CDL Combination Vehicles TestDriving Combination Vehicles SafelyMedium
A commercial driver is preparing to descend a long, steep grade with a fully loaded combination vehicle. The driver has downshifted and is using engine braking. What is the maximum safe speed for this descent?
- AA speed that requires constant, heavy application of the service brakes to maintain control.
- BA speed that is 5 mph below the speed used for ascending the same grade.
- CThe posted speed limit for trucks.
- DA speed that allows the use of only the service brakes for occasional short, light applications.
Show answer & explanationAnswer & explanation
Correct answer: D. A speed that allows the use of only the service brakes for occasional short, light applications.
The maximum safe speed on a downgrade is one that allows the driver to control the vehicle using engine braking and requires only occasional, light applications of the service brakes to maintain speed. This prevents brake fade.
Why the other options are wrong
- A. Requiring constant, heavy application of service brakes indicates the speed is too high, leading to brake fade and potential loss of control.
- B. The ascending speed is not a reliable indicator for safe descending speed, as vehicle dynamics and braking requirements are different.
- C. The posted speed limit may be too high for a fully loaded combination vehicle on a steep downgrade, even with engine braking.
Safe Downgrade Speed
On a long, steep downgrade, select a speed that allows engine braking to control the vehicle, with only light, occasional service brake use to prevent fade.
- Use engine braking.
- Avoid constant service brake use.
- Prevent brake fade.
Memory trick: Downhill, slow, engine takes the load, service brakes just show.