The braking distance requirement at 55 mph states that the tractor-trailer needs twice as much stopping distance as a car. Which option correctly reflects this?

• A. The tractor-trailer requires half as much distance
• B. The tractor-trailer requires twice as much distance
• C. The tractor-trailer requires the same distance
• D. The tractor-trailer requires four times as much distance

Answer: (B)

The key idea is that stopping distance depends on how much energy the brakes must dissipate. A tractor-trailer has much more mass than a car, so at the same speed it carries more kinetic energy and forward momentum. The braking system and tires can only convert that energy into heat at a finite rate, and road friction limits deceleration. Because there’s more energy to shed, more distance is required to come to a complete stop. Under normal conditions at 55 mph, this extra energy translates to about twice the stopping distance compared with a car. That’s why trucks need more room to stop, and why you should give them a larger following distance. If road or brake conditions are worse, the required distance increases even more.