Lighting short road tunnels 29 The newly formed ‘tunnel’ was required to be lit to cater for users transiting by both day and night in comfort and safety – and this was the principal reason for the requirement of a tunnel lighting installation. An initial design (Figures 7 and 8) was conducted by a potential supplier of equipment following the guidelines of the then current tunnel lighting standard
what, if any, alternatives to the initial proposed scheme could be available. To this end, I was commissioned to undertake a review of the structure’s lighting requirements and advise on any potential alternative practical lighting solutions. Friargate tunnel had that contradiction highlighted earlier of being physically short while visually ‘long’. It was only 100m, but
The light pool involves the formation of a bright transverse strip of light within the core region of a short tunnel illuminating both the road surface and sidewalls. This method results in a visual impression corresponding to the lighting conditions of two short successive underpasses, in effect introducing a section of the tunnel as if there were natural light entering from above, in other words a skylight within the roof structure, as shown in Figure 10 below.
p Figure 10. Lighting layout for the light pool and
basic day/night units. Calculation of scheme to BS 5489-2:2016
p Figure7. Plan of initial scheme proposed to BS 5489-2:2003
p Figure 8. Calculation of initial scheme proposed to BS 5489-2:2003
using latest LED technology. However, as stated earlier, such ‘short’ tunnels were not dealt with very well within this standard. As such the direct logical interpretation made for this particular structure was that a full tunnel lighting scheme would be required. This resulted in some 112 luminaires being required with a substantial associated support structure from the new decking beams. Securing such a support structure to the new deck beams was also quite limiting, with luminaires being positioned within the beam spacing voids to maintain traffic envelope clearance, whilst no drilling of the beams was permitted. These restrictions led to a significant bracing arrangement (Figure 9) for the support structure being required to install any lighting scheme. Because of client doubts that the most effective scheme had been proposed, independent expert advice was sought to evaluate the newly formed tunnel and proposed lighting scheme tabled and
p Figure 9. The luminaire support bracing
had a critical feature in that drivers could not see the full extent of exit portal from the stopping distance (SD) on approaching the entrance portal because of the horizontal curvature of the roadway. This, again, was the principal issue that led the initial proposed design towards a full ‘long tunnel’ lighting solution. Following evaluation of the structure and what potential alternative options were available, I considered using the light pool approach.
The light pool method is used for contrast enhancement by illumination to the rear of potential obstacles within the tunnel. The target lighting values of the light pool are in accordance with the requirements for the entrance of a long tunnel using the same portal luminance values and factors. The level of the light pool strip would continually adapt to the changing external daylight conditions at the entrance portals. Supplementary lighting was installed in addition to the light pool to provide for the night and basic daytime lighting throughout the structure. Uniformity of this supplementary lighting complied with the requirements for long tunnels. It was considered the specific geometry of the Friargate tunnel lent itself well to the application of the light pool methodology, which led to the number of required luminaires being halved together with a significant reduction of support steelwork and of course ongoing reduced energy costs and reduced CO2 emissions. Friargate was the first structure in the UK to utilise this specific methodology of lighting for a short tunnel structure, with the noteworthy credibility that this method has been adopted by the German DIN standards authority. The now-issued revised UK BS standard for the lighting of road tunnels and underpasses incorporates this methodology and an option for consideration.
p Figure 11. Resultant installation image
taken during commissioning
John Rands is project manager at Designs for Lighting
Lighting Journal February 2017