People are bad at risk management. I see oodles of people making wild decisions in their cars (sweeping across 4 lanes at once, backing up on the highway shoulder to return to a missed exit). The norm for trailing the car in front of you is also way way too short based on real physical limitations of braking.

Speed limits can also exist for reasons beyond the safety of drivers. There are roads around me that are wide, straight, and empty enough to comfortably drive 40mph but are right next to a school. The school-zone limit exists to provide extra safety for children who might foolishly step into the road very suddenly.