Low bridges and truck restrictions: how to stay on legal roads

Trucks need truck routing because a road that works for a car can be illegal or impassable for a loaded tractor-trailer. Low overpasses, weight-posted bridges, roads closed to trucks and hazmat restrictions all sit on routes that phone map apps suggest without a second thought. The fix is a route built from your rig's real dimensions, checked against the signs you see on the road.

What a car route doesn't check

Low clearances

There is no federal height limit for trucks. FHWA lists state height limits ranging from 13.6 to 14.6 feet, and older bridges, rail overpasses and parking structures can sit below either number. A van that is legal on every highway in a state can still meet an underpass it can't fit under.

Weight-posted bridges

On the Interstate System, the federal gross limit is 80,000 pounds, with axle limits of 20,000 pounds for a single axle and 34,000 pounds for a tandem. Off the Interstate, states set their own weight limits, and an individual bridge can be posted far lower. Some weight signs give one gross limit, while others list a separate limit for each truck type, so read the line that matches your configuration.

Truck bans and restricted roads

Parkways, residential streets and some city routes can be closed to trucks, with the ban posted at the entrance. Even Interstates have exceptions. The federal route list in 23 CFR 658 notes that vehicles over 4 1/2 tons, other than passenger buses, are prohibited on the MacArthur Freeway section of I-580 in Oakland, and that the Theodore Roosevelt Memorial Bridge on I-66 in Washington, D.C., carries a 24-hour truck ban.

Hazmat routes

If you haul placarded hazardous materials, the route rules change again. Under 49 CFR 397, states and tribal governments can designate routes that hazmat loads must use and restrict routes they may not use, sometimes by hazard class. FMCSA publishes these designations in the National Hazardous Materials Route Registry.

A well-known example is the Eisenhower-Johnson Memorial Tunnel on I-70 in Colorado. CDOT's truck guide states that hazmat vehicles must bypass the tunnel on US 6 over Loveland Pass. Only when US 6 is closed does CDOT take hazmat loads through the tunnel, at the top of the hour.

The National Network and the last mile

The National Network exists because of the Surface Transportation Assistance Act of 1982 (STAA). It is the set of highways where STAA-size trucks may operate: the Interstate System plus other routes listed in Appendix A to 23 CFR 658, about 200,000 miles in all according to FHWA. On it, no state may set a width limit other than 102 inches, a semitrailer length limit under 48 feet, or a limit under 28 feet for each trailer in a twin combination.

The network ends at the exit ramp, but your delivery doesn't. Under 23 CFR 658.19, states may not deny STAA-size trucks reasonable access between the network and terminals or facilities for food, fuel, repairs and rest, and they must publish their access rules for drivers. Local roads beyond that access can carry restrictions the Interstate never did, and that is where many bad routes go wrong.

Where the network itself has unusual conditions, 23 CFR 658.21 requires them to be signed. That includes vertical clearances, posted bridge weight limits and time-of-day prohibitions.

How to read clearance signs

The Manual on Uniform Traffic Control Devices (MUTCD), the national standard for road signs, uses three low clearance signs:

• The Low Clearance Advance sign, a yellow diamond warning sign placed ahead of the structure.
• The Low Clearance Overhead sign, mounted on the structure itself.
• The Low Clearance Lane Overhead sign, used when the clearance differs by lane.

The advance sign is required where clearance is less than 14 feet 6 inches, or less than 12 inches above the state's legal maximum vehicle height, whichever is greater. That rule has a practical edge. In a state at the low end of the height range, a structure at 14 feet 8 inches may carry no warning sign at all, so if you run taller than the legal maximum on a permit, a missing sign tells you nothing. Follow the route on your permit.

Habits that keep you clear:

1. Know your height from a measurement, not a spec sheet. Measure the tallest point, which may be the reefer unit, the load on a flatbed or an antenna, and keep the number where you can see it.
2. Recheck your height whenever you change trailers or loads.
3. Leave a margin. Repaving and packed snow or ice can reduce the space under a structure after it was signed.
4. Watch arched structures. On an arch, the clearance near the edges is lower than at the center.

What to do if you're routed wrong

The time to act is when you first see a sign that doesn't fit your rig, not when you reach the structure.

1. Stop before you commit. If a posted clearance or weight limit is below your rig, don't attempt it. Pull over where it is legal and safe, and turn on your hazard flashers.
2. Look for a posted truck route or detour. Truck route signs override any app.
3. Get help if you can't turn around safely. Backing a combination vehicle into traffic or along a highway shoulder is dangerous, so call local police (or 911 if you are blocking traffic) to have traffic held while you back out or turn.
4. Re-plan from where you are with your correct truck profile, and check the new route on the map before you move.
5. If you strike a structure, stop and call 911 so the structure can be checked before other traffic uses it.

Common mistakes

• Trusting a phone map app built for cars. It doesn't know your height, weight or hazmat class.
• Leaving an old truck profile in place after switching trailers or loads.
• Assuming every Interstate is open to you. The federal route list itself contains truck bans.
• Taking a shortcut off the network to a truck stop or customer through residential streets.
• Reading the wrong line on a weight sign that lists limits by truck type.

How Fort Route routes for your rig

Fort Route's truck routing builds each route from the truck profile you enter: height, width, length, gross and axle weight, number of axles and hazmat, plus whether to avoid tolls and ferries. The router checks those limits against the restrictions recorded in its map data, prefers truck routes over neighborhood shortcuts, and offers up to 3 route options with miles, drive time, tolls and stops for each. If you miss a turn, the reroute doesn't start with a U-turn.

The honest limit is that map data can be wrong or missing. A new restriction, a temporary closure or a bridge that was never mapped won't show up until the data catches up, so posted signs always win over the route on your screen. Treat the route as a plan you check, not an order. For more on the difference, see truck GPS vs. phone maps, and for grades on the same trip, read mountain driving and steep grades.