Under Colorado Driving Regulations:
"When vehicles meet on a steep, narrow road which is not wide enough for two vehicles, the vehicle going downhill must yield the right-of-way by backing up to a wider place or by stopping to leave sufficient space for the vehicle going uphill, except where it is more practical for the vehicle going uphill to return to a wider space or turnout."

I kept waiting for a car to come up the road and the truck had to stop and back up.
Now, that would be a sight!