[entropicgravity]

I would say no they are not and that's because after driving for about five years a driver acquires an expert sense of the safe speed for the stretch of road they happen to be on. In fact sometimes the speed limits are set at close to the 85 percentile speed of the of the traffic, so in effect the drivers set the speed limit.

Snow on the road? No one is driving close to the limit. In my area there are even curves that are purposefully slanted away from the curve of the road so that drivers will slow down on that curve. For a lot of drivers their slowing is subconscious but it still works.

A wide, straight road beside an elementary school and traffic will fly by unless the serious steps are taken to alert the subconscious brains of the drivers to slow down. Narrow the road, put a huge red square across the road with a huge "20 mph" written in it and some drivers will slow down. More will slow down if there are kids around. Almost all drivers speed is set by their subconscious.

There is one particular situation where the driver's subconscious expertise doesn't work very well and that's on highways with intersections. It's raining, the light turns red, you're driving at a speed that's safe for the highway but not safe for stopping at the light and you rear end the car in front. On such highway near me they put in a long strip of high traction pavement in front of these intersections so when the light turns red drivers can stop safely and quickly.