speed limit, it will be zipping along at about 30 metres per second. Drivers are also instructed to maintain a three-second following distance (although in practice they usually stay closer to two). In any case, with these following distances, at these speeds, there is plenty of room between vehicles to fit a third one in -- even a large truck. This makes passing quite easy (whereas on a city street, it is often the case that someone must "let you in").

But while there is usually enough physical space to move in between vehicles without hitting either one, there is often not enough "time" space between them to do so without causing disruption. When you zip in between two cars, you often force the motorist behind to hit the brakes in order to maintain proper following distance. So while the pass may shave a few seconds off your commuting time, it adds a few seconds to the commute time of maybe a dozen cars behind you.

Suppose that after making the pass, you are able to travel 10 km per hour faster. However, the pass has the effect of slowing down cars behind you. In order for the pass to generate a net increase in the average travel time on the road, you would have to maintain the increased speed for quite a while. Chances are, however, that you will not have an unobstructed road in front of you. Within a few minutes you will encounter another vehicle moving slower than you, forcing you to decelerate or, worse, execute another pass. If you choose to pass, it may again slow down the traffic behind you. As a result, your passes will speed up your trip, but they will slow down all the vehicles behind you, and thus reduce the average speed of vehicles on the road.

Of course, if you were the only one passing, you would not suffer any negative consequences. But just as your passing slows down everyone behind you, the people passing in front of you slow you down. In effect, commuters all slow each other down, even though not once of them slows him or herself down. Each motorist who speeds through, trying to maintain a velocity higher than the average for the road, generates a wake of disruption and delay, like ripples emanating from the back of a boat.

The Dutch truckers, of course, had no idea that they were doing this to one another. it seems logical to suppose that if everyone drives as fast as possible, the traffic as a whole will move as fast as possible. But this turns out to be false. It is in fact a paradigm case of individual efficiency generating collective inefficiency.

What makes these inefficient outcomes especially perverse is that even if everyone recognizes the problem and sees how his or her own behaviour is contributing to it, this may have absolutely no effect. Having learned from the Dutch example, we now know that passing slows down traffic. Does this mean that we will all stop passing? Absolutely not -- because we have no individual incentive to stop. After all, passing still helps you to get where you're going faster; it only slows down the people behind you. As economists say, the costs of your behaviour are externalized. So why not do it?

Anyway, Heath goes on from there. It doesn't offer solutions to traffic problems, but uses this problem as a good example of the prisoner's dilemma, where coordinated group action gives a better result than each of us going for our own goals. It sets up the book, which ultimately uses the prisoner's dilemma to explain both why markets sometimes work and why sometimes they don't.

This is an idea I hope to explore more of as I write for this site. Heath certainly recognizes that there are many ways in which personal self-interest can promote the greater good -- e.g., within competitive markets that have been made possible through collective action like the establishment of the rule of law -- but there are also many ways (frequently overlooked) in which we all have the potential for a better life through some sort of cooperation.


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