What does "low clearance" mean?

"Low clearance" refers specifically to a situation where an overpass or any overhead structure has a restricted vertical space that may not be sufficient for certain vehicles, particularly taller ones, to pass underneath safely. When encountering a "low clearance" sign, drivers are informed that the height of the overhead structure is lower than the normal clearance needed for larger vehicles like trucks or buses.

This is crucial for ensuring safety on the road—vehicles that exceed the height restriction indicated by this sign risk damage if they attempt to pass under the overpass.

Although the other options touch on different aspects of roadway conditions, they do not directly address the specific meaning of "low clearance." For instance, while a height restriction ahead might indicate potential low clearance, it is less direct than stating that the overpass specifically has low overhead clearance. Thus, the clarity and directness of the provided answer make it the correct interpretation of "low clearance."