I think that a single speed limit is too inflexible. It is set for the lowest common denominator. A better solution would be to use a variable speed limit. The criteria could be:
- Time of day or night
- Weather conditions
- Amount of traffic
- Type of road
- Type of car
- Age of car
- Age of driver
- Accident history
These factors could all be totted up and scored, and a resultant weighting factor produced. Then the road signs could display a speed ( 70), but the would apply the weighting factor to them. E.g.
Excellent weather conditions, afternoon, new high performance car, motorway, no other traffic: Weighting +50%.
Rainy, built up area, nighttime, old car, old driver with history of accidents: Weighting -30%
Normal weather, 3.30, in a school zone: Weighting -50%
Normal weather, midnight, in a school zone: Weighting 0%.
A problem would be the calculating the speed. However, I usually drive according to these criteria anyway. Why restrict people to 70 if the motorway is empty, and you are in a decent car? Allowing people to drive according to the road conditions would allow people to concentrate more on their driving, and less on trying to spot speed cameras, police at the side of the road, and laser guns hidden just over the top of hills or just round corners.