For lighting purposes, a tunnel is defined as a structure over a roadway which restricts the normal daytime illumination of a roadway section such that the driver's visibility is substantially diminished. Tunnels cover roadways and produce a shadow that limits the ability of the driver to see objects or obstructions within the tunnel.

Tunnel lighting can require both daytime and nighttime lighting. Tunnel lighting design is outside the scope of this manual. For more information, see the AASHTO Roadway Lighting Design Guide and ANSI/IES RP-22-11 Tunnel Lighting.