What determines a “speed limit”?

Is it the speed at which the traveler feels safe? On a city street, the driver might feel safe at 35 or 40. On the interstate, the driver feels safe at 75 or 80… So that’s what they drive, even if the limit is 30 or 65, etc.

The factors that go into this “feeling of safety” include vehicle and road characteristics — steering and suspension; braking and engine power; road quality and contour — all vastly improved.

The most important abstracted factor is what I call “interruptions.” Every driveway, every intersection, every bump… is an interruption. The chance of a tree branch falling on the road is an interruption. City streets have a lot. Interstate highways have fewer. The Bonneville Salt Flats have none.

Better car suspension lessens the value of an interruption. A bump that causes a vehicle to veer off course or lose contact with the road is a major interruption. But a better suspension lessens the interruption-value of that bump. I’m convinced that better handling, braking and powertrain are major reasons people drive too fast.