Downgrade Speed Creep on a Snowy Mountain Pass
You are driving a 38,000-pound, 40-foot bus with 45 passengers down a 6% Colorado mountain grade. The road is snow-packed, the temperature is 28°F, and the posted speed for buses is 45 mph. At the top you selected “D” because the bus was accelerating normally; the transmission has a manual low range and the engine has a retarder, but you have not used either. Halfway down, your speed has climbed from 35 to 58 mph despite steady pressure on the brake pedal. You smell hot brake lining, the air pressure gauge reads 105 psi and is steady, and the next brake check area is 1 mile ahead. A runaway truck ramp is signed 1.5 miles ahead on the right. Traffic is light but there is a curve at the bottom.
1. What is happening, and what is the correct immediate sequence?
Read the worked answer
The brakes are overheating from continuous application, and you are getting brake fade: the pedal is still down but the bus is accelerating. Do not keep pressing harder and do not continue to the bottom. Immediately select a lower gear range, use the retarder if it is safe on the snow, and begin snub braking: apply the brakes firmly enough to drop about 5 mph below your safe target speed, then release them fully and let the brakes cool. Repeat as needed. If speed still climbs and you cannot control it, use the runaway ramp; a controlled stop on the ramp is better than a curve at excessive speed. Keep monitoring air pressure and watch for a soft pedal. If the bus will not slow, warn traffic if possible and choose the escape route early rather than late.
2. What mistakes led to this, and how should you have set up at the top of the grade?
Read the worked answer
The main mistake was descending in “D” and relying on the service brakes instead of gearing down before the grade. At the top, you should have selected a low range that holds the bus at a safe speed without continuous braking, tested the brakes, and used the retarder according to the manufacturer’s snowy-road guidance. You should also have checked the posted bus speed and the brake-check area. A 6% grade with 45 passengers and a 38,000-pound bus needs a lower gear from the start; once the speed is already 58 mph, downshifting may be difficult or impossible without over-revving. The smell of hot lining confirms the brakes were already too hot before you acted.
3. How does the snow-packed surface change your braking and retarder choices?
Read the worked answer
Snow reduces traction and lengthens stopping distance, so hard brake applications can lock wheels or start a skid. Use gentle, progressive snubbing and leave more following distance. The retarder acts on the drive wheels and can also cause a skid on a slick surface, so use it on a lower setting or not at all if the rear wheels begin to slip. A lower gear gives engine braking without relying only on the brake pedal. Avoid sudden steering and braking on the curve. If the bus is sliding, release the brake to regain rolling traction rather than holding a locked wheel. The goal is smooth control at a speed below the posted limit, not simply reaching the brake check area.