Stopping at 40 mph

• A. 4's safety cushion, 240ft traveled, 210ft distance to stop
• B. 4's safety cushion, 320ft traveled, 340ft distance to stop
• C. 5's safety cushion, 400ft traveled, 340ft distance to stop
• D. 3's safety cushion, 200ft traveled, 100ft distance to stop

Answer: (A)

The concept here is how stopping distance is composed and what a safe following distance looks like at a given speed. At 40 mph, a 4-second safety cushion translates into the distance you would travel before you start braking. Since 40 mph is about 58.7 feet per second, four seconds of space equals roughly 58.7 × 4 ≈ 235 feet, which rounds to about 240 feet. That is why the option lists 240 feet traveled for the cushion.

Once you start braking, the distance you need to come to a complete stop from 40 mph is the braking distance. Using a reasonable emergency braking deceleration (roughly 0.25 g, or about 8.0 ft/s²), the braking distance from 40 mph is about v²/(2a) ≈ (58.7)²/(2×8) ≈ 210 feet. That matches the 210 feet distance to stop.

So this choice makes sense: a 4-second safety cushion, about 240 feet of travel before braking, and about 210 feet of braking distance. The other options don’t align with these calculated distances for 40 mph, so they don’t fit the expected stopping behavior.