The braking distance of a tractor-trailer at 55 mph relative to a car is which of the following?

• A. One time
• B. Two times
• C. Three times
• D. Four times

Answer: (B)

When you brake, you have to dissipate the vehicle’s kinetic energy. A tractor-trailer carries far more mass than a car, so at the same speed it has more energy to shed once the brakes are applied. Even if the braking system can pull roughly the same deceleration, the heavier vehicle takes longer to lose that energy, meaning a longer stopping distance. At 55 mph, a tractor-trailer typically needs about twice the distance a car needs to stop. This refers to braking distance only, not the distance traveled before you start braking (the reaction distance). So two times is the best answer because the heavy truck requires roughly twice the stopping distance of a car. The other options would imply equal or much greater stopping distances than is generally observed.