Low clearance: how to keep your truck out of a bridge strike
A bridge strike rarely starts at the bridge. It starts with a truck whose height nobody measured, a route built for cars, or a sign read too late. Here is what low clearance means, how far to trust the number, and what a strike does to your record.
Written by Rigo Diaz
Low clearance in numbers
Low clearance means the space under a bridge, tunnel, or other overhead structure is too short for some vehicles. A warning sign is required under 14 feet 6 inches, or higher where state limits are taller, but no federal truck height limit exists, so avoiding a strike depends on knowing your loaded height and routing around anything lower.
Most of the risks I talk about with carriers are hard to see coming. A low bridge is the opposite: it has a number painted on it, and it has been in the same place for decades. That is exactly why a strike is so hard to defend afterward.
I spent more than six years in risk management before I founded Simplex, and I have led it for more than 25 years. This guide covers what the low clearance sign promises, where strikes actually happen, a routine that prevents them, and what a strike costs your safety record and your insurance.
The full briefing, read aloud
8 chapters. Select one to jump to it.
Generated with AI text-to-speech from this article's key insights, in order, rather than reading it word for word. Chapter times come from a transcript of the audio. The article itself was written and reviewed by Rigo Diaz.
Key takeaways
Low clearance has a legal trigger
The federal sign manual requires a Low Clearance warning for clearances under 14 feet 6 inches, or under 12 inches above the state's legal vehicle height, whichever is greater.
The posted number leans in your favor, until it doesn't
Signs show the actual clearance rounded down to the inch, and up to 3 inches less in frost areas, but resurfacing after the sign was set can take height away.
No federal height limit exists
States set their own, and FHWA puts them between 13.6 and 14.6 feet. A legal height does not mean every road will clear it.
Most strikes happen where trucks shouldn't be
New York research found most of its roughly 200 strikes a year were on bridges over parkways or local roads closed to trucks.
A car app doesn't know your truck
FMCSA warns that non-commercial navigation apps may not warn of height and weight limits or other truck restrictions.
A strike is usually a recordable crash
If the truck has to be towed, it meets FMCSA's accident definition, goes in your register for 3 years, and counts in your Crash Indicator BASIC.
In this articleContents
What does low clearance mean?
Low clearance means the vertical space under an overhead structure, such as a bridge, overpass, tunnel, or railroad underpass, is lower than some vehicles need to pass. On a sign, the number is the clearance from the road to the structure above.
The federal Manual on Uniform Traffic Control Devices decides when the warning is mandatory. Under Section 2C.25 of the 11th edition, a Low Clearance Advance sign (W12-2) must warn of any clearance under 14 feet 6 inches, or under 12 inches above the state's legal maximum vehicle height, whichever is greater.
| Sign | Where it goes | What it tells you |
|---|---|---|
| W12-2 Low Clearance Advance | Ahead of the structure, and with a distance plaque at the nearest road or wide point where you can detour or turn around | The lowest clearance ahead, and your last easy way out |
| W12-2a Low Clearance Overhead | On the structure itself (optional) | The clearance at the structure, which should match the advance sign |
| W12-2b Low Clearance Overhead | Over the lane or shoulder it applies to | That clearance differs across the structure, as under an arch |
Off a freeway exit or on an intersecting road, the manual calls for a rectangular warning sign with a word message instead of the W12-2. The warning will not always look the same, so drivers need to read for the number, not just the shape.
How accurate is the number on a low clearance sign?
The posted number is designed to err on your side, but it is only as current as its last measurement. MUTCD guidance says the sign should show the actual clearance to the nearest inch without exceeding it, and in areas where frost action moves the road, the figure may be reduced by up to 3 inches.
The same section tells road agencies to re-evaluate clearances periodically, particularly after resurfacing. A new layer of pavement raises the road under the bridge. If the sign was not updated, the number overstates what is left.
Two more gaps are worth teaching. A sign cannot account for packed snow or ice on the road beneath it, and under an arch or uneven structure the clearance changes from lane to lane, which is why the manual adds lane-specific signs where clearance varies greatly.
Is there a federal height limit for trucks?
No. FHWA confirms that federal law imposes no vehicle height limit. Each state sets its own, and FHWA reports state standards ranging from 13.6 to 14.6 feet.
California shows where the responsibility sits. Its Vehicle Code section 35250 caps vehicles and loads at 14 feet, and any vehicle or load over 13 feet 6 inches may only run on highways where the owner deems it safe. A legal height is not a promise that every road will clear it.
Interstates give you the most room. FHWA's Interstate standard calls for at least 16 feet over the whole roadway on rural sections and on one routing through each urban area, while other urban Interstate routes need only 14 feet. Lower structures need an approved design exception, so even the Interstate has low spots.
How do you know your truck's real height?
- Measure the tallest point of the loaded unit, not the spec sheet: the trailer roof, a reefer unit, freight stacked on a flatbed, or equipment on a lowboy.
- Measure again whenever the trailer, the tires, or the load changes.
- Post the measured height in the cab where the driver sees it, and enter the same number in the navigation profile.
- Anything over a state's legal height needs that state's permit, covered in our oversize load permit guide.
Where do bridge strikes happen?
Most bridge strikes happen where trucks were not supposed to be. Research summarized by the USDOT in 2013 found New York bridges took roughly 200 strikes a year by over-height trucks, mostly on low bridges over parkways or local roads closed to truck traffic. One bridge in Rye Brook was struck more than 100 times in 10 years.
