A routing engine built for cars finds the fastest legal path for a car. Put a 13-foot-6-inch box truck or a loaded tractor-trailer on that path and the plan can send it under a bridge it will not clear, across a structure it is too heavy for, or down a residential street where it cannot turn. The fix is not a better algorithm. It is a road network annotated with the restrictions that apply to commercial vehicles, and a vehicle profile for every truck in the fleet.
The federal limits
The Federal Highway Administration summarizes the federal size and weight framework. On the Interstate Highway System:
| Limit | Federal figure |
|---|---|
| Gross vehicle weight | 80,000 lb |
| Single axle | 20,000 lb |
| Tandem axle | 34,000 lb |
| Axle spacing | The bridge formula, which may require a lower gross weight depending on the number and spacing of axles |
On the National Network — the Interstate system plus highways certified by states as able to carry larger commercial vehicles — no state may set a width limit of more or less than 102 inches, or a trailer length limit below 48 feet for a semitrailer (more where grandfather rights apply) or 28 feet for each trailer in a twin combination.
There is no federal height limit. FHWA notes that state limits range from 13.6 to 14.6 feet, which is exactly why height is the restriction most often wrong in a routing database: it varies by state, and low clearances on local roads are signed but rarely mapped well.
Federal width and length protections apply on the National Network. Leave it — to reach a customer on a local road — and the state and local rules apply.
Weight is more than the gross figure
A routing profile that stores one number, gross weight, misses two things.
Axle weights. A truck under 80,000 lb gross can still exceed an axle limit, depending on how the load sits. That is a loading problem as much as a routing one — see load securing and weight compliance — but routing needs the vehicle's configuration to check posted limits.
The bridge formula. FHWA's bridge formula ties allowable weight to the number of axles and the distance between them, so the same gross weight can be legal on one configuration and not another. Posted bridge limits on local roads are often lower still. A routing engine that knows only "truck: yes/no" cannot evaluate either.
Hazardous materials
Hazmat adds rules about where a vehicle may go, not just whether it fits.
- Designated routes. Under 49 CFR 397.67(a), a carrier transporting non-radioactive hazardous materials must comply with routing designations made by states or Indian tribes.
- Avoidance where no route is designated. Under 397.67(b), placarded vehicles must use routes that do not go through or near heavily populated areas, places where crowds gather, tunnels, narrow streets or alleys — unless there is no practicable alternative, a reasonable deviation is needed to reach terminals, loading points, fuel, food, repairs, rest or a safe haven, or an emergency or law enforcement requires it.
- Convenience is not a reason. Section 397.67(c) states that operating convenience is not a basis for deciding that an alternative is impracticable.
- Explosives in Class 1, Divisions 1.1, 1.2 and 1.3 require a written route plan, a copy of which goes to the driver (397.67(d)).
What a commercial routing engine needs to hold
For each vehicle profile:
- Height, width and length (including trailer)
- Gross weight and axle count, plus axle weights where loads vary
- Hazmat class carried, if any
- Turning constraints that rule out certain streets and U-turns
For the road network:
- Low clearances and posted bridge weight limits
- Truck-prohibited roads and designated truck routes
- National Network status
- Hazmat-designated and hazmat-restricted segments, including tunnels
- Time-of-day restrictions, such as delivery curfews and city-center windows
The algorithm on top — covered in route optimization algorithms explained — is only as good as these layers. A plan that is 3% shorter and crosses a 12,000-lb bridge is not an optimization.
The last 500 feet
Restrictions data covers public roads. What it does not cover is the customer site: the dock height, the gate width, the yard a 53-foot trailer cannot turn in. That information belongs in the stop record, alongside the delivery instructions — which is the same discipline as address data quality, extended to vehicle access. Drivers know these sites; capture what they know in the driver app so the next plan uses it.
Testing a vendor
- Run your own known problem routes. Every fleet has a list of bridges, underpasses and streets that have caused trouble. Put them into the demo with your real vehicle dimensions.
- Change one dimension. Raise the height by six inches and check that the route changes where it should.
- Test a placarded load through a city with tunnels or designated hazmat routes.
- Ask about update cycles for restrictions data, and how drivers report a wrong restriction.
- Check the driver navigation uses the same truck profile as the planner. A planned truck route followed with car navigation undoes the work.
These belong in any route software requirements checklist for a fleet running anything larger than a van.
Frequently asked questions
Is there a federal height limit for trucks?
No. FHWA states that no federal vehicle height limit is imposed; state limits range from 13.6 to 14.6 feet. Low clearances on local roads are posted individually.
What is the federal gross weight limit on Interstates?
80,000 lb gross vehicle weight, with 20,000 lb on a single axle and 34,000 lb on a tandem, subject to the bridge formula, which may require a lower gross weight depending on axle number and spacing.
Can consumer navigation apps route trucks?
Apps built for cars do not hold vehicle dimensions or commercial restrictions. A commercial routing engine needs a vehicle profile and a road network annotated with clearances, weight limits, truck routes and hazmat restrictions.
Do hazmat trucks need a route plan?
For explosives in Class 1, Divisions 1.1, 1.2 and 1.3, yes: 49 CFR 397.67(d) requires a written route plan furnished to the driver. Other placarded loads must follow state or tribal designations and the avoidance rules in 397.67(b).
