Most people believe that lowering speed limits and posting more speed limit signs will cause drivers to drive slower and reduce collisions. Facts indicate otherwise. Decades of research reveals that drivers are most influenced by roadway conditions, NOT the posted speed limit.
Although speed limits are not always posted, drivers are expected to know the BASIC speed limits of 15 miles per hour in alleys and 30 miles per hour on residential streets. Most school zones are between 20 and 30 miles per hour and have the effective hours posted at the beginning of the zone.
All other speed limits are established on the basis of Traffic Engineering surveys. The surveys include analysis of roadway conditions, collision records, and the prevailing speed of prudent drivers.
The prevailing speed of prudent drivers is established by taking a sample of vehicle speeds and determining the speed at or below which 85% of the drivers are traveling. The 85th percentile speed assumes that most drivers are reasonable and will drive at a safe speed, not wishing to endanger themselves or anyone else and that they wish to reach their destination in the shortest possible time without having an accident.
If speed limit signs are posted for a limit lower than most drivers feel is reasonable, many drivers will simply ignore the signs. At the same time, other drivers will stay within the posted limits. This increases the conflicts between faster and slower drivers, reduces gaps in traffic, and makes it difficult for pedestrians and drivers to judge the speed of approaching vehicles. Studies have shown that where uniformity of speed is NOT maintained, traffic collisions increase.
Section 102-108. Altered Prima Facie Speed Limits
Section 102-109. Authority to establish Temporary Speed Limits
The City’s traffic engineering division may determine and declare temporary reasonable and safe speed limits to the maximum extent allowable pursuant to state law. Such change shall be effective at all times when appropriate signs giving notice thereof are erected.