Speed Limits in Montana

The Biggest Differential in America

Montana enforces the largest truck-to-car speed differential in the United States: 65 mph for trucks versus 80 mph for cars on rural interstates. That 15 mph gap is not just a number — it fundamentally changes the driving experience. You will be passed constantly by cars traveling 15 to 20 mph faster than you. The dynamic requires patience, lane discipline, and awareness that closing speeds on approach are much higher than in states with smaller differentials.

Speed Limit Breakdown

The “Reasonable and Prudent” Legacy

Montana is famous for its former “reasonable and prudent” speed law — before 1999, daytime interstate travel had no numeric speed limit. The law was struck down and replaced with numeric limits, but Montana’s driving culture still carries its DNA. Expect traffic to move briskly. The 80 mph car limit is treated as a floor by many Montana drivers, not a ceiling.

For truck drivers, the “reasonable and prudent” standard still applies in a practical sense: you must drive at a speed that is safe for conditions regardless of the posted limit. In a Montana winter storm, 65 mph is not reasonable and prudent. Adjust for conditions — the speed limit is a maximum, not a target.

Corridor-by-Corridor Guide

I-90 (East-West, Southern Montana)

I-15 (North-South, Western Montana)

I-94 (East-West, Eastern Montana)

US-2 (Hi-Line, Northern Montana)

Comparison With Neighboring States

Montana’s differential is the widest in the country. Idaho has the second-largest at 10 mph. North Dakota has no differential at all. The transition matters — a trucker entering Montana from North Dakota goes from matching traffic speed to being 15 mph slower than cars.

Seasonal Speed Restrictions

Montana imposes seasonal weight restrictions during spring thaw (late winter through early spring) that can also include reduced speed limits on specified routes. These restrictions protect road surfaces as frozen ground thaws and roadbeds become vulnerable.

Check mdt.mt.gov/travinfo/restrictions.aspx for current seasonal restrictions. Spring thaw speed reductions are temporary but strictly enforced, and violations during restriction periods carry enhanced penalties.

Practical Tips

• Stay in the right lane on interstates. You are the slowest traffic at 65 mph.
• Be aware of closing speeds. A car approaching at 80 mph closes a gap 15 mph faster than you expect.
• Do not try to keep up with car traffic. The fine for a truck exceeding 65 mph on a rural interstate is real, and the speed differential enforcement is active.
• The mountain passes have reduced limits through grade areas. Posted limits on approaches take priority over the general 65 mph.
• At night, truck limits on two-lane highways match daytime limits (65 mph), but conditions often warrant slower speeds. Montana’s wildlife density — deer, elk, moose — makes night driving on two-lane highways genuinely hazardous.