There is a missing piece of information here. Lower vehicle speeds are actually worsen pollution per mile. The optimum fuel economy for most ice vehicles is around 40 miles/hour. Anything below or above that will increase the pollution per mile. So what cities should do is: lower speeds for all personal transport. Speed up busses to 40miles/hour. Give the busses special lanes everywhere. Increase the amount of public transport. Increase it so much that you can just travel without an itinerary. Make public transport faster that private transport.