Which statement best describes the braking distance relationship at 55 mph between a tractor-trailer and a car?

Which statement best describes the braking distance relationship at 55 mph between a tractor-trailer and a car?

• A. The tractor-trailer stopping distance is shorter
• B. The tractor-trailer stopping distance is about the same
• C. The tractor-trailer stopping distance is twice as long
• D. The tractor-trailer stopping distance is four times as long

Answer: (C)

Stopping distance at a given speed depends on how much kinetic energy the vehicle has and how quickly that energy can be dissipated by the brakes. A tractor-trailer weighs far more than a car, so it has much more kinetic energy to shed and greater momentum to overcome. The braking system and tires can only provide so much friction to slow you down, and that friction doesn’t scale up enough to match the truck’s extra energy. Because of this, at 55 mph the tractor-trailer needs more distance to stop than a car, typically about twice as long. The other options don’t fit because the extra weight means it isn’t shorter, isn’t roughly the same, and isn’t four times longer under normal conditions. Always leave extra following distance behind heavy vehicles to account for that longer stopping distance.

Stopping distance at a given speed depends on how much kinetic energy the vehicle has and how quickly that energy can be dissipated by the brakes. A tractor-trailer weighs far more than a car, so it has much more kinetic energy to shed and greater momentum to overcome. The braking system and tires can only provide so much friction to slow you down, and that friction doesn’t scale up enough to match the truck’s extra energy. Because of this, at 55 mph the tractor-trailer needs more distance to stop than a car, typically about twice as long. The other options don’t fit because the extra weight means it isn’t shorter, isn’t roughly the same, and isn’t four times longer under normal conditions. Always leave extra following distance behind heavy vehicles to account for that longer stopping distance.