You're right, I should have been way more specific. I mean number of casualties in the one specific incident. Terrorist acts often involve tens, if not hundreds, of people, both killed and injured. In the vast majority of cases, a drink-driving accident will affect a handful of people.

Nothing about any of this is very rational, but there's something in our collective nature that draws more attention to a single incident involving X casualties than X/Y unrelated incidents each involving Y casualties.