Final answer:
On packed snow, the minimum safe following distance while driving at 40 mph greatly exceeds the normal recommendation of one car length per 10 mph, likely requiring up to 10 seconds or more to ensure adequate stopping space in icy conditions.
Step-by-step explanation:
When driving on a packed snow road at 40 mph, the minimum safe following distance is not a fixed number but rather depends on several factors. These include the driver's reaction time, the vehicle's condition, the coefficient of friction between the tires and the road, overall visibility, and road conditions. The general rule of thumb for normal conditions is to maintain at least one car length for every 10 mph of speed. However, for packed snow or icy conditions, it's recommended to increase this distance significantly. Considering the braking distance increases on slippery surfaces, a safe following distance could be up to 10 seconds or more behind the car in front of you.
Without exact calculations, and given that reaction times might be slower on such roads, it's safe to say that following distances should be vastly increased to accommodate for the reduced traction. As a good practice, drivers should adjust their following distance to ensure there's ample space to come to a stop without collision.