Friday, 7 December 2012

Traffic Rules: It's not about you.

     People often complain about traffic regulations, particularly speed limits (and rules against following-too-close, one of my pet peeves), on the basis that they don't improve safety. "I'm a good driver," they say. "I'm alert and I pay attention, and I can judge my stopping distance and control my vehicle at much higher speeds that the posted limit."

     That may well be true, although it probably isn't, considering how many people consider themselves to be above-average drivers. But even if it is, it is based on a narrow and unreflective conception of why we have traffic laws. They aren't just about preventing accidents; they're also about maintaining an efficient and orderly flow of traffic so people can actually get where they're going.

     Let's start with a very simple example. You might feel more comfortable driving down the middle of the road, with more room to spare in avoiding parked cars or other obstacles at the side of the road, but we have a convention of driving just on the right side of the street (or left if you happen to live in England or Japan). This isn't really about preventing collisions, because presumably if we had no rules about what side of the road to drive on, people would be alert to the possibility of oncoming traffic and usually be able to stop in time. Rather, it's a way of sharing the road so that traffic can move in both directions, smoothly and efficiently. Instead of stopping to get into arguments about who was there first and who should get out of the way, we just instinctively move over to the right and drive past each other without incident, arriving at our destination sooner than we would otherwise

     We rarely get people demanding the right to drive on the left if they should choose, and complaining about the The Man telling us what side of the road to use. But the safety rationale for speed limits seems fuzzier; while there is a very clearly marked line down the middle of the road that we all recognize we should not cross if we don't want a head-on collision, it's a lot easier to rationalize that driving a mere 20 klicks over the posted limit isn't all that much more dangerous than 5 or 10 klicks.
     Even if that were true (which it isn't), it doesn't matter. The speed limit isn't there because The Man thinks you're not a good enough driver to handle your vehicle at higher speeds. Like driving on the right side, it's a way of sharing the road so that we can all get where we're going with minimal delay.
     Consider: You're at a stop sign, waiting for a break in traffic to cross the street or merge into traffic. How big a gap do you need? Well, obviously that depends on how fast traffic is moving, and how fast you can accelerate. The faster traffic is moving, the bigger a gap you need, and consequently, the longer you'll have to wait. The longer you have to wait, the less your total travel time benefits from higher speed limits, and there is a point at increasing the speed limit actually decreases total average speed.
     The same principle applies to lane changes. If you find yourself stuck behind someone going slower than you'd like, it's reasonable to want to pass them by moving into another lane. But if traffic in that lane is going very much faster than you are, it will be much harder to find a safe opportunity to do so. So you are delayed longer, driving slower than you'd like to, because people in the other lane are allowed to drive as fast as they want. Again, a lower speed limit in this situation is to your benefit, because it gives you more chances to actually drive at that speed, rather than being delayed by the difficulty of merging into the higher speed lane.

     So, counterintuitive though it seems, speed limits are actually intended to speed you up, to get you and everyone else where you're going as fast as possible by sharing the road. The road is a scarce resource, and traffic laws are as much about fairly distributing that resource as they are about saving lives.


  1. On most freeways there seems to be a difference between defacto and dejure speed limits. What problems do you see this to cause, and what would you suggest is the solution?

  2. For the most part, I don't think it's a serious problem. It's of greater importance to me that the traffic is all moving at close to the same speed, so that everyone can safely change lanes, pass each other and so on. It's only if the de facto limit is demonstrably unsafe or is otherwise creating disproportionate delays that enforcement of the de jure limit should become a priority.

    I suspect that traffic engineers select speed limits with the knowledge that the de facto limit will often tend to be a little higher than whatever they post, and take that into account. Engineers are accustomed to building in tolerances, after all. So long as traffic is generally flowing within the design parameters, the system should work efficiently.

  3. I drive the Whitemuc freeway at least 3 days a week. I've noticed that on days when the weather is good it's fairly easy to get above the speed limit, and several drivers will. However, I also noticed how long it takes me to get from the west end of the city to the east end - I typically spend about 20 minutes on the freeway. We had some bad snowfall a couple of weeks ago, and the goign was such that it was literally 60kph the entire way - no speeding up, no slowing down. It took me 13 minutes on the exact same distance. Tells me something there...