I don't buy that 'the purpose of speed traps is revenue'. I'll buy that sometimes it is; others it's for safety; there can be both motives.
What if officers were doing a prostitution sting, or a bait car, or a drug buying sting, and someone started warning everyone in the area to prevent them doing it?
At what point is it 'interfering with a police operation' and a crime?
I think the people who want to warn others of a 'speed trap', to the extent they ignore the issue of safety the tickets help with, are idiots. It reminds me of the sympathy a lot more people used to have for drunk drivers, treating it like some big joke. It's misplaced sympathy with people who put others' safety at risk illegally.
The issue, however, is whether this is 'free speech'. Arguably, it is 'speech', it's communicating a message the same as if they held up a 'cop ahead' sign.
I'm tempted to say 'it's wrong to warn the speeders, but it's free speech'. But that 'interfering with police operations' is an issue, too.
This is one of those times it'd be hard as a Supreme Court Justice weighing the different interests. I have a hard time explaining why the flashing light warning is any different than yelling about "it's a bait car!", why the latter 'seems' more prosecutable. When in doubt, which I am, err with free speech, but I don't feel great about this position to say 'protect the right to interfere with the speed enforcement'.
Let's make the issue moot, by requiring devices attached to all cars with GPS's and speed trackers that report all illegal speeding automatically. Happy now?