True or false: The braking distance for a tractor-trailer at 55 mph is two times that of a typical automobile.

• A. True
• B. False
• C. Not enough information
• D. It depends on conditions

Answer: (A)

Braking distance grows with both speed and the vehicle’s mass, because more momentum and energy must be dissipated to come to a stop. A tractor-trailer carries much more weight than a typical car, so it has far more momentum to overcome when you brake. Even though the truck may have more braking capability, the extra inertia means it typically needs considerably more distance to stop. At 55 mph, it’s common to treat the stopping distance for a big rig as roughly twice that of a car under dry, good-condition road surfaces, which is why this statement is considered true.

Keep in mind that actual distance varies with road conditions, tire and brake condition, load, weather, and road slope. Wet, icy, or gravel surfaces, or a poorly loaded or poorly maintained system, can increase the gap well beyond that twofold.