How average luminance with good uniformities and limited glare in tunnel lighting enhances motorists’ vision.
The contrast in light between daylight and the luminance within a tunnel (brightness) causes motorists to slow down as they approach tunnel entrances. They are also confronted with the problem of visual adaptation during daytime.
Motorists’ eyes are adapted to the high level of daytime luminance when they approach the tunnel entrance. Consequently, if the luminance level inside the tunnel is much lower than outside, no details of its interior or any objects in it will be visible from the outside and the tunnel will appear as a “black hole” (see Fig. 1).
The motorist’s eyes adapt to the sudden change in lighting level as they go from bright daylight outside to the comparative dark inside the tunnel. To keep the level of visual performance and the motorist’s sense of confidence high, the luminance in the tunnel must be such that the adaptation problem is minimised, so that good visual performance is ensured over the whole distance, from the brightness outside the tunnel entrance, through the darker tunnel interior, and back to the brightness outside the tunnel exit.
Good tunnel lighting ensures good visibility for the motorist. This requires lighting levels matched to the adaptation level of the motorist’s eyes. This adaptation level changes gradually as the motorist travels through the tunnel. For lighting purposes, the tunnel can be divided lengthwise into five zones: the access; threshold/entrance; transition; interior and the exit zone (see Fig. 2).
The figures show the luminance variation for a motorist passing through a long tunnel. Coming from the access zone outside the tunnel, the driver passes through the threshold/entrance zone and the transition where the eye is allowed to adapt to the low luminance inside the tunnel. The level of luminance in the interior zone of the tunnel is similar to the levels used in exterior road lighting.
The exit zone provides the transition from interior zone to the outside at the end of the tunnel.
Fig. 1: Where the luminance levels outside and inside the tunnel differ greatly, the tunnel entrance appears as a “black hole”, causing motorists to slow down.
Access zone
The zone on the open road before the tunnel entrance where the approaching motorist should be able to see into the tunnel is called the access zone. The access zone is not part of the tunnel itself, but of the approaching road immediately before the tunnel entrance.
The length of the access zone is equal to the safe stopping distance (SSD), the distance needed by the motorist to stop the vehicle, given a certain speed. When approaching the tunnel, the motorist should be able to detect any obstacles inside and to react before reaching them. Given a maximum speed, the lighting should be designed to ensure that obstacles are detected at a distance from the tunnel portal that equals the SSD at speed limit.
The maximum light adaptation condition of the motorist’s vision in the access zone determines the luminance in the threshold/entrance zone at the beginning of the tunnel. Daylight is not constant; it changes continually with the time of the day, seasons and weather conditions. The entrance/threshold lighting level must be adjusted according the outside brightness.
L20 portal luminance measurement generates the input to regulate the different stages (see Fig. 3).
When driving normally, the motorist’s field of vision is relatively wide (about 20 degrees of aperture). When approaching a tunnel, this field of vision narrows, and is limited to a field corresponding more or less to the aperture of the tunnel’s portal, i.e. about two degrees.
CIE 88-2004 Guide for the lighting of road tunnels and underpasses defines the adaptation state as L20, which is defined as the average luminance contained in a conical field of view, subtending an angle of 20° with the apex at the position of the eye of the approaching motorist and aimed at the centre of the entrance portal.
This is a means of assessing how bight the tunnel portal appears to an approaching motorist and, therefore, directly affects how bright the entrance of the tunnel must be to allow a motorist to enter the tunnel safely (see Fig. 4).
Under identical daylight conditions, the L20 value will vary greatly between tunnels due to differences in approaches and surroundings. For the design of a tunnel installation, we need to know the highest value of L20 measured with sufficient frequency during the year.
Fig. 2: The tunnel is divided into five zones in terms of lighting.
The L20 value is affected by the speed of the road, as it is measured from the stopping distance, the shape of the tunnel portal, its surrounds, the approach road and its orientation. The orientation of the tunnel can have a dramatic impact upon the L20 value as the position of the sun in relation to the tunnel directly affects how bright the entrance portal appears to the motorist on approach.
Tools are available to calculate an accurate L20 value.
