The paper gives an outline of the influence of some important factors on the power requirements for lighting in vehicular traffic tunnels. Some of these factors are: the "critical" tunnel length, the vehicle speed, and the driver''s adaptation. The adaptation differential depends on the topography of the access area of the tunnel.
General Rules for the Lighting of Vehicular Traffic tunnels Lighting in vehicular traffic tunnels must always permit a driver - when approaching and driving through the tunnel - to discover objects on or close by the road at a distance which enables him to stop in front of the object. This means that tunnels, which from a lighting point of view are long, need power-intensive, complicated day-time lighting installations, where the luminance level in the entrance zone must be adapted to the variations of daylight outside the tunnel. For power-economy reasons, one should always try to design the tunnel in such a way that it is considered short from a lighting point of view. Short tunnels mean tunnels, whose exits and areas behind the exits can be seen from a point situated at a distance equal to the stop distance in front of the tunnel portal. All other tunnels are long. From power point of view, a "critical" tunnel length is the length at which a tunnel is considered to "pass" from short to long.
The critical tunnel length varies between approx. 40 m and 100 m, depending on the tunnel profile and the traffic conditions. A length of 40 m applies to curved tunnels, high vehicle speeds and mixed traffic. The critical tunnel length can be doubled, if a shaft is opened up in the middle of the tunnel to permit daylight to enter. As an example can be mentioned, that the power required in a short tunnel with nighttime lighting might be in the range of 3–5 W per linear meter, while the power required in a long tunnel with day-time lighting might amount to approx. 1 kW per linear meter tunnel. The power requirements could thus be 200–350 times larger in a day-time lighted tunnel than in a night-time lighted one.
The power requirements in a day-time lighted tunnel decreases by approx. 10% at a reduction of the allowed vehicle speed by 10 km/h.
The topography of access areas of tunnels is very important with regard to the power required in a day-time lighted tunnel. In order to avoid the "black-hole" effect and enable drivers to discover objects immediately inside the tunnel entrance, the luminance level in the threshold zone (see Fig. 1) must be at least 1/15, or preferably 1/10, of the luminance adaptation level at the stop distance point. After the threshold zone, the luminance level is reduced in the transition zone with regard to the eye's ability to adapt itself to the low luminance level in the inner zone. The whole course of luminance applicable to a driver on his way into a tunnel is shown in Fig. 1.