Not many and some experts say they’re the cause of accidents.

Here’s more from Priconomics.com:

One reason is that a minority of drivers do follow the speed limit. “I’ve found that about 10% of drivers truly identify the speed limit sign and drive at or near that limit,” says Megge. Since these are the slowest share of drivers, they don’t affect the 85th percentile speed. But they do impact the average speed — by about 2 or 3 mph when a speed limit is changed, in Lt. Megge’s experience — and, more importantly, the variance in driving speeds. 

This is important because, as noted in a U.S. Department of Transportation report, “the potential for being involved in an accident is highest when traveling at speed much lower or much higher than the majority of motorists.” If every car sets its cruise control at the same speed, the odds of a fender bender happening is low. But when some cars drive 55 mph and others drive 85 mph, the odds of cars colliding increases dramatically. This is why getting slow drivers to stick to the right lane is so important to roadway safety