If the purpose of establishing speed traps is to discourage speeding, the police should welcome these private efforts to warn drivers of their presence. Would not such information cause motorists to slow down in the very areas where the police regard speeding to be particularly dangerous?
Of course, the purpose of speed traps is not to get drivers to slow down, but to catch them in the act of speeding - sometimes generating car-chase scenes - so that the state can collect hundreds of dollars in fines from those caught. It is part of the same syndrome that attends parking-meter violations: not to collect "rents" for parking spaces, but to collect money from those who overstay their allotted time and are given tickets. (This is why it is a misdemeanor, in many cities, for one to put coins in the parking meters where other persons' cars are parked.)
Even in automobile usage, the essence of state action always comes back to the line made popular in Watergate days: "follow the money!"