Road tunnel lighting is probably not something you see every day, although the Bell Common and Holmesdale tunnels on the M25 near Waltham Cross carry tens of thousands of commuter drivers. They were Britain’s first illuminated motorway tunnels and were built in the 1980s.
Tunnel lighting has always been considered a bit of a black art, but it just requires different techniques. One of the main ideas is that of transition zones. It would be prohibitively expensive and energy hungry to provide an illumination level throughout the tunnel that was equal to the daylight outside. Daylight can easily exceed 10,000 lx and 20,000 lx is not unknown, even in the UK. (Most traffic tunnels are designed using luminance, cd/m2, as the criterion. For simplicity, I am using illumination level, lx, because it is more readily understood).
To avoid excessive lighting levels/energy consumption and to prevent drivers entering a ‘black hole’, the entrance to the tunnel is lit to a high level and reduced in stages. The length of each zone depends on the speed of the traffic – the zones can be 50-100m long. As the exit to the tunnel is approached, the illumination increases. This can be done over a shorter distance because the eye adapts more quickly to high levels of illumination than from high to low.
Luminaires in tunnels are also subject to vibration and grime. Gusts of air from high-sided vehicles can severely buffet the luminaires, and the concentration of diesel fumes and oily exhausts means dirt can accumulate rapidly. Directly related to this is maintenance, and the longer life of LEDs offers a distinct advantage over HID or fluorescent sources. The luminaires also have to be resistant to the detergents used in pressure jets for cleaning.
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A single line of luminaires running down the centre is common for narrow tunnels or where you don’t need high levels of illumination. One advantage of Holophane’s T·Max is the huge range of light outputs available. The lowest is just 6,000 lm so it can be used for quite small tunnels or even subways.
At the higher end, the 40,000 lm version means that instead of having multiple rows of luminaires in the entrance zone, you can continue with a single row. Similarly, the intermediate transition zones can use the same luminaire with differing light output. This option shows the 12,000 lm version in the entrance zone and 6,000 lm in the running zone.
Tunnels are rarely the same shape or size and the combination of luminaire orientations, fixings and cable routes can be daunting. Holophane has a range of standard mounting options. For example, in this overhead arrangement, the control gear can be remotely mounted in the flanks of the tunnel where it would be easily accessible. The luminaire itself is less than 75mm deep so you can use it in quite confined spaces or recesses.
- Luminaires Holophane T·Max
- Optical control Symmetric lenses
- Arrangement Overhead typically 8m spacing
- Average horizontal illuminance 365 lx entrance, 130 lx running.
- Pros Clear visual guidance for the driver
- Cons A single line may not be sufficient for wide tunnels
For wider tunnels, lateral mounting from both sides is usually preferable. This option uses LED lenses with an asymmetric distribution. There is enough to light the walls and pavement but most of the output is directed on to the road surfaces. The Holophane T·Max is designed to be used with single-sided or twin-sided layouts – it has three beam options.
Again, instead of having multiple rows in the entrance zones, you simply use a higher wattage luminaire.
Unlike most luminaires, the T·Max has been designed to exploit the movement of air inside the tunnel. The rear face has enclosed fins and air movement convects heat away from the LEDs. This is accentuated by the Bernoulli effect – the air speed increases where the flow is constricted by the fins.
Tunnels can get hot, especially with slow-moving traffic; even more so in summer. Holophane deserves credit for not only designing a luminaire suitable for an ambient temperature of 60°C but also publishing critical data such as life, lumen output and wattage in its general brochure. I wish more manufacturers would do the same.
- Luminaires Holophane T·Max
- Optical control Asymmetric lenses
- Arrangement Lateral, typically 8m spacing
- Average horizontal illuminance 400 lx entrance, 125 lx running
- Pros Better for wide tunnels, easy access
- Cons A bigger tunnel means more units and higher installation costs
There are still a lot of tunnels illuminated like this using high-pressure sodium sources. Until a few years ago, the advantage of HPS over other light sources was the high lumen output. Entrance zones to tunnels require bright illumination and the 30-35,000 delivered lumens from a single HPS luminaire was far greater than that of other sources.
Even so, there were inevitable early lamp failures. You can see that one has failed in the entrance zone. Banks of luminaires were often used to boost the illumination. HPS lamps, especially the larger wattage ones with a long arc tube, can suffer early failure as a result of vibration. And tunnel luminaires get a lot of wind buffeting.
These failures meant that a lot of maintenance was required. With an overhead system, the tunnel would often have to be closed.
Of course, at the time, HPS was the most efficient light source available, but now it has been overtaken by the better quality LED luminaire manufacturers in terms of overall efficiency and installed load.
- Luminaires 250/400W HPS
- Optical control Aluminium reflector
- Arrangement Overhead, with multiple at entrance and 6m elsewhere
- Average horizontal illuminance 286 lx entrance, 83 lx running
- Pros None nowadays
- Cons Lamp replacement and maintenance