Threshold zone
The length of the threshold/entrance zone is equal to the stopping distance and we need lighting in this zone to avoid the “black hole” effect at the tunnel entrance. The amount of light needed to do this depends on the brightness outside the tunnel. It is determined by the brightness of the tunnel portal at the stopping distance, at a viewing angle of 20° (L20) .
Our eyes cannot adapt to different lighting levels in an instant: they will adapt automatically to the high outside brightness. To avoid the black hole effect, the entrance or “threshold” zone must be lit brightly over a length equivalent to the safe stopping distance. The luminance level at the end of the threshold zone is decreased to 40% of the luminance in the initial level. If this threshold lighting is designed correctly, it will result in:
Transition zone
In the transition zone, the lighting level is reduced gradually towards the level required in the interior zone. The lighting creates the transition (reduction) from the high light level at the entrance to the lower interior lighting level, at the “speed” at which the eye adapts.
The amplitude of the reduction from high to low light levels determines the time it takes the eye to adapt. At a given speed, the greater the difference between the lighting level outside and inside the tunnel, the longer the distance (and time) required for the eye to adapt.
Interior zone
The interior lighting is provided by a row of evenly spaced luminaires along the length of the tunnel.
The luminance in this zone is kept constant because the motorist’s eyes are reasonably well adapted, except at the beginning of this zone, where adaptation is not yet complete. It is therefore necessary to arrange for a level of luminance that is fairly high compared with the level needed on an open road at night with a static adaptation. The reason for this is that in a tunnel there is less space to correct an error and less space to avoid obstacles; if an accident does occur, the consequences are likely to be more severe. Practical experience has shown that the levels of 2 – 10 cd/m², are to be recommended for the interior zone, the lower levels being acceptable for tunnels with low traffic density and driving speed.
Fig. 3: L20 portal luminance measurement allows the regulation of the different tunnel stages.
Exit zone
In the exit zone, the lighting must facilitate the transition from the low illuminance of the interior zone to the high illuminance level outside.
The exit zone begins at the end of the interior zone and ends at the tunnel exit, where the motorist’s vision is influenced by the brightness outside the tunnel during daytime.
Visual adaptation from a dark environment to a bright one is quick. When going from a dark environment to a bright one, the visual adaptation is very quick and, therefore, extra lighting (in addition to the interior zone lighting) is usually not needed at tunnel exits. However, lighting must be reinforced if the traffic is dense or when the motorist is faced into a low, setting or rising sun while driving out of the tunnel.
Uniformities and glare
Good uniformity of luminance should be provided on the road surface and the walls up to a height of 2 m. The lower parts of the walls act as a background for traffic, as does the road.
The recommended luminance in clear conditions is a ratio of 0,4 to the average on the road surface and on the walls up to 2 m height. A longitudinal uniformity of 0,6 along the centre of each lane is recommended for the road.
If the access road is lit, the lighting inside the tunnel should be at least equal to the average level uniformities and threshold increment (TI) level of the access road. The uniformity of the tunnel at night shall fulfil the same requirements as the daytime lighting.
Fig. 4: L20, the average luminance in a conical view, is a means to assess how bright the entrance appears to the motorist.
If the access road is not lit, the average luminance in the tunnel must not be less than 1 cd/m², an overall uniformity Lmin/Lav ≥0,4 and a longitudinal uniformity Lmin/Lmax ≥0,6 inside the tunnel, for each lane.
As visibility decreases because of glare (TI), it is important to reduce glare by as much as possible. In terms of glare, increase of threshold contrast should not be greater than 15% for all lighting levels and lighting zones of the tunnel.
Conclusion
Average luminance with good uniformities and limited glare in tunnel lighting will enhance motorists’ visual comfort, visual performance and sense of confidence.
Acknowledgement
This article is based on a presentation delivered at the 2018 IESSA Conference and AGM, and is published here with permission.
References
[1] W van Bommel: Road lighting: Fundamentals, technology and application, Springer International Publishing, Switzerland, 2015.
[2] CIE 88 Guidelines for the lighting of road tunnels and underpasses), 2004.
[3] Philips Lighting internal materials.
Contact Nelisiwe Nkosi, Philips Lighting SA, Tel 011 471-5066, nelisiwe.nkosi@philips.com
