Driving scales horribly. That's its first problem. If there are a few cars on the road, it's safe at reasonable speeds. At congestion, it becomes unsafe, slow,

Driving scales horribly. That's its first problem. If there are a few cars on the road, it's safe at reasonable speeds. At congestion, it becomes unsafe, slow, and miserable. It's also a leaky abstraction. It's surprisingly safe for what it is-- humans moving at speeds no land animal, under its own power, can. The risk of death on the highway is about 0.5 micromorts per hour. That makes it safer than skydiving or scuba diving, and comparable to marathon running. Ultimately, those aren't bad odds. (By the way, running marathons is safer than a sedentary lifestyle by far. I'm doing a half-triathlon in October with no qualms about the risk.) The real issue isn't speed (although that's a part of it) but complexity. Accidents don't usually happen when one thing goes wrong but when several things go wrong together. That's why they're more common on low-speed roads and in urban settings (where more random shit can pierce the abstraction of driving) than on the highway. Speed does mean, however, that the accidents that do occur will be a lot worse. Speeding on low-speed roads is actually a lot worse than on the highway. If you hit a pedestrian at 40 in a 25 zone, you'll probably kill him.