Hardly a week goes by without something in the news about streetlighting. So why all the interest? The reason is that hot topics such as energy saving, personal safety, circadian rhythms, human centric lighting, light pollution and the environment and, finally, digitally connected cities all combine together in the humble lamp post.
Streetlighting has to address all these issues so let’s try to disentangle them.
There is no doubt that you can save energy by using LEDs over conventional light sources. Whilst the lamp efficacy of high wattage LPS, SOX and HPS, SON, is higher than most LEDs, the delivered efficiency in terms of light on the roadway and pavement is less. Besides, the commonly used 70W SON lamp is only about 85 lm/W and that’s before luminaire losses and beam control are taken in to account.
Far bