It only takes a careless second, a single moment of inattentiveness. And that's saying nothing of the level of malice required to attack another human being with a ton and a half of metal and glass.
Every single time this issue comes up, I find myself flabbergasted by how anyone could come to the point where they're effectively willing to risk another person's death. My cousin struck a pedestrian once, late at night. She had been drinking (working up the courage, I believe). She jumped in front of his car while he was driving at about 45 MPH. He killed her. And he felt terrible. Hasn't been the same since then.
And that was an accident.
How could someone--anyone--take even the slightest chance that their actions, whether careless or malign, might kill another person? Can you imagine the guilt?
If I am fortunate enough to have any non-cyclists in my audience, let me clarify for you: As cyclists, we are almost defenseless. Yes, we can carry mace or an airhorn or one of those ridiculous little bells. We can bike defensively. Hell, I've been known to give motorists the finger.
But when it comes down to it, you are encased in several hundred pounds of metal. And we are encased in lycra. And if you hit us, whether by accident or design, whether by inattentiveness or ignorance or frustration or downright hatred, you run the risk of killing us. Not making us angry. Not damaging our precious bicycles. Killing us.
Even if you think we shouldn't be on the road in the first place, do you really want to take that kind of risk?
Thanks to J for publishing on this topic in the first place. As more and more people trade cars (ahem, gas) for bikes, this issue will become more and more important for all of us.