Police Cars: Why?

Ever notice how everyone on the road will automatically slow down when they see a police car? Even if people are going the speed limit, it's still such a habit to slow down because police cars are scary. They are. The worst part of this is the fact that although they're on the road to keep people safe (I assume), policemen end up often being the cause of near-accidents. I cant say how many times I've seen the cars around me have to put on their brakes just a little too fast because they didn't notice that the car in front of them was decelerating at, I dunno, about 20m/h^2 (that's physics). Maybe it's just me, but I really feel like Policemen might further their cause a little farther by driving a car that doesn't scream criminals hide now because you see me before I see you (it's a lot to scream, but those are some impressive cars). I respect policemen. It takes a lot of guts to be made fun of your whole life by the teenage community. But really, I feel like that whole "undercover" thing should catch on a bit more. Everyone goes the speed limit if cops are around. Yet, I know of very few people that will otherwise.