--- In carfree_cities@y..., Andras Toth <toth_andras@y...> wrote:

> Is it true or false that a car trapped in a traffic jam

> or slowed down by sleeping policemen pollutes more on a

> given distance than a car doing the same journey at an

> optimal speed/rpm whatever? If true, how significant is

> the difference?

Yes, it is true. And you can prove it yourself. For a

given car, a good first-order measure of how much it

pollutes on one trip compared to another trip is the

ratio of gasoline burned on the two trips. The more

fuel burned, the more pollution emitted. How much does

your mileage improve when driving on the highway, over

driving on the city? For most cars, it's quite a bit.

If you look, you will find articles on the optimum

driving for gas mileage, and this is close to optimum

driving to minimize pollution. A typical car will

get the the best mileage by being driven at a steady

speed somewhere around 45 mph. It varies from vehicle

to vehicle, as does pollution per gallon burned.

"First-order measure." In fact, the pollution increase

from stop and go traffic is even worse than reduced

gas mileage indicates, because a car starting from a

dead stop generates more pollution per gallon burned

than the same car cruising at steady speed. (Some

decades ago, I programmed engine control computers for

Ford.)

Of course, the pollution generated by walking or

bicycling is MUCH less than by driving! The question

asked is far from the only, or even the primary, factor

in urban planning, or even in figuring out how to

minimize pollution. Asked that way, yep, it's true.

Don't try to buck the facts. But don't let one narrow

fact distract from other issues.