What is the design feature of many expressway signs when struck by a vehicle?

The design feature of many expressway signs that allows them to break away upon being struck by a vehicle is primarily a safety measure. This design helps to reduce the risk of injury to the vehicle occupants as well as minimize damage to the vehicle itself. When a vehicle collides with a breakaway sign, the sign is designed to fall over or detach rather than remaining rigid and potentially causing severe harm.

This feature is part of a broader approach in traffic safety that aims to enhance the survivability of crashes by minimizing hazardous impacts. In contrast, signs that remain intact could pose a greater risk in terms of potential injuries or vehicle damage during a collision. The other options, such as lighting up or sounding an alarm, are not common features of expressway signs and do not contribute to the primary goal of traffic safety in the event of a vehicle collision.