Quote:
Originally Posted by Luciom
It takes longer to reach your destination. Most air pollution for urban car use is at traffic lights anyway.
Certainly you don't reduce urban air pollution by going slower. If lucky nothing big changes, otherwise it's a lot worse.
I wasn't giving my opinion on this, I was stating what research has shown. Fuel consumption/exhaust emissions are somewhere between slightly higher and slightly lower, and particulate pollution (e.g. brake dust) is consistently lower.
What you might be mistaking is the fact that traffic calming methods such as speed bumps do tend to result in increased pollution because they result in much more accelerating/braking, which reduces fuel economy. A consistent lower speed limit has been found to noticeably reduce the amount and severity of acceleration/braking, which is why it can result in fewer emissions despite engines generally being a little less efficient at the lower speed (this is what actually matters, not how long the same journey takes).
To expand this to the original topic of speed limits on highways, pollution would definitely be reduced by a moderate amount if the limit was reduced from 70mph to 55mph because 99% of car engines are more efficient at these speeds so that's another possible thing to consider.