Hazmat Hazardous materials, as classified by the U.S. Environmental Protection Agency (EPA). Transport of hazardous materials is strictly regulated by the U.S. Department of Transportation.

Headache Rack Heavy protective barrier mounted behind the tractor's cab. Designed to prevent "headaches" caused by load shifting forward from the trailer and crushing the cab.

Horsepower (hp) Measure of power (the amount of work that can be done over a given amount of time). One horsepower is defined as 33,000 foot-pounds of work in one minute. Example: Lifting 33,000 pounds one foot in one minute, or lifting 3300 pounds ten feet in one minute.

Hours-Of-Service U.S. and Canada have set rules that govern the hours of service of commercial vehicle drivers engaged in interstate and international trucking operations.

Hydroplaning When the tires lose contact with a wet road surface.