A tunnel is a structure over a roadway which restricts the normal daytime illumination such that the driver's visibility is substantially diminished. The information in this Recommended Practice assists engineers and designers in determining lighting needs, recommending solutions, and evaluating resulting visibility at tunnel approaches and interiors.

The designer's task is not simple, particularly for a new tunnel, when only partial information about the portal and about the approach roads is available. Important variables include geographic orientation, geometric design, traffic volume, traffic speed, service levels, light source used, and modes of light application. The design criteria and procedures included in this Practice are based on theory and on information drawn from practical experience and engineering judgment. For example, this Practice clearly explains why maintenance should be a prime concern for any new tunnel lighting project. A good design not only provides acceptable initial results, but also enables the maintenance staff to sustain system performance.