What Dave says is eminently reasonable; in a logical world you'd quantify danger by multiplying the probability of an event happening by
some number describing the seriousness of that event. We don't operate in a logical world, though. The amount of effort and money expended to reduce a risk is much more a function of how dramatic that risk seems to be. Expenditures involved in mining safety, say, are much greater than those for pedestrian safety. I won't even discuss terrorism.