An average freight train traveling at __________ mph needs a stopping distance of more than 1/2 mile.

The statement about the average freight train needing a stopping distance of more than half a mile highlights the significant momentum that trains possess due to their heavy weight and high speed. An average freight train moving at a speed of 30 mph will have a relatively shorter stopping distance compared to those moving at faster speeds.

At 30 mph, the physics of motion allows for a deceleration that is sufficient to stop within that distance. As the speed increases—like at 40, 50, or 60 mph—the stopping distance increases dramatically, often exceeding half a mile as momentum builds. Freight trains require longer stopping distances because of their mass, and the braking systems are designed to handle such large weights but still need ample distance to come to a complete stop safely.

Therefore, 30 mph is the correct answer, as it is at this speed that the need for a stopping distance greater than half a mile does not apply, while the higher speeds do require significantly longer stopping distances.