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SECTION 7 - STREETLIGHTING
Streetlights are generally operated throughout the hours of darkness as a service to the residents and road users of the area. There are estimated to be over 6 million lighting columns in public ownership in the UK. The principal role is to promote safe movement of traffic and to provide for the security to people and property.
Road lighting is designed to create an even luminance on the road surface as it is seen by the motorist.
Road lighting schemes are generally designed using British Standard BS 5489. The standard is split into ten codes of practice that cover the principal different categories of lighting. It contains flow charts to guide the design of road lighting schemes.
The design process involves careful consideration of the specific locations for lighting columns to give the best distribution of light. For example, on straight roads a staggered arrangement will produce an optimal effect, but on curves the luminaires are placed on the outside of the curve to ensure reflection off the road surface and at T-junctions a column is always positioned opposite the minor road approach.
Lighting is normally required at at-grade roundabouts for reasons of safety. BS 5489 concludes that a reduction in lighting at isolated junctions is unlikely to affect road safety but would result in less overall environmental intrusion.
In general local highways authorities use high pressure sodium lamps with flat glass horizontal cut-off ( HCO) luminaires for new rural road and junction lighting schemes. Columns are usually 10-12m high, with 8m high columns sometimes being preferred at isolated locations. Individual lamp light outputs normally vary between 12,000 and 50,000 lumens. Non- HCO low pressure sodium luminaires may require fewer columns and be more energy efficient, but high pressure sodium luminaires normally have a lower whole life cost if the columns are over 8m high and may therefore be preferred. Most road lamps are installed in fixed angle luminaires, whose main beams of light radiate out, up and down the road at between 60 and 75 degrees to the downward vertical. There is normally no need for any light to radiate above the horizontal and it is becoming more common to stipulate that luminaires have a full HCO.
Lighting installations for pedestrian pathways are normally scaled down versions of the above, although many use more decorative luminaires. Mounting heights are usually between 4-6m and light outputs vary between 3,000-6,000 lumens.
Control measures can include:-
- The use of front and / or back shields to improve illumination on the road whilst reducing intrusion elsewhere.
- As most street lighting is alight throughout the night, the obtrusive light levels to be adhered to should be those given for all night, i.e. after curfew.
- In addition, the Government's 'Lighting in the Countryside: Towards good practice' includes street lighting, and is applicable to towns as well as country.
- Reduction of the number of columns to a minimum - a single column may be sufficient on many small roundabouts;
- consider colour of lighting columns in relation to surrounding landscape, ie use a dark colour if the columns are set against backdrop of vegetation;
- give priority to the use of high pressure sodium lights which give some degree of colour rendition, and to the use of luminaires with full horizontal cut-off, wherever a lit junction is necessary;
- carry out a visual appraisal and design lighting scheme to minimise visual intrusion of light at night and of structures by day.
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