Good points, IOWA, especially about "speed range rather than limits." But
:
That already exists, in the real world of driving, as we all know. On a highway or even city surface street we are accustomed to seeing a range of speeds around the posted limit.
The only groups of people going the exact same speed with no passing or causing slowdowns are on a train, not driving.
Also, if slow drivers are involved in collisions it is a lot less likely to be their fault than the speeders who are darting around in anger at them: just slow down, he's likely going within 5mph of the posted limit anyway.
Drivers all over the world have something in common, as I've seen, no matter the city, state or country: most seem to consider the posted limit as a minimum, rather than the maximum. Thus the carnage. Human nature can be nasty.