The three-second rule is generally safe to implement at speeds below 40 miles per hour. It’s possible to use this rule to maintain a safe following distance on many different roads in California.   If you drive faster than 40mph, it’s a good idea to add an extra second for each 10mph of speed. For example, at 40mph, you should leave roughly five seconds of space between your vehicle and the vehicle in front of you. At 50mph, you may want to increase this to roughly seven seconds.