What does a low clearance sign primarily warn trucks about?

A low clearance sign is designed to inform drivers, particularly those operating larger vehicles like trucks, about the vertical space available under an overpass, bridge, or other structures. This is crucial for ensuring that a vehicle can safely pass underneath without risk of colliding with the structure.

For truck drivers, being aware of such height restrictions is vital because exceeding the height indicated could result in serious damage to the vehicle, as well as creating traffic hazards. Understanding the height of the bridge directly relates to safety and navigation for large vehicles.

The other options, such as maximum weight, distance to the next exit, or traffic conditions, do not pertain to height restrictions and therefore do not relate to the purpose of a low clearance sign. These other factors are important for overall driving but are not relevant to the specific warning provided by a low clearance sign.