Final answer:
The rule of thumb for a safe following distance is commonly the 'Three-Second Rule,' not a specific number of car lengths per 10 mph as the options suggest. Car lengths can vary, and the three-second rule adapts to different driving speeds and conditions to maintain safety.
Step-by-step explanation:
In ideal driving conditions, the rule of thumb regarding following distance is often stated as the 'Three-Second Rule', which can be adapted to equate to a certain number of car lengths. Although not explicitly listed among the options provided, this guideline is crucial for maintaining a safe distance between vehicles. However, the specific answer options provided seem to suggest a different method based on car lengths for distance. While the options mention one, two, three, or four car lengths per 10 mph of speed, this is not considered the standard rule of thumb.
As the length of the average car is 4 m (based on the reference information provided), the equivalent in car lengths could be calculated if a specific time-based distance like the Three-Second Rule were to be converted into distance. However, those exact options don't directly correspond to the recommended practices for safe following distances. It's important to note that the three-second rule can adjust the following distance based on car lengths according to the speed being traveled, conditions of the road, and the driver's reaction time.