In California traffic law, evidence obtained from speed traps
is not admissible. A speed trap usually means a speed limit that is arbitrarily set, not related to prevailing speeds or hidden dangers, or a speed limit that is enforced by manually measuring travel times over a measured distance. Photo enforcement for traffic signals measure vehicle speeds to set the beginning of the yellow signal indication phase. Some courts have ruled that this is not a speed trap.The term speed trap is usually used by motorists, not by enforcement officers. It may be considered pejorative, and use of the term may suggest the appearance of speed enforcement by concealed means or excessively strict speed enforcement.
Speed traps have been used since the beginning of the 20th Century as a means to enforce speed limits, and Britain's Automobile Association was set up specifically to notify members of speed traps.
Cities or road sections become known as speed traps where police have a reputation for writing an unusually high number of traffic tickets, especially speeding tickets. Sometimes the posted speed limits are not easily seen; in other places, the limits might be set excessively low.
Speed traps often are found in small towns, often near major highways, in which travelers are less likely to return and challenge a ticket. Speed trap towns typically have an unusually large percentage of their local workforce dedicated to traffic law enforcement or judiciary occupations. Furthermore, traffic fines make up an unusually large percentage of income for speed trap towns.