Speed limits are SUPPOSED to be set by actually measuring the rate of traffic (before a low limit is set) and then setting the rate to what a large percentage of the drivers do. But politics gets involved and the rate is set arbitrarily, usually too low. So the vast majority of the people driving on the road will speed, risk the ticket, and actually be driving a safe appropriate speed for the road. Google simply needs to measure the average traffic speed for a road and adjust accordingly if they want it to be safe.