Mountain driving and steep grades: a guide for truck drivers

Safe mountain driving in a truck comes down to a decision you make before the crest: choose a speed and a gear low enough that the engine holds you back all the way down. On a long downgrade, the service brakes are only a supplement. Lean on them hard and continuously and they overheat and fade, which is why runaway truck ramps exist.

Gear down before the crest

The CDL manuals put it plainly: use the braking effect of the engine as the principal way to control your speed on a downgrade. That effect is strongest when the engine is near its governed RPM and the transmission is in a lower gear. So the shift happens at the top, before the truck starts to gain speed.

Your safe speed for a grade depends on the total weight of the truck and cargo, how long and how steep the grade is, and the road and weather conditions. If a speed limit or a Maximum Safe Speed sign is posted, never exceed it.

Selecting the gear early matters because once speed builds on the way down, a downshift may no longer be possible. A missed shift can leave you in neutral with no engine braking at all, which is the start of most runaway situations.

Brake fade and snub braking

Brakes slow the truck by turning motion into heat. They are built to absorb a lot of it, but continuous use on a long grade produces more heat than they can shed. Heat changes the brake lining so it grips less, and it expands the drums so the shoes have to travel farther to reach them. The result is brake fade: you press harder and get less.

Brake adjustment makes this worse. Brakes that are out of adjustment stop doing their share first, which leaves the rest to overheat and fade. Check adjustment before a mountain run, and use the brake check areas where they are provided.

Once you are in the right low gear, use snub braking:

1. Let the truck reach your safe speed.
2. Apply the brakes firmly enough to feel a definite slowdown.
3. When you are about 5 mph below your safe speed, release the brakes. The application should take about three seconds.
4. When the speed climbs back to your safe speed, repeat.

If your safe speed is 40 mph, you hold off the brakes until 40, brake firmly down to 35, release, and repeat as often as the grade requires. Releasing completely between applications gives the drums time to cool. Riding the brakes lightly all the way down does the opposite.

Reading steep-grade signs

The yellow Hill warning sign shows a truck on a slope. Plaques below it often give the percent grade, the length of the grade, or both, and some grades add a TRUCKS USE LOWER GEAR plaque or a posted truck speed limit.

The percent is the drop per 100 feet of road. A 6 percent grade drops 6 feet for every 100 feet traveled, about 317 feet per mile, so a 5-mile descent at 6 percent loses roughly 1,580 feet of elevation. The length plaque matters as much as the percent: a short 7 percent pitch is a different problem from 7 percent for 15 miles.

RUNAWAY TRUCK RAMP signs tell you how far ahead the next ramp is. Note each one as you pass it, so you always know where your next way out is.

Runaway truck ramps

A runaway truck ramp is an escape lane built beside a long downgrade to stop a truck whose brakes can no longer hold it. CDOT built its I-70 ramps after a number of runaway trucks and crashes on the steeper grades in the 1970s.

Use a ramp the moment you realize your brakes can't control your speed. Waiting to see whether the brakes recover only adds speed, and the next ramp may be miles away. Damage to the truck is always better than a crash at the bottom.

1. Signal and steer straight onto the ramp, lined up with its center.
2. Hold the wheel firmly with both hands as the truck slows.
3. Once stopped, secure the truck and call for recovery.

Never park on a runaway truck ramp or block its entrance.

Worked example: I-70 in Colorado

I-70 west of Denver is one of the best-known truck grades in the country. CDOT's truck guide lists these grades:

The same guide lists runaway truck ramps on westbound I-70 at mileposts 182, 185, 209 and 212, and on eastbound I-70 at milepost 257. The Eisenhower Tunnel sits at 11,158 feet between mileposts 214 and 215, and the Vail Pass summit is at 10,666 feet at milepost 190.

CDOT also lists commercial vehicle speed limits on two westbound descents: 45 mph on Vail Pass between mileposts 180 and 190, and 30 mph on the tunnel's west approach between mileposts 208 and 214. Hazmat loads can't use the tunnel at all and must go over Loveland Pass on US 6, a 5 to 6 percent grade with tight switchbacks. Conditions and limits change, so follow the signs posted on the day you drive. If you also face snow there, read our guide to winter driving and chain laws.

Common mistakes

• Starting down in too high a gear and trying to downshift halfway.
• Riding the brakes lightly from top to bottom.
• Skipping the brake check area to save a few minutes.
• Passing the last runaway ramp in the hope that the brakes will cool.
• Planning the climb but not the descent on the other side of the summit.

How Fort Route warns you about steep grades

Fort Route's steep-grade warnings come from the elevation profile of your planned route. Only sustained grades count: 5 percent or steeper for at least a mile, or 6.5 percent or steeper for at least half a mile. Each one appears on a card in the route sheet and in a list, and tapping it highlights that stretch on the map.

While you drive, you hear a voice warning 2 miles before each steep downgrade, for example: "Steep downgrade in 2 miles: 6 percent for 4.9 miles. Gear down before the top." That gives you time to pick your gear before the crest.

The limits are worth knowing. Grades are calculated from elevation data, so they are estimates and can differ from the posted percent, and shorter pitches below the thresholds aren't flagged. Posted signs win.