At 55 mph, a tractor-trailer requires how many times the braking distance of a typical automobile to stop?

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

Answer: (B)

The main idea is that stopping distance grows with the amount of energy the brakes must dissipate. A tractor-trailer has far more mass and momentum than a typical car at the same speed, so there’s more kinetic energy to remove when you brake. Even though the brakes on a truck are powerful, they have to handle that extra energy, and tire grip limits combined with weight transfer mean the deceleration isn’t as large as in a lighter car. All of this adds up to needing more distance to come to a stop. In practice, at 55 mph a tractor-trailer needs about twice the braking distance of a typical automobile, under normal dry-road conditions. Of course, road conditions, cargo load, brake condition, and slope can change the actual distance.