Safe space between cars varies with speed. At 40 mph or less, a 3 to 4-second following time is sufficient, but at higher speeds, a 4-second rule is recommended, to account for increased stopping distances.
When determining the safe space between cars while driving, we must take into account the vehicle speed and the necessary following time to stop safely. If you are driving at 40 mph or less, a following time of 3 to 4 seconds may be sufficient. However, as your speed increases, the recommended following time increases as well to compensate for greater stopping distances required at higher velocities. At higher speeds, it is best to use the 4-second rule.
Applying physics principles, we understand that stopping distance is a product of speed, reaction time, and road conditions. Reaction time largely remains constant, but speed significantly alters the distance needed for a car to come to a complete stop. A car initially traveling at a speed such as 30.0 m/s (equivalent to 67 mph), for instance, would require considerably more distance to stop than a car traveling at lower speeds. Thus, increasing the recommended following time as speed increases enhances safety.
So, while a 3 to 4 second gap may be sufficient at speeds of 40 mph or less, higher speeds demand a greater following time to allow for safe stopping distances. It's crucial for a driver to adjust following distance based on their current speed to ensure they have enough room to stop in case of an unexpected event.