(Portions courtesy of the City of San Antonio) By state law and city ordinance, the speed limit on McKinney streets is 30 mph unless otherwise marked. Residential streets are mostly 30 mph except in marked school zones. Municipalities and other agencies responsible for traffic control are sometimes asked by citizens to reduce the posted speed limit in hopes that the lower speed limit will reduce operating speeds and accident potential. Although public agencies share the desire for low driving speeds and improved safety, studies have shown that arbitrarily reducing the speed limit does not accomplish the desired result. The safest traffic condition occurs when all vehicles travel approximately the same speed, creating a low speed differential. When speed limits are set too low, accident potential may increase due to the greater speed differential. Many years of research and experience have shown that lowering regulatory speed limits does not result in lower driving speeds or in reduced accident potential. By and large, drivers choose their speed independent of the posted speed limit. Drivers select their speed intuitively based on the environment around them and the speed that feels comfortable and safe. Studies have shown that there are no significant changes in average vehicle speeds following the posting of revised speed limits. In most cases, drivers operate their vehicles at speeds that they consider safe and prudent, despite posted speed limits. Most speed limits are based on the 85th percentile speed. It has proven to be an effective theory used by the majority of safety agencies that set speed limits throughout the country.