Underpass lighting is mounted to the bridge structure and is detailed on RID(3). Luminaires are typically 150W EQ but may be larger for heights above 16'. Each lighting circuit on a bridge requires a lockable fused disconnect mounted at least 10 feet above grade on the bridge column or bent cap.

Type 1 underpass assemblies are mounted with 2 in. diameter arms on the bent cap of the bridge. This is the simplest mounting and may be used when the bent is close enough to the roadway for the underpass lighting to be effective.

Type 2 underpass assemblies are mounted on the bridge beams, and are used when lighting mounted on the bent would be too far from the edge of the roadway to be effective. Type 2 arms are attached near the top of the beam to avoid the beam's internal reinforcing steel strands. If the height of the beam is less than 54 in. then the luminaire is mounted on a 2 in. diameter arm. If the height of the beam is greater than 54 in. then the clamped part of the arm is 2 ½ in. diameter, and reduces down to 2 in. where the luminaire is attached.

Each underpass luminaire is protected with internal 10 amp time delay fuses to remove the luminaire from the circuit in case of failure. Underpass circuits that use a neutral require one fuse per fixture, and circuits with two hot conductors require two fuses per fixture.

Standard sheet RID(3) has details for Type 1 underpass, Type 2 underpass, and general notes applicable to most underpass installations.