The one-second rule is similar to the three-second rule but refers to vehicle length. This rule states to leave one second of space for every ten feet of vehicle length below 40mph. Above 40mph, add an extra second for every10 feet of vehicle length. This is because longer vehicles like tractor-trailers require much more time and space to slow down and stop to avoid crashing.   Another commonly used following distance rule is to leave about one second of space per 10mph of following distance. This would mean three seconds of following distance at 30mph, six seconds at 60mph, and so on. Faster speeds mean drivers should leave greater following distances.