sussex2 wrote:It soon became apparent that it wasn't working as collisions (especially fatal ones) increased and pollution hardly changed.[...]
As a result traffic flowed more smoothly and collisions were reduced.
The lack of reduction in pollution would suggest that people simply didn't change speed. But even if people we to be forced to drive at a slower speed, it doesn't always have an effect, although such instances are quite rare.
The biggest Motorway in Israel, the M6, had the speed limit in it's northern section increased from 110km/h to 120. These was a (small) reduction in collisions, and that's with speed enforcement that is liberal enough for the actual speed to be within 10k excess of the posted limit.