My bike, my bag and me are under 200 lbs (yeah, I'm a skinny lil' bastard) and I generally stay under 20 mph. Cars are more like a few thousand pounds, and most cars can reach 100 mph or more. With that weight and power comes responsibility. You can easily take someone's life with a car, but it's a bit harder to do with a bike, and much more risky for the biker. Sadly, the privilege of driving is not as tightly controlled as I think it should be. I constantly encounter severely inattentive drivers.

So basically, bikes have a right to plow through the streets irresponsibly because we rock hard and cars should watch they azz. (note: sarcasm)