New speed limits must be established in accordance with strict guidelines. Experience of the Los Angeles Department of Transportation shows that enforcement is compromised when speed limits are not established according to guidelines. Studies have repeatedly shown that motorists tend to drive at a speed they feel is safe, regardless of posted speed limits.

Speed Traps

The California Vehicle Code defines “speed traps” to be unreasonably low speed limits that many motorists violate. Speed traps are illegal.

		The California Vehicle Code requires that speed limits posted along each segment of roadway be justified by four factors:
		Prevailing speed Traffic accident history Pedestrian and bicycle usage (including at nearby schools) Conditions not readily apparent to drivers

Setting the Speed Limit

Prevailing speeds are measured by calculating a speed that less than 15 percent of motorists exceed. The prevailing speed is slightly less than the 85th percentile speed.

Unless there are unusual circumstances, streets should have speed limits that are within a five mile-per-hour increment of the prevailing speed. If this is untrue, radar enforcement becomes illegal.

Sometimes the approved speed limit on a street may be too fast for residents or too slow for motorists. If changes are made to the speed limit, public officials must be prepared to justify their decision, according to State laws and guidelines.

		Consequences occur when speed limits are not set in the prescribed manner.
		Some drivers obey the lower speed limit while others ignore it. The gap in speeds creates erratic traffic flow and greater crash potential. Some drivers develop a disregard for traffic regulations. An unrealistic speed limit is not self-enforcing. Since police officers cannot always be present to enforce traffic regulations, voluntary compliance by a majority of drivers is needed.

Speed Limit versus Speed Advisory

Some people confuse “Speed Limit” signs and “Speed Advisory” signs. Regulatory speed limit signs are enforceable. Speed advisory signs warn drivers to unusual conditions and are not enforceable.

Regulatory speed limit signs are posted in black letters on a white background. Speed advisory signs are written in black on a yellow background.