According to AAA, a "speed trap" is a speed enforcement zone that illegally induces drivers to speed so that a ticket can be issued, or a zone with a speed limit set specifically to induce speeding instead of for safety reasons. For example, the speed limit drops from 55 to 35 MPH but there is a tree in front of the sign making it impossible to see. The AAA used to list Speed traps in the USofA on their web site. I don't believe there were ever more than a couple on the list at any time.
What the OP is referring to should be called a "Speed Enforcement Zone". It isn't a "trap". A cop can sit where ever they like to catch speeders and other scofflaws.
According to Wiki, California outlaws "speed traps" and uses a different definition. In this case "trap" means a defined distance over which the speed is determined by timing. (A "speed trap" in a laboratory uses two gates and a timer to measure speed.)
The average joe probably uses "speed trap" to refer to "NFBSKing police officers who have the audacity to ticket'm for excercising their god given right to speed".