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Speed Psychology

When you're driving along on a highway, how do you know how fast you're going? Sure, you can look down at the speedometer occasionally, but what about the rest of the time?

Well, obviously, you can look outside the car and see how fast the scenery is going by. That works, but it's not very accurate, at least for me. Among other things, I find that there is a lot of hysteresis in my judgement. If I've just gotten out of stop-and-go traffic and have sped up to 55 mph, it feels pretty fast; but if I've been going 80 for a while, and then had to slow down to 55, it feels quite slow.

Another possibility is that you can listen to the pitch of the engine. In fact, if you had a good ear, knew the relevant note frequencies and gear ratios, and were good at mental arithmetic, you could probably pin down your speed pretty well. (3600 RPM, for example, should produce a nice 60 Hz hum.) I can't do that, but once I've settled on a desired speed, I can stay more or less at the same speed by keeping the engine at the same pitch.

Similarly, you can feel how far you've pressed the accelerator down. You probably can't map accelerator positions to speeds, but you can at least maintain a more or less constant speed by not moving your foot. Actually, I don't know about you, but I can do better than that—I can remember a single accelerator position by feel and return to it after having accelerated to go up a hill, or to pass someone.

The above are all interesting to think about, and true as far as they go, but I think there is a much better answer to my question. I think that most people, most of the time, gauge their speed by observing the other cars on the highway—in other words, by observing relative speed and distance. That one little fact has some interesting consequences.

For one thing, you can mess with people's minds! If someone is driving behind you, you can gradually speed up or slow down, and ve'll probably do the same, within limits. You can try to do the same thing when you're the one in back, but it doesn't work nearly as well, because the driver isn't paying as much attention to you, and only sees you in vis rear-view mirror.

Another consequence is that inattentive drivers are sticky. Typically, they'll come up behind you, but then instead of passing, as they should, they'll stick, staying behind you and matching any change in speed you may make, as long as it's not too abrupt.

Inattentive drivers can be sticky in the reverse situation, too. They'll be driving along at some moderate speed, but when you catch up and start to pass, they'll see you and match your speed, so that you can't finish. Then, when you decide not to pass after all, they'll just slow right back down.

I don't like having other cars stuck to me. Partly that's just a quirk of mine, a kind of claustrophobia, but it's not completely irrational—the arguments against allowing other cars to hover that I gave in Relative Speed apply here as well. In particular, I don't think being surrounded by inattentive drivers is a desirable state of affairs.

Fortunately, it's not too difficult to get rid of sticky drivers, or, even better, to have them not stick to you in the first place. Perception of relative speed is the key. If a sticky driver thinks ve's going about the same speed as you are, ve will match speeds, but if ve thinks you're going slower or faster, ve won't. So, if you're passing, pass decisively, with vigorous but not excessive relative speed, perhaps 10–15 mph; and if you're being passed, be passed decisively.

There are also more subtle approaches, with the emphasis on the perception of relative speed. The best example, I think, is the combination of speeding up and slowing down that I described in Via Change in Speed, but I know another example that's almost as good.

Suppose you're driving along the highway and are slowly catching up to a potentially sticky driver; and suppose you want to pass with correct relative speed. What do you do? The natural thing, I think, is to speed up when you change lanes and then slow down when you change back. Unfortunately, that leaves you sitting a short distance ahead of the other car, with low relative speed—an ideal target for a sticky driver. What I like to do, instead, is start slowing down as soon as I've pulled completely ahead, like so.

At that point, I imagine, the other driver will have noticed the large relative speed, and will slow down a bit to help me maintain it.


  See Also

  Cruise Control
  Foolish Consistency
  Looking Ahead
  Preserving Options

@ April (2002)