cops walking (note that walking and driving are NOT the same) a beat
I've heard the same thing. I wonder if it's because walking is slower than driving (stop beating the guy for a second as the car passes), or if it's because seeing 'people' has more of an effect.
Some of both, I think. Cars are very anonymous, and the driver will (hopefully) be concentrating on driving rather than observing.
A police officer on a bicycle can be a good halfway: they're still very much human (can speak and be heard, can stop immediately without blocking the road) but they can cover a wider area. Depending on local geography, they can get to some places faster than by car. About half the police I see around here (London, but not the centre) are on bicycles.