Practical considerations
Applying lighting to the tunnel – thoughts on design parameters
As traffic and infrastructure move underground to allow humans more space and a better environment above, the understanding of road tunnel lighting assumes more importance.
A typical example of this is that of the new ring road that is being constructed around Stockholm, with much of the new portions underground to avoid existing structures and make it less vulnerable to the extremes of weather. The Southern Link (Sodra Lanken) has about 4.5 km of tunnel with multiple tunnel tubes and interchanges, creating a complex challenge for our lighting team.
At the same time, as traffic volumes increase, the complexity of roadway interchanges is demanding more underpasses to be constructed to maintain traffic flow. These also require lighting for safety and efficiency.
Underpasses
Underpasses differ from tunnels largely in terms of dimension and complexity. An underpass is typically no more than 100 metres or so, with both entrance and exit visible to the driver simultaneously. It may also incorporate pedestrian walkways and pavements.
Usually the lighting is kept at a uniform level, at a brightness similar to the threshold levels for a road tunnel. Control systems and photometer control are generally simple.
The method used for determining whether an underpass requires daytime lighting is called the look through percentage, or LTP, which evaluates how a car or pedestrian is silhouetted against the exit portal at a distance equal to the stopping distance from the entrance portal. The program downloadable free from the link below can calculate LTP for you.
LTP Calculator
Tunnels
There are various forms of tunnel construction, all of which have an influence on the lighting scheme design and its implementation. Many road tunnels are a combination of techniques.
The archetypal tunnel is bored painstakingly through solid rock. This is both time consuming and expensive, and the cost is magnified by the number of bores or tubes that have to be made. All services have to be run within the bore.
Tunnels can also be bored through mixed ground, requiring a tunnel lining to be cast in concrete as it advances through the relatively soft matrix. Although provision for services can be made within the lining, this may be restricted by need for strength and structural integrity.
A very common tunnel is a fairly shallow one which is relatively inexpensive to install, and can also be used as access to a fully bored tunnel or the tube tunnel. This is the ’cut and cover’ tunnel, effectively a very deep trench with a cover. Here it is easier to include a small service tunnel that carries the electrical cabling etc.
A fourth type of tunnel, often found in marine installations, is the sunken tube tunnel. Again services have to be contained within the bore.
Tunnel design and dimensions
The design of the tunnel may also have to consider whether the bores are single direction, contra-flow, single or dual carriageway, the emergency and carriageway shutdown provisions, interchanges and traffic flow restrictions (traffic lights and barriers outside the tunnel).
As well as the roadway, the cross-sectional dimension of a tunnel has to include the traffic (with tall vehicles), possible walkways/kerbs and foot access, safe havens and fans, detectors, cameras, warning and directional signage, lighting installation, control and switching equipment.
The tunnel’s length and cross-sectional dimensions also have a bearing on the volume of electrical equipment needed and, therefore, on the amount of cabling and control equipment required. A long tunnel with old-fashioned switched lighting stages needed massive amounts of heavy duty cabling especially at the entrance points, all of which was carried along the side walls and or cable trays overhead.
Addressable control of lighting like i-TunneL® B-Scout has greatly improved this aspect and the advanced system can manage 4000 lamps and up to 4km of tunnel.
The concentration of electrical equipment means that the tunnel is an electrically ‘noisy’ place in EMC terms, so signals must be well shielded and hardened.