I've also done the "flash your lights" thing to warn people and have slowed down when I see others signal with it.
I use the 'flash the lights' warning myself. True story - I had just driven through a known speed trap area, and sure enough two cop cars were sitting in the shadows, waiting (or maybe just eating donuts) After I passed them I watched cars coming from the other direction. Anyone who seemed to be in danger of going too fast, I gave them the 'flashing' signal. One car came around a curve flying. At least 10 over the limit. I started flashing to warn him. Then as he got close I realized it was a cop.
Which isn't surprising. I get passed by more cars with blue light bars on top than any other kind of car. They routinely drive 10+ over the limit, run red lights, stop signs... all the things we would get tickets for. They also waste a lot of gas. They don't have to pay for it...and they've got a souped up car. Light turns green and it's pedal to the metal.
My favorite thing to do is follow em. Most everyone slows down and gets out of their way. They plow through traffic, I tuck in behind them, match their speed and whizz by everyone. I try to hang back just enough to not be obvious. On the highway/interstate, you have to stay back about a 1/4 mile. On city streets, just a few car lengths... It doesn't always work, but when it does, you can make good time through city traffic.