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Here I want to collect some random thoughts about trucks. When I say “trucks”, I'm really thinking of tractor-trailers, but everything I'm going to say applies to some extent to anything that's larger than a car. Cement mixers? Definitely! FedEx vans? Sort of. Pickup trucks and SUVs? Maybe a little. So, do think of tractor-trailers, but keep the rest in mind too.

The first thing to note is that truck drivers are professionals. They have a lot of experience driving, and you can learn some interesting things by watching them. Here's how I put it in Via Brake Signal.

My experience has been that the average truck driver is much more aware of the dynamics of traffic than the average car driver, …

Truck drivers are required to have commercial driver licenses, so I suppose it's possible that some of their knowledge comes from training rather than experience. However, given how easy it is to get a regular license, I hardly think that's likely.

The next thing to note is that trucks have different capabilities than cars … in particular, they can't accelerate or decelerate as quickly. That's obvious when I say it, I know, but it's easy to forget when you're driving. So, what are some consequences of that?

  • You really don't want to be stuck behind a truck at a traffic light, it will take forever to come up to speed when the light changes.
  • In stop-and-go traffic, trucks need to maintain larger gaps than cars. You can use these gaps to move around; on the other hand, you should also be nice (see below).

On hills, trucks tend to slow down more on the up side, and speed up more on the down side … so much so, in fact, that on steep hills it's necessary to have countermeasures to stop runaway trucks, like emergency ramps and barrels of sand. That variation in speed leads to a few more consequences.

  • You don't want to be stuck behind a truck at the start of a hill.
  • You shouldn't pull in front of a truck just as it comes over the top of a hill.

Basically, although the reason is different, you have to do the same kind of thinking as if you're driving up and down on cruise control.

One other thing … deadheads (tractors without trailers) are entirely different. They are surprisingly speedy! But, you really don't see them very often.

Another consequence of not being able to accelerate or decelerate quickly is that driving trucks in traffic is difficult and annoying. Or so I imagine. Truck drivers know that, and try to help each other out; and now you know it, so you should be nice and help them out too. If you make room in a merge, for example, I'm sure that's much appreciated.

Similarly, if you're about to pass a truck, make sure it's not approaching a slower car, otherwise you might cut the truck off (as in the diagram in Via Lane Change). If you do cut it off, the truck will have to slow down, and, not having enough acceleration to merge with the passing cars, could be stuck there for a long time, even in light traffic.

Since trucks are large and heavy, and can't decelerate quickly, if it happens that you're right in front of one in dense traffic, and the traffic stops suddenly … well, it will go poorly with you. That's why truck drivers have to maintain larger gaps, of course, but I wouldn't want to rely on that. It's like when I'm walking across the street in a crosswalk … the cars are supposed to avoid me, but if they don't, I'm the one who will be sorry. So, you never want to be right in front of a truck … not in dense traffic, certainly, but really there's no reason ever to be there.

By the way, I apply the same kind of logic to cars as well as trucks. If I see that someone is talking on a cell phone, or putting on makeup, or driving erratically, I definitely don't want to be right in front of ver. The car can decelerate quickly if needed, but I have no confidence that it actually will. (If I'm forced to be there, I'll open a large gap, brake early and often, and keep an eye on the other car to make sure it's following suit.)

Conversely, if you're right behind a truck, and traffic stops suddenly … actually that's not entirely bad. You can stop faster than the truck, so you have a few extra milliseconds available in the event of a crash, just as if you had a larger gap between you and the truck. On the other hand, the visibility is terrible. The driver can't see you, which is bad, and you can't see ahead, which is worse, because it loses you the extra milliseconds and more in reaction time.

So, in short, you never want to be too close to a truck, either in front or in back, but if you have to, in back is better.

Speaking of trucks and visibility, here's a fun example of indirect observation that I ran into recently. I was stuck in slow-moving traffic behind a tractor-trailer … several cars back, I think, but the road was straight, and I couldn't really see what was going on. But then, all of a sudden, I saw a big cloud of exhaust come up out of the tractor's vertical exhaust pipe, and I knew we were going to be speeding up pretty soon. (I probably also heard the engine rev.)

Finally, I know I haven't said much about models yet, but I do think about them, and one of the things I think about is how trucks fit in. And, it seems pretty clear to me that there ought to be two levels of description. On the one hand, maybe you can think of the trucks as fixed obstacles, and the cars as a fluid, and solve for how the cars flow around the trucks; on the other hand, maybe you can think of the cars as a fixed background, and the trucks as (quasi)particles, and solve for how the trucks move through the cars.

Actually, since the truck density is usually pretty low, that ought to work nicely … the gas phase of trucks in a car background should be almost as tractable as the gas phase of cars “in vacuum”. And of course we should be careful to distinguish the bare trucks from the renormalized trucks that carry cars with them!

I also like to think of trucks as rocks in a stream.


  See Also

  Long-Distance Driving

@ June (2004)