Speed limits are established on a basis of traffic engineering surveys. These surveys include an analysis of roadway conditions, accident records, a sampling of the prevailing speed of traffic, amount of side traffic access, and traffic volumes.
Generally speaking, a safe and reasonable limit is set at or below the speed at which 85% of the drivers drive. Posting an appropriate speed limit simplifies the job of enforcement officers, since most of the traffic is voluntarily moving at the posted speed. Blatant speeders are easily spotted, safe drivers are not penalized, and patrol officers aren't expected to enforce unrealistic and arbitrary speed limits.