What is a safe following distance behind the vehicle ahead under good conditions, using the 'three-second rule'?

The three-second rule provides a simple way to keep a safe following distance: watch the vehicle ahead pass a fixed object (a sign or pole), then count 'one-thousand-one, one-thousand-two, one-thousand-three.' If you reach the object before finishing the count, you are following too closely and should drop back. Because the rule is time-based, it automatically scales with speed. Increase the gap to four or more seconds in rain, fog, on slippery roads, when following large vehicles, or when being tailgated. Adequate following distance gives time to perceive a hazard and brake safely.