What is the economic cost of slowing everyone down?
If moving from 30mph roads to 20mph roads reduced average traffic speed, then this would be a valid argument.
However, for most urban streets during the daytime the average speed is set not by the roads themselves but by the traffic capacity of the intersections along the road. It doesn't really matter how fast you can drive along a road if it only means that you get to the back of the queue for the next set of traffic lights a little earlier!
If moving from 30mph roads to 20mph roads reduced average traffic speed, then this would be a valid argument.
However, for most urban streets during the daytime the average speed is set not by the roads themselves but by the traffic capacity of the intersections along the road. It doesn't really matter how fast you can drive along a road if it only means that you get to the back of the queue for the next set of traffic lights a little earlier!