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Comment by xvedejas | Hacker News Reader

Surely the reduction in vehicle count is more than enough to cancel this out, but a moving vehicle does emit more exhaust and tire dust per unit of time than does a vehicle idling. For the environmental improvements it's more about the reduction in the number of cars than about the better traffic flow.

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The better traffic flow reduces the amount of time they’re operating for as well (assuming start/end of planned route is independent of travel speed)

Right. Presumably a car idling for ten minutes produces less pollution than a car being driven for ten minutes, but a car that is driven for ten minutes and idled for an additional ten produces more pollution than either of them. Any pollution produced by cars idling in bad traffic is superadded to the pollution produced in transit so improving the flow of traffic should reduce pollution even if the total number of cars remains steady.

It's worse than that.

If the trip costed 10 minutes moving, yes the comparison would be between a car moving for 10 minutes and one that idles for some time and then moves for 10 minutes. But congestion makes the cars move slower, and at congestion speeds the amount of pollution increases very quickly with reduced speeds.

Pollution per time doesn’t make any sense as a metric. A trip that includes a lot of idling will pollute more than a trip that doesn’t.

I think that depends on the motivations of the driver. You (and I) are probably thinking of a trip that is motivated solely by getting from A to B (or A to B to C to A). In that case, any pollution from idling is strictly additive.

But a taxicab working an 8 or 12 hour shift is about the only case where I think GP's math/logic applies. (And to be fair, there are a damn lot of yellow cabs in Manhattan.)

The stop and start conditions of highly congested traffic produce more brake and tire dust

And more emissions. Idling is pretty efficent, as is driving at a constant speed. Repeatedly stopping or slowing, then accelerating is not. This is also an unintended consequence of "traffic calming" devices e.g. speed bumps or chicanes. People slow down, then hit the gas again which is awful for emissions.

I’ve sometimes pondered if a traffic calming device could be made which would allow vehicles to pass unimpeded if they are at or below the speed limit, but subject to an increasingly large bump if they exceed it. The problem, I suppose is that it must be extremely robust which would make it expensive and potentially more complex than a simple passive bump on the road.

Won't those idling vehicles also end up moving?

A moving car from point a to point b will always emit such "moving vehicle" pollution. The idle pollution is just extra.

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