Roadside barriers are important components in road and bridge project designs when a hazard is perceived alongside the roadway. Hazards include fixed objects such as nonbreakaway light and sign posts, telephone poles, bridge piers and abutments, retaining walls and culverts, trees with diameters of more than 6 inches, rough rock cuts, boulders, embankments, streams and permanent water bodies. Roadside barriers are also used to separate roadways from pedestrians, bicycle paths and steep grades, to separate opposing traffic lanes, and to define medians. They are typically set in the roadway's “clear zone” or “recovery zone”, the area beyond the travel lane that needs to be kept clear of potential fixed-object hazards. This area's depth varies with traffic volume and design speed.
There are three general types of roadside barriers, also known as guardrails: flexible, semi-rigid, and rigid. These categories refer to the system's deflection characteristics upon impact. Safety and aesthetic characteristics vary depending on the type of system.