In most cases, lowering the speed limit on certain streets will not slow traffic. The majority of motorists travel at a speed in which they feel is reasonable speed given the surroundings of the street, regardless of what the posted speed limit signs read. The basic speed law states that no person shall drive at a speed greater than is reasonable or prudent.
Therefore, the State of California has established the 85th percentile, or critical speed as the sensible speed. This is the speed at which 85% of the motorists are traveling at or below.