Final answer:
A safe following distance at 40 mph under ideal road conditions is represented by the '3-second rule'. This distance should be increased in less than ideal weather or road conditions to ensure enough stopping space.
Step-by-step explanation:
The proper following distance for a vehicle that is traveling at 40 mph under ideal road conditions is often referred to as the '3-second rule'. To determine the appropriate following distance, a driver can wait until the rear of the vehicle ahead passes a stationary object, like a sign or tree, and then begin counting seconds. If it takes at least 3 seconds before the front of their own vehicle reaches the same object, that distance is generally considered safe for a traveling speed of 40 mph. It's important to adjust this following distance if road or weather conditions are not ideal, such as in rain, fog, or snow, where a greater distance would be necessary for safe stopping.