So the idea of the micromort is about conceptualizing risk. A one in a million probability that something will kill you? That’s 1 micromort. Not the most difficult math, but it helps normalize risk (as for quantifying it, you’re still on your own), so (at least in some sense) the idea that hang-gliding generates the equivalent risk of death as driving 3000 km (~1800 mi) in a car. There does seem to be a time element missing, as well as some way to account for mitigating elements (I can drive a lot better than I can hang-glide). I saw this concept referenced on Bruce Schneier’s blog, who in turn credits the Stubborn Mule blog.