Interesting to know about the turning radius issue. Bicycles can cut very sharp, and I suppose that's a function of both mass and velocity. (Possibly the lighter bicycle is easier to tilt over into a tight turn, driven down by the rider's weight, while the motorcycle has to tilt under its own weight.)

Ultimately, though, on a city street my gut says that motorcycles are in pretty much the same situation as bicycles, or better. Not sure how the stopping distance compares, but what a motorcycle lacks in being able to swerve, it should make up for in power. (Different benefits in different situations.) I believe part of why drivers don't see bicycles is that bicycles are usually ridden on the right side of the right lane. But I have never ridden a motorcycle, so my gut could well be wrong. :-)

(Highway, that's a different beast. I care less about how loud motorcycles are out there.)

So I guess here's a question: Should bicycles be equipped with constant noisemaking devices?