I've heard one of the reasons police let people drive above the speed limit is it helps get them home and off the road sooner, thereby thinning out the amount of traffic on the road at any one particular time. The man telling me this used the analogy of water pipes: If you need to move more water (car traffic), you can do only two things - increase the size of the water pipe (build larger roads) or increase the velocity of the water (let cars drive faster). Most of the major roads are simply too small for the amount of traffic on them (at least during rush hours).