Choose the right gear before you start down
The single most important rule in mountain driving is to select the correct gear before beginning the descent. Modern trucks no longer have the rule of thumb that you should descend in the same gear you used to climb — engine and transmission designs have changed enough that this guideline can leave you under-geared. Consult the vehicle manufacturer's recommendation, the route advisory signage, and your own experience with the specific load and vehicle combination.
Use engine braking; save the service brakes
Engine brakes (a "Jake brake," compression release engine brake, or exhaust brake depending on the vehicle) are designed to absorb the kinetic energy of the descent so that your service brakes do not have to. Service brakes that are forced to do all the work overheat, lose effectiveness through brake fade, and in the worst case fail entirely.
The proper technique is to apply the service brakes briefly and firmly to slow from your safe speed plus five miles per hour back down to safe speed; release; and let the engine brake hold the vehicle steady at safe speed. Riding the service brakes lightly is the wrong technique because it heats the brakes without giving them time to cool.
If your brakes fail: the runaway ramp
Runaway truck ramps are gravel- or sand-filled escape lanes designed to absorb the kinetic energy of a vehicle whose brakes have failed. If your brakes fail and a ramp is available, use it without hesitation — the cost of using the ramp (vehicle damage, recovery fees) is trivial compared to the alternative. The CDL exam will test whether you can recognize the warning signs of imminent brake failure (an unusual smell, a soft brake pedal, the sound of the brake fade) early enough to act.
For drivers preparing for Mountain Driving and Runaway Ramps, additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.