How much more stopping distance do you need at 40 mph compared to 20 mph?

When determining stopping distance, it’s important to understand that stopping distance increases with speed due to the physics involved in deceleration and the time it takes for a driver to react. Stopping distance is generally calculated using the formula that incorporates reaction time and braking distance, both of which are influenced by speed.

At 20 mph, a vehicle will require a certain distance to come to a complete stop. When speed is doubled to 40 mph, the stopping distance doesn't just double; it increases significantly due to the equation of motion. The braking distance is proportional to the square of the speed. This means that when speed increases, the amount of stopping distance needed increases much more rapidly.

Thus, the stopping distance at 40 mph is approximately four times longer than the stopping distance at 20 mph. This significant increase is due to factors such as increased kinetic energy at higher speeds, which causes a longer distance to dissipate that energy safely. Therefore, understanding this relationship between speed and stopping distance is crucial for safe driving, particularly when transporting passengers.