Which method does the California Driver's Handbook recommend for keeping a safe following distance behind the vehicle ahead?

Why

The handbook's tool against tailgating is the three-second rule: stay three seconds behind the vehicle ahead. The glossary explains how to measure it: pick a fixed point such as a sign, start counting when the vehicle ahead passes it, and if you reach that point before you finish counting to three, you are following too closely. The handbook uses the same figure elsewhere, when merging onto a highway and when following a motorcyclist. That space is what gives you time to react if the driver ahead brakes suddenly or makes a mistake. The one-car-length-per-10-mph guideline is an older rule of thumb that the handbook does not use. Seeing the other driver's mirrors is a rule about large trucks, and it works the other way around: when a truck's side mirrors are out of your view, you are out of the driver's. Add space when a tailgater is behind you and when following a motorcyclist over metal surfaces or gravel.