Relative Speed

Of course, this concept is designed for restricted-access highways, where the inner lanes can have higher velocities and passing is desirable; it does not apply to city streets, not even those with two or more lanes going the same direction.

By the way, the velocity pattern shown above is one of the canonical examples of a vector field that has curl, or vorticity. It makes an interesting example because it's counterintuitive—it is surprising that a field of velocities all in the same direction can have vortex-like characteristics. The same velocity pattern is also known as a shear flow.

Using a rule based on relative rather than absolute speed can sometimes be more restrictive. When, for example, traffic in the outer lane or two is slowed or stopped at a highway entrance or exit, the traffic in the inner lanes should slow down as well. (And, in practice, it does … people do this instinctively.)

Although the picture above doesn't show it, the same rule about relative speed is valid in low-density traffic, or even in situations one wouldn't normally describe as “traffic” at all. In such situations I like to think of the same rule in terms of passing: you should know at all times who is passing whom, and get it over with promptly. There's no practical reason to hover right next to another car, or, worse, to hover in another car's blind spot.

Sometimes you'll find another car hovering next to you, but just because you're not the one doing the passing doesn't mean you can't help get it over with, by slowing down. Slowing down gradually is safest, and sometimes I do that, but other times, when there aren't any other cars behind me, I like to slow down abruptly, to use the sudden change in relative speed as a medium of communication. Ideally, at that moment the other driver would become enlightened, and realize both the immense potential of inter-car communication and the error of vis previous ways.

Or, if you're feeling contrary, or simply don't want to get bogged down in a cluster of cars, you can accelerate, zoom out ahead, and adjust your default speed so the other driver won't catch up again. This also has some potential as a means of communication.

I said above that there's no practical reason to hover next to another car. In fact, there are practical reasons not to.

• The further away from other cars you are, the less likely you are to run into them by accident.
• Driving near other cars requires part of your attention, and if you move further away, you can use that attention for other things.
• If someone is willing to hover next to you, they're probably clueless, and the further away from them you are, the better.

I also like to remind myself that there's no prize for driving close to other cars. Actually, there's a very specific situation in which that exact thought always comes back to me. If I'm driving late at night, so that the road's almost empty, and I'm coming up on a tractor-trailer that's in the right lane, and there are three lanes, so that I have a choice of which lane to pass in … that's exactly when I remember about there being no prize, and I always (well, almost always) pass in the leftmost lane.

Another benefit of driving with correct relative speed is that it helps keep you alert. If you're stationary relative to other cars, nothing is changing, and you're likely to become bored and inattentive; but if you're always moving, you're always faced with exciting new situations. I guess there's a fact about the mind in there somewhere.