It&#x27;s not about some particular kind of skill-based estimate of the road&#x27;s design speed. It&#x27;s much simpler than that: people drive at the fastest speed at which they feel safe.<p>If people are driving on a large separated highway with big lanes, plenty of light, and it&#x27;s straight with no curves, and no traffic for miles, then they might feel safe going 80 or 100 mph if their vehicle can handle it. People on a small, narrow, windy street with pedestrians around will drive another speed. The point is that people make these judgment calls primarily based on instinct, rather than based on signs.<p>Imagine that you were walking down a path on foot, and one sign said the walking path was supposed to be 1 mph, and another said 2 mph, and another said 3 mph. Would those signs mean much to you, or would you trust your two feet, sense of balance, and situational awareness to find the right speed? People do the same in cars. Dylan16807 is right.<p><i>Most</i> people do, anyway. A small subset of people, I believe it&#x27;s about 5%, follow the sign regardless. These people create traffic problems since they move at a different speed than the rest of traffic. I read a good article about how traffic planners are re-evaluating the idea that slow speed limits are a good thing or beneficial for safety. The conclusion was that slower speed limits do not actually benefit safety; if the posted speed limit is out of alignment with the road, and whether people feel safe, it creates more problems than it helps. The takeaway from the research was that if traffic planners want people to drive more slowly, then they need to create smaller, narrower roads.