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Traffic’s a gas

May 21, 2016

The physics of traffic – that is, trying to model and understand how cars interact on the road – is an idea that’s occurred to me several times but I’ve never pursued it. My offhand thoughts always tend to comparisons of cars on a highway to molecules in a pipe. Under what conditions does traffic flow change phase from a gas/liquid to a solid: a traffic jam?

I’ve often wondered if the people who design roads have ever studied these topics. Has anyone done any experiments on this? Has anyone "ground-truthed" any theories for smooth traffic flow? Or do road designers just use rules-of-thumb for planning new highways (as I suspect from seeing the results)?

So this video from The Mathematical Society of Traffic Flow naturally piqued my interest. Since it’s hosted by Nagoya University in Japan, I assume the authors are associated with the school.

I’ve often wondered how much difference it makes to traffic flow whether drivers regulate their speed using their brakes to ‘actively slow’ as opposed to using their throttles (accelerators) to ‘passively slow’ by coasting.

Coasting to slow down is something a lot of American drivers just don’t seem to get. Their feet are always on one pedal or the other – or sometimes both, as my mother used to do. But they rarely drive with a foot on neither pedal.

When you find yourself in a group of cars that doesn’t drive that way – in a group that coasts to slow down – then traffic seems to flow much more smoothly. But that observation may be due to traffic density: maybe people are more likely to use their brakes in denser traffic.

In the U.S. this problem’s worse because there’s very little lane discipline on the freeways, in contrast to British motorways or German autobahns. This is despite the fact that it’s a law in most of the states that drivers should keep right unless passing. It’s not unusual to find people in the left-most lane doing the speed limit. Technically, that’s legal1 but it completely defeats the self-organizing design feature of a freeway.

The first time I drove on an English motorway, I thought I’d died and gone to Drivers’ Heaven. On the other hand, the result of typical lane usage in these parts is that you’ll find all five lanes of a freeway coming to a complete halt with no apparent reason before resuming speed again. It’s just like the video except five lanes wide. How in the world does that happen?

Sometimes that pattern repeats, giving you the feeling that you’re in a kind of "traffic accordian." Stop, speed up, slow down, stop, speed up, slow down, stop, et cetera ad nauseum.

It makes me think we’re all just lemmings on the pavement.

1Except in California AFAIK.

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