I believe there are speed traps. However, I also believe that many people complaining about getting caught in a speed trap, simply got caught speeding.
Oftentimes an officer is handing out tickets because of complaints by the local residents. I got a ticket like that once. It was a common commute route and the homeowners complained enough about speeding that the PD set up an officer to hand out tickets over the course of a week. I got caught speeding and got a ticket. It wasn't a speed trap, it was a focused application.
The officer even apologized to me for the ticket. There was no plan to make money, just to stop the speeding.
Amazing how many people told me it was a speed trap and I should fight it because "it was illegal". Nothing illegal about it. I knew I was speeding. The signs were proper. There just happened to be an officer watching with a radar gun.