A speed trap is a section of road which
police have staked out with the intention of stopping, and issuing fines to, drivers exceeding the local
speed limit. In its basic form, an officer will wait by the side of a road, clocking passing traffic by means of
radar,
LIDAR,
VASCAR, or other methods, and when a vehicle determined to be worth dealing with passes by, they or a compatriot will pursue and stop it, usually issuing a fine. Speeding fines are used as a revenue generator in many small municipalities, especially those lucky enough to have a major transportation artery pass through its boundaries at some point, and the speed trap thus can prove to be a particularly profitable enterprise for local government.
The term "trap" implies some subterfuge, and not without due cause. While the visible presence of a squad car may cause drivers to slow down, achieving the nominal goal of reducing speeds and increasing motorist safety in the targeted area, a driver who is made to adhere to the law is one fewer driver who can be fined for violating it. Prevention thus often takes a back seat to punishment, and this requires surprise. At the least questionable, speed traps are often camouflaged, with police lying in wait behind billboards, foliage, blind corners, or other coverage to ambush unsuspecting speeders. Less on the up-and-up, traps can be located at the bottom of hills, on straightaways, and in other areas where drivers are likely to be driving faster than the average for the road. Even more reprehensible, officers sometimes lie in wait in areas featuring a sudden decrease in the speed limit, ideally (from a revenue-producing point of view) if that decrease is sudden and poorly indicated.
As geography and local road conditions will tend to dictate that police rely on a few set positions to conduct their operations from, area residents will tend to be aware of the locations of local speed traps and avoid them. This likely does not trouble the police much, as out-of-towners are less likely to return later to challenge any fines in court, or have friends at the local courthouse or police station able to get the whole thing called off.
Between the ambush tactics and the blatant mercenary goals, you might notice some similarities to traditional, literal, highway robbery. Well, yes - let this be a lesson to you that granting the state a monopoly on the use of force is no guarantee that force will be used with any particular restraint. However, you're not the first person to make such a connection, and neither legislators nor their constituents (that's you) particularly like being the targets of government banditry. Thus, the location and operation of speed traps is in many areas regulated by law in an attempt to reign in such attempts at predatory policing and return the issuance of speeding tickets to a matter of public safety. This is no guarantee that police will actually follow these constraints, but a knowledge of the laws governing speed traps can prove useful in getting fines and speeding tickets dismissed as invalid in the courts.