We're often asked to accept some kind of parity between cyclists and motorists with regard to how much harm we can do. "Cyclists are just as dangerous as cars" and other such crap. Its nonsense, of course, for all sorts of reasons, and it doesn't take a genius to understand why.
Kinetic energy. We've not got very much of it. Really, we haven't. Stay with me here, we're going to do some maths.
Kinetic energy, in Joules (J) is calculated by taking half of the mass of an object (in kg) and multiplying it by velocity (in meters per second) squared. So a car, lets say 1500kg, travelling at shall we say 40mph (just shy if 18 meters per second) has 238,814 J kinetic energy. Lets say a cyclist is 90kg with the bike and is travelling at a rather rapid 15mph, that works out at 2023 J. Or, in other words, the car has 118.5 times more kinetic energy than the bike.
This all assumes of course that when you're hit by the vehicle, its still going at full speed and thats the whole story. If you're hit by a car its actually likely to be worse - you bend in the middle, you see, and your head will therefore be travelling MUCH faster when it hits the bonnet. And while much of the cyclist/bike equation is just a squishy as the pedestrian, every single component of the car is harder and less breakable than your fleshy bits are. But despite this, somhow a cyclist would somehow have to overcome more than two orders of magnitude difference in nastiness to be as dangerous as a car. This is why thousands of people are killed by cars, but cyclists cause fewer UK deaths than trouser donning accidents do. Bluntly the cyclist would probably have to cover himself and his bike with chainsaws to catch up with the harm a car can do.
I wanted to work out a good comparison for this, and the first thing I came up with was comparing a gun shot to a paper airplane. That doesn't work though - paper planes fly much more slowly than you think even if you put your back into it. A bullet with a muzzle velocity of 1000 meters per second and a mass of 0.0042kg has a kinetic energy of 2100 J. A better comparison is a tennis ball struck by a good amateur, flying at 60mph - this has 98 times less kinetic energy than the bullet (21.37 J, travelling at 26.8 meters per second and weighing 0.0549kg if you must know).
So there you have it - if someone tells you that being hit by a cycle is as dangerous as being hit by a car, or that cyclists are as hazardous as cars, tell them they're talking shite. To compare being hit by a car to being hit by a bike is mathematically comparable to comparing being hit by a bullet to being hit by a tennis ball.
So, what do you fancy? Bullets flying about on the roads or tennis balls? I know which I'd rather be hit by.