Heavy traffic dramatically increases the accident rate but decreases the fatality rate because it prevents people from building up enough speed to overcome their vehicle's safety features.

However, there is a hidden fatality rate in slow traffic. The number of person-hours wasted waiting for the traffic to move. It can build up to a lifetime pretty easily with enough traffic, so cumulatively the traffic jams in a major metropolitan area can waste the 750,000 hours or so of people's lifetime and virtually kill a person.

In short, commuting sucks.