Montana Semitrailer Length Limit

Montana is grandfathered at 53'0". Every state must allow 48 feet on the National Network; Montana kept the right to allow 53'0" and cannot drop below it.

Appendix B length53'0"

Federal floor48'0" (all states)

Kingpin conditionNone

Tractor + 2 units93' 137.8K

Tractor + 3 units100' 131.06K

This is a floor, not your length limit. Appendix B lists what a state was grandfathered to keep under the STAA of 1982 and must go on allowing on the National Network. It is not the state's own limit, and it is not permission to run that length on any road you like. Most states allow 53-foot semitrailers under their own law — that permission comes from state statute, not from this table — and every state can still restrict length on specific routes, in cities, and off the National Network. Use this to know the federal baseline, then check the state limit and the route.

01 The rule

What Montana has to allow

Montana kept 53'0" under the STAA grandfather, so anything up to that length is federally protected on the National Network and Montana cannot legislate below it. A standard 53-foot van sits comfortably inside it.

The number comes from 23 CFR Part 658 Appendix B, the STAA grandfather list. It binds Montana on the National Network and cannot be lowered. It says nothing about what Montana allows beyond it, which is where state law takes over.

02 The details

Length, kingpin and combinations

Appendix B semitrailer length: 53'0"
Kingpin-to-rear-axle condition: None in Appendix B for this state.
Tractor plus two trailing units: 93 feet at up to 137,800 lb
Tractor plus three trailing units: 100 feet at up to 131,060 lb
Other grandfathered combinations: State submission includes multiple vehicles in this category—see individual State listings.
Past the legal length: a Montana oversize permit — see Montana oversize permits.

Montana Trailer Length FAQ

What is the legal trailer length in Montana without a permit?

The federally protected length on the National Network is 53'0" (23 CFR 658 Appendix B). Montana was grandfathered above the 48-foot floor and must keep allowing it.

Can I run a 53-foot trailer in Montana?

Yes. Montana is grandfathered at 53'0", so 53 feet is inside the federally protected length on the National Network.

Are doubles or triples allowed in Montana?

Montana holds grandfathered longer-combination-vehicle rights under Appendix C. Tractor plus two trailing units: 93 feet at up to 137,800 lb. Tractor plus three: 100 feet at up to 131,060 lb.

Does this length apply on every road in Montana?

No. Appendix B binds the National Network — the Interstates plus the designated primary routes. Off the network, and inside cities, Montana can and does restrict length, and posted route restrictions still apply. Check the route as well as the number.

Reference information for planning, not legal advice. Traffic laws change and this can be out of date, so always confirm the current statute and obey posted signs before you rely on it. Last reviewed July 2026. Source: eCFR, 23 CFR 658 Appendix B and C (title 23 as of 2026-07-23). See our Terms & Disclaimer.

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Montana Semitrailer Length Limit

What Montana has to allow

Length, kingpin and combinations

Montana Trailer Length FAQ

More for Montana

Montana Weight Limits

Montana Oversize Permits

Montana 511

All States

Check Montana before you roll