Turns out none of us are right. (Oh, okay, I'll say it: I was wrong.)
First of all, this is a graph of the numbers I was thinking of:

And this compares the figures according to hours travelled:

So it turns out the figures for biking, driving, and walking aren't all that different from each other--all in the same general ballpark.
Here's a good overview (from Treehugger.com) of the subject of bicycle safety and helmets. Here's another (from BicycleSafe.com) that makes some of the same points. And here's a nice analysis and debunking of the most widely-cited study on the subject, the one that claims that bicycle helmets "reduce head injuries by 85% and brain injuries by 88%."
This (from that last source) seems to be a pretty good takeaway on the whole subject:
- Even if helmets are effective, it does not follow that all cyclists should wear them. Racing car drivers wear helmets, but not people driving to work. The difference is the level of risk. For the same reason, racing cyclists and mountain bikers often choose to wear helmets, but riding down a quiet road to the corner shop is a generally safe activity. Driving and cycling have similar risks per hour of serious head injury.
So...a good general rule is to be aware of the level of risk and then just use common sense. Who'da thunk it?



