What is the recommended following distance at 30 mph?

The recommended following distance at 30 mph is primarily expressed in terms of time rather than a physical measurement like car lengths. The two-second rule is a commonly accepted guideline, which suggests that a driver should maintain a distance that allows them to count two seconds from the time the vehicle in front of them passes a specific point until their own vehicle reaches that same point. This method accounts for the vehicle's speed and provides a buffer for safe stopping distances.

While measuring a physical distance of one car length (approximately 20 feet) could be relevant, it often does not provide an adequate safety margin at higher speeds or under varied conditions, such as rain or heavy traffic. This is particularly true because the stopping distance increases with speed, necessitating a longer following distance to allow for safe reaction times.

By integrating both concepts, the idea of two seconds serves as a more reliable measure of safe distance, irrespective of vehicle length or the conditions on the road. Thus, the choice that includes both the two-second guideline and one car length (20 ft) recognizes the comprehensive understanding of following distances, even though the two-second rule forms the essential guideline for safety at that speed.