That makes a truck ban a safety rule, not just a traffic rule. Under 49 CFR 392.2, every commercial motor vehicle must be operated under the laws and ordinances of the place it is driving.
Navigation is the usual reason a truck ends up there. FMCSA's tips for truck drivers warn that non-commercial navigation systems and apps may not provide warnings of height and weight limits or other truck restrictions. A 2014 survey cited in the National Academies review found about a third of motor carriers blamed GPS errors for sending vehicles toward inadequate clearances.
The same review lists the other recurring causes from a UK study: drivers unaware of their vehicle's height, no designated low-bridge routes, and poor signage. Every one of them is a planning gap, not bad luck.
How do you avoid low clearance strikes?
Low clearance avoidance works as a routine with checks at the yard, at dispatch, and on the road. Six steps cover it, and each one has an owner.
Step 1: Measure and post the height
The person who hooks the trailer or builds the load records the loaded height and puts it on a cab label. A driver should never have to guess.
Step 2: Use a truck navigation profile
Enter height, length, and weight in a commercial navigation profile, and keep it current when equipment changes. A car app routes for a car.
Step 3: Check the route before dispatch
Look for parkways, local truck bans, and underpasses, especially near the pickup and delivery, where drivers leave the main truck network. Our commercial driver safety tips cover the wider trip-planning habits.
Step 4: Decide at the advance sign
The W12-2 sign with a distance plaque marks the nearest road or wide point where you can detour or turn around. That is the place to decide, not the bridge face.
Step 5: Stop when in doubt
If the posted number is within a few inches of your height, or you cannot read it, stop where it is safe and call dispatch for a detour. A detour costs time. A strike starts a crash report.
Step 6: Plan over-height loads lane by lane
A permit is not a clearance check. In the 2013 collapse of the I-5 Skagit River Bridge in Washington, the NTSB found a permitted oversize load struck the bridge because it did not travel in a lane with adequate overhead clearance, citing insufficient route planning, a pilot vehicle driver distracted by a cell phone, and inadequate permit review with no advance low-clearance signs.
What does a bridge strike cost a carrier?
A bridge strike is usually a recordable crash. Under 49 CFR 390.5T, an accident includes any occurrence in which a vehicle has disabling damage and must be towed from the scene, as well as injuries treated away from the scene and fatalities. A peeled-open trailer that leaves on a tow truck qualifies.
Recordable crashes go in the accident register you must keep for 3 years under 49 CFR 390.15, and they count in your Crash Indicator BASIC. FMCSA's Crash Preventability Determination Program can remove not-preventable crashes from that measure, but its eligible types are mostly cases where the truck was struck by someone or something else. Hitting a low structure is not one of them.
Then come the claims. Damage to the bridge is a third-party property claim on your auto liability, damage to your own truck falls to physical damage coverage if you carry it, and the freight is a cargo claim. Each one lands in your loss history, and crash history is part of how insurers price your premium.
If a strike does happen, the first hour matters for both the record and the claim. Our guide on what to do after a truck accident covers the scene.
How Simplex helps you manage bridge strike risk
Prevention lives in your routine. The record and the coverage behind it are where we come in.
Your crash record, watched
Our CSA program team monitors your CSA data, including crashes, and helps fix the safety programs behind it.
Coverage for your own equipment
Auto liability pays for the bridge. Physical damage insurance is what repairs or replaces your own truck and trailer after a collision.
Permits for over-height loads
When a load is taller than the legal limit, our oversize permit services handle the state permits for each leg of the move.
Frequently asked questions
What does low clearance mean?+
Are low clearance signs accurate?+
Is there a federal height limit for trucks?+
What is a low clearance avoidance system?+
Is a bridge strike a DOT recordable accident?+
Where this information comes from
The sign rules, height standards, and crash definitions below come from federal regulators and research bodies, checked on October 16, 2026. Insurance treatment of a claim depends on your policy, so confirm it with your agent.
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1
FHWA — MUTCD 11th Edition, Chapter 2C
Section 2C.25: when Low Clearance signs are required, how the clearance is posted, the 3-inch frost reduction, and re-checks after resurfacing.
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2
FHWA — Federal size regulations overview
No federal vehicle height limit; state limits range from 13.6 to 14.6 feet.
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3
FHWA — Vertical Clearance on the Interstate System
16-foot minimum on rural Interstates and one urban routing; 14 feet on other urban routes.
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4
FMCSA — Tips for Truck and Bus Drivers
Non-commercial navigation apps may not warn of height and weight limits.
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5
eCFR — 49 CFR 390.5T
The definition of a recordable accident, including tow-away for disabling damage.
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6
FMCSA — Crash Preventability Determination Program
The crash types eligible for a not-preventable review.
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7
NTSB — I-5 Skagit River Bridge collapse (HWY13MH012)
Probable cause of the 2013 bridge collapse after an oversize load strike.
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8
USDOT — Strikes on Low Clearance Bridges in New York State
About 200 over-height truck strikes a year, mostly where trucks are prohibited.
Written by
Rigo Diaz
Founder & CEO, Simplex Group
Founder and CEO of Simplex Group, with more than 25 years leading the company and helping trucking entrepreneurs navigate compliance, insurance, and the day-to-day demands of running a carrier. Before founding Simplex, he spent more than six years in risk management, experience that continues to shape how he approaches insurance, compliance, and carrier risk today.
Published · Updated
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Every bridge strike is a planning gap you can close
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If you want help reviewing your crash record or the coverage behind your trucks, talk to our team.