The maximum grade allowed on any US interstate highway is 6% or a gain in elevation of 6 ft in 100 ft of roadway. Sections of highway with such grades almost invariably have 'truck' lanes for slower traffic, but not having driven the section of I-80 through the Rockies, I can't guarantee that that is the case there. I will note, however, that I-80 follows the same basic route as the first transcontinental railroad, a route chosen for its grade. Any other interstate routing would result in more miles without (I don't think) any significant easing of climb over the mountains.

AZBuck