28 Sources to have slowed recently, says Webster. ‘Placing a value on the maintenance cost saving is not straightforward and will depend on the maintenance arrangements that are in place. Energy savings are often the main contributory factor in residential streets, but on traffic routes, while energy saving is important, it is not the only factor. Where traffic management is required for cyclical maintenance, fault diagnosis and repair and so on, maintenance savings can become greater even than energy, and it is difficult to attribute a cost to less disruption experienced by the road user.’ Ultimately, whatever factors are put into the equation, it still all comes down to basic quality and trust in the product, says Higgins. ‘Irrespective of whether capital cost is lower, equal or higher, any whole-life cost analysis is based on the assumption that the product will perform as anticipated and where any failures occur, the manufacturer is still able to honour any guarantee and ensure the authority is not subject to any risk. ‘LEDs are new and so always have an element of the unknown,’ he continues. ‘However, we have limited that exposure by using a highquality product from an established manufacturer.’ One of the problems is nailing the specification, an issue that was a key discussion point at the last ILP PIP Forum (see Lighting Journal September). ‘Our concern is that there’s still an education gap with regards to LED specification,’ says Grant. ‘Low drive currents are often specified and religiously stuck to due to lack of understanding – with
efficacy now being linear, droop characteristics are no longer at the levels seen two or three years ago, for example. Some specifications are based on products that are two to three years old, in other words what is now old technology.’ Moving to solid state should not be problematic if certain criteria are fulfilled when fittings are specified, says Marques. ‘I have no concerns with the transition to LEDs if the correct colour temperature is specified, first-class components from major global suppliers are used, the correct glare category is specified, and the luminaires are tested by UK laboratories. The question remains though, says Webster, does it make more sense at this point to go for LEDs or can traditional options achieve the same results for a lower outlay? ‘New technology has changed the market completely in the past six years and a considerable amount of contracts in place did not have access to viable LED technology. If LED technology were not available today, and we needed to save the amount of energy that we do, the real question would be, could it be achieved with conventional light sources? ‘The answer is perhaps it could, but undoubtedly it would have to involve strategies such as partnight operation, deep dimming, downgrading of light levels and potentially decisions to no longer light certain areas. ‘I sometimes struggle with this as I personally liked the Cosmopolis. However, compared to the LED units we currently use, the energy savings are lower as it is less energy efficient and the associated maintenance
The US Transportation Research Board (TRB) recently initiated a project to evaluate new lighting technologies and identify new metrics for comparison. Lighting Research Center (LRC) scientists John Bullough, principal investigator, and Leora Radetsky co-authored the report, Analysis of New Highway Lighting Technologies. One of the problems in assessing new roadway lighting technologies is that comparisons are difficult because information for different systems is given in different forms. Bullough and Radetsky systematically analysed the performance of a number of representative luminaires of each type, and developed a consistent data sheet format, allowing direct comparisons. They found that many commercially available LED, ceramic metal halide and plasma discharge roadway lighting systems can meet existing standards for lighting the US equivalent of link roads and motorways, achieving
Lighting Journal October 2014
costs can be considerably higher.’ Manufacturers, committed to investment in solid state technology, are understandably less ambivalent. It is about the long-term view, says Rice. ‘LED technology, particularly LED luminaires, are designed to provide years of trouble-free performance. When built correctly an LED luminaire is designed to be more or less maintenance free. ‘The LEDs, and drivers, all operate on significantly lower power outputs than HID – while the trend to use power controls significantly increases the lifetime of the LEDs further because they only operate on full power for a short time, with the introduction of more lighting control, dimming and PIR operation. They are also easy and safer to recycle, containing less hazardous waste such as mercury.’ However, says Rice, the reality is that often local authorities are adopting a pragmatic, piecemeal approach. ‘We have to appreciate that this is still a relatively new technology and some may see it as prudent to observe rather than to invest in a full change over. What is happening is LAs will coordinate LED lighting trials in a particular street or roads within a town...Where a local authority decides to completely change over to LED they may do this over a set period of time, so often when an HID unit fails they may only use a direct LED replacement.’ Which reflects the case in Suffolk, says Webster. ‘Often there is a mixture of light sources as technology develops and funding is not in place to keep abreast – especially when existing lanterns are not at the end of their serviceable life.’
comparable or greater pole spacing than HPS systems and, in many cases, resulting in lower energy use. An important factor, says the report, was that not all systems of each type performed equally well, underscoring the importance of consistent data-reporting formats. The authors found that pole height was an important factor in the overall effectiveness of roadway lighting. Luminaire system application efficacy (LSAE), a metric developed by the LRC, can be used to optimise pole height and spacing for optimal economic performance of different schemes. Bullough and Radetsky also recommend that transportation agencies consider new benefit metrics for roadway lighting, including photometric quantities based on mesopic vision, brightness perception and visual comfort. http://onlinepubs.trb.org/onlinepubs/nchrp/docs/ NCHRP20-07(305)_FR.pdf