The typical driver
has no idea what a safe speed feels like. They
routinely travel beyond the safe speed for a given environment. Charitably, they are unaware of the connection between their actions and the increased risk to pedestrians and cyclists. Uncharitably, they just don't care.
For example, did you know that the risk of severe injury for a pedestrian struck by a moving vehicle is about 25% at 23 MPH but jumps all the way to 75% at 39 MPH. Do the people who routinely travel down my street at or near 40 MPH know this? They do not behave like they know this. They behave like they think 39 MPH is more or less the same as 23 MPH. They behave like they aren't aware of the non-linear increase in risk to everyone around them (because they aren't and they're operating climate-controlled machines that are designed to abstract away the environment outside the car).
Not to mention just how goddamn unpleasant it is to be around cars driving that fast on residential streets, even if they never hit anyone. It's loud, aggressive, and just downright uncomfortable for everyone not also in a car.
