The Thelma brakes are no longer effective below what speed?

• A. 2 mph
• B. 5 mph
• C. 8 mph
• D. 12 mph

Answer: (A)

The main idea here is that some braking systems only work when the vehicle is moving above a very low minimum speed. Thelma brakes cannot generate meaningful braking force if the wheels are turning too slowly, so they become ineffective as you drop toward the lowest speeds.

In practice, once you’re moving even a little, the Thelma brake can engage and help slow the vehicle, but as you approach around two miles per hour there isn’t enough wheel speed to produce reliable braking action. That’s why they’re considered ineffective below that threshold.

The other speed options imply higher speeds where the system would still have some usable action, so they don’t describe the point at which the brakes stop working. Therefore, the speed at which the Thelma brakes are no longer effective is about two miles per hour.