Low Clearance Sign: CDL Test Question & Meaning
Road signs on the CDL knowledge test: practice with the Low Clearance Sign
Whether the Low Clearance Sign appears on your CDL knowledge test depends on your state’s question bank, so we don’t claim it will. Every CDL applicant is expected to read standard road signs correctly, though, and this one is worth knowing by shape, color and meaning before test day.
Try the 5-question mini-test below, then take the full road-signs practice test or the CDL general knowledge practice test.
The low clearance sign warns drivers that an overhead structure ahead — such as a bridge, overpass, tunnel, or overhanging structure — has limited vertical clearance. The sign displays the maximum height of vehicles that can safely pass underneath, shown in feet and inches (for example, "12'-6" means 12 feet, 6 inches). This sign is critically important for drivers of tall vehicles including trucks, buses, RVs, and vehicles carrying tall loads. Hitting an overpass is a serious and surprisingly common accident that causes catastrophic damage to the vehicle, the structure, and can be deadly. The posted clearance is measured under ideal conditions — in winter, ice buildup or repaving can reduce actual clearance by several inches. If your vehicle is too tall, do not attempt to pass under the structure. Look for alternate route signs or turn around and find another way. Some low clearance signs include flashing lights or electronic warnings that activate when a vehicle too tall to pass approaches. Low clearance impacts vary by location — many urban areas have old railroad bridges or historic overpasses that were built before modern truck heights were standardized.
Quick check: 5 road-sign questions
What does this road sign mean?
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CDL & Commercial Driver Notes
Low clearance is one of the most critical signs for CDL drivers. Know your vehicle's exact height including any rooftop equipment, tarps, or raised containers. Post the height inside the cab. The posted clearance may be reduced by road repaving — subtract 3-6 inches from the posted clearance to be safe. Many bridge strikes happen on unfamiliar routes or when drivers follow GPS designed for passenger vehicles. Use truck-specific GPS and routing. A bridge strike can result in CDL disqualification, criminal charges, and career-ending liability.
Also studying for your regular driver's license? See this sign on DMV Help
Rules & What to Do
Know your vehicle's height before driving. Compare it to the posted clearance. If your vehicle is within 6 inches of the posted clearance, find an alternate route. Do not attempt to squeeze under. Look for alternate route signs. Be aware that repaving and weather can reduce actual clearance below the posted height.
Penalties
Hitting an overpass or bridge due to height violation can result in fines of $500-$10,000+, points on your driving record, and liability for structural damage repair which can cost hundreds of thousands of dollars. Criminal charges may apply.
Frequently Asked Questions
What does a low clearance sign mean?
A low clearance sign warns that an overhead structure ahead has limited height. The posted number shows the maximum vehicle height that can safely pass. If your vehicle is too tall, find an alternate route.
How tall is a standard truck?
Most standard semi-trucks are 13 feet 6 inches tall. The legal maximum height varies by state but is typically 13'6" to 14'. Always know your specific vehicle's height before driving.
What happens if a truck hits a bridge?
A truck hitting a bridge can cause severe vehicle damage, bridge structural damage, injury or death to the driver and others, and massive traffic disruption. The driver faces large fines and potential criminal charges, plus liability for bridge repairs.
