Which statement about braking at 55 mph is correct?

Which statement about braking at 55 mph is correct?

• A. A tractor-trailer needs about twice the stopping distance of a car
• B. A tractor-trailer needs about the same stopping distance as a car
• C. A tractor-trailer needs shorter stopping distance than a car
• D. A tractor-trailer needs four times the stopping distance of a car

Answer: (A)

When you brake, you must dissipate the vehicle’s kinetic energy, which depends on speed and mass. A tractor-trailer has far more mass than a car, so it has much more momentum to overcome at the same speed. Even with strong brakes, the heavy truck can’t decelerate as quickly as a car because there’s more energy to shed and the braking system (including air brakes and the trailer) takes time to respond. This combination means a tractor-trailer requires a longer distance to stop, typically about twice the stopping distance of a car at the same speed. So at 55 mph, the truck’s stopping distance is roughly double, rather than the same, shorter, or four times as long. Keep in mind road conditions, load, and brake condition can affect the exact distance, but the general idea is that heavy vehicles need significantly more room to stop.

When you brake, you must dissipate the vehicle’s kinetic energy, which depends on speed and mass. A tractor-trailer has far more mass than a car, so it has much more momentum to overcome at the same speed. Even with strong brakes, the heavy truck can’t decelerate as quickly as a car because there’s more energy to shed and the braking system (including air brakes and the trailer) takes time to respond. This combination means a tractor-trailer requires a longer distance to stop, typically about twice the stopping distance of a car at the same speed. So at 55 mph, the truck’s stopping distance is roughly double, rather than the same, shorter, or four times as long. Keep in mind road conditions, load, and brake condition can affect the exact distance, but the general idea is that heavy vehicles need significantly more room to stop.