Here in Vermont, when people are speeding and they see a cop up ahead they slow down. Clearly, by the time they can do that, the cop has seen them; in fact, slowing down, if anything, draws attention to the fact that they were speeding. But I guess the assumption is, the cop doesn't care as much that you were speeding, as that you stop.
Back where I grew up on Long Island, no one slowed down. If you're speeding and you're going the same speed as the cars around you, the assumption is, he can't catch all of us. Maybe if you're the car at the very back of a group, you might slow down, but otherwise, the odds of you being the one he pulls over are too slight. In fact, slowing down might be what makes him pick you out from the crowd.
In Vermont, when people who are speeding see a cop has pulled someone over ahead, they slow down. And not just because there's a car on the side of the road so they need to be slower for safety's sake. Even if the road's two lanes and wide open, they slow down just because there's a cop there.
Back where I grew up on Long Island, people kept the same speed, or might even speed up, when they see a cop having pulled someone over. After all, if he's standing there talking to some other poor sucker, he can't be chasing you and pulling you over. So this is the best time to be speeding because they know at least one cop (and probably the only cop doing traffic duty in the area) is busy.
I make no judgments about either approach, or which one is better.