07-01-2014, 10:59 PM
Drivers' speed on motorways is determined by social norms of which the law is an important part. Consider a police car than drives along a motorway for a short distance at 55mph. If the police car merely ensured that the law was followed then drivers would merrily overtake the police car at 68 mph and when it was out of sight bomb on at 100mph. This doesn't happen. Everyone slows down to 55 miles a hours and when the police car departs continues at 80mph.