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Cruise Control

Cruise control is a nice feature to have when you're driving on an open highway, but it's also very easy to overuse. Here's a rule of thumb I use to think about it. Suppose you drive without cruise control over a certain stretch of road, with a certain pattern of cars on it. Naturally you're driving as well as you can, right? Now suppose you drive with cruise control over the same stretch of road, with the same pattern of cars on it. The cruise control probably makes your speed more stable, but if your driving changes in any other way, it's almost by definition a change for the worse.

I'll go into detail later, but for now, just to give you an idea, here are some ways that cruise control is likely to make your driving worse.

  • It tempts you to violate the principle of relative speed.
  • It tempts you to violate the principle of safe distance, either by coming up too close behind another car, or by allowing another car to come up too close behind you. So, in short, it encourages non-communicative tailgating.
  • It also encourages foolish consistency in speed. Actually, that's not putting it strongly enough good driving doesn't just allow varying your speed, it requires it.

Now, to approach the matter from the other side, I'd like to describe what I think is the one correct use of cruise control. First of all, as I said above, you should be on an open highway, by which I mean that the following conditions should hold.

  • The road should have at least two lanes (in the same direction).
  • You shouldn't be in or near a city.
  • The traffic density should be so low that you only interact with one other car at a time. In other words, the traffic should be in the gas phase.

Furthermore, you still need to vary your speed at certain times. If you're passing someone (who may well also be on cruise control) and your relative speed is low, you need to speed up for the duration of the pass. Fortunately, that's easy I think with any kind of cruise control, you can press the accelerator to speed up, then simply release it to resume cruising.

Similarly, if you're being passed by someone and your relative speed is low, well, ideally ve should speed up, but in practice you may need to slow down to force the pass. That's more of a nuisance. If you want to do it smoothly, what you can do is match the accelerator position, turn off the cruise control, decelerate a little for a little while, then turn the cruise control back on. Of course you can also just brake, but that's likely to wake up any sleeping passengers you might have.

In both cases, passing and being passed, if the other driver isn't on cruise control, the low relative speeds make speed psychology an important factor. Often, for example, if you slow down, the other driver will slow down too, so that when you try to resume cruising after the pass, you'll immediately catch up to ver. Or, if you pass and then immediately resume cruising, your pass will have sped up the other driver, so that ve'll immediately come up right behind you. That is especially rude, because to the other driver it looks like you passed and then slowed down. Anyway, the point is, because of speed psychology you may need to make your passes longer and more definitive than usual.

That's particularly true when driving up and down hills. For anyone who's not on cruise control, the natural behavior on a hill, I think, is a compromise between maintaining constant speed and maintaining constant accelerator position. So, for example, if you're cruising up a hill and make a short pass around someone, it's quite likely ve will speed up on the other side and hence immediately want to pass you back. (That's a complication I forgot to mention in Who Is Passing Whom?.)

That assumes you're cruising at constant speed, of course. I don't know if this is still true, but I'm pretty sure there used to be two kinds of cruise control, one that maintained constant speed and one that maintained constant accelerator position that is, one for each of the extremes that humans tend to find a compromise between. So, there are all kinds of possibilities one can consider. What kind of cruise control do you and the other car have? Are you passing or being passed? Is the terrain is a hill or a valley? Fortunately, the reality isn't as complicated as I make it sound.

I said at the beginning that it's very easy to overuse cruise control. Why is that? In other words to overuse cruise control, you have to have it on when it should be off, but the conditions above are fairly straightforward; you're not going to turn it on in the middle of rush hour. So why is overuse a problem?

Actually, I've set you up the bomb it's a trick question. The problem isn't that you might turn it on at the wrong time, it's that you might fail to turn it off. When you're cruising along, it's easy to build up a kind of mental inertia that makes you want to keep the cruise control on, even after the situation has become unsuitable for it. Don't! You have to keep your mind flexible, watch what's going on, and be ready to turn the cruise control off before you get in the wrong situation.

That's the last point I wanted to make about the correct use of cruise control. Now, getting back to the idea that cruise control is likely to make your driving worse, here are some examples.

  • I've probably already beat this one into the ground, but it's also by far the most common error I see, so here it is one more time: just because you're both on cruise control doesn't mean it's a good idea to pass someone at a relative speed of 0.01 mph.
  • Probably the second most common error is letting the cruise control bring you right up against a barrier. You don't even want to be that close; even worse, you might still leave the cruise control on, in which case you'd be so focused on not actually running into the car in front of you that you wouldn't be able to pay attention to anything else.
  • Finally, I think it's a mistake if you don't turn off cruise control when you approach a city or town. Maybe there's no traffic on the road, and maybe there are no traffic lights, but there will always be side roads, so cars can enter at any time, and can be moving at wildly varying speeds. (In particular, they can slow down to turn.) In that kind of situation, you don't want to have cruise control interfering with your response time.

Speaking of response time, yes, it's true that braking automatically turns off cruise control, so there's no interference there. However, not everything you might want to do involves braking. When a car turns onto the highway, for example, you might want to change lanes and take your foot off the gas to slow down a little but, oops, you don't seem to be slowing down! That is the kind of interference I'm talking about.

A final point to remember about cruise control is that even if you don't overuse it, other people are probably still going to. There's not much you can do about that, except learn to recognize it and adjust your behavior accordingly.


  See Also

  Great Idea, A

@ June (2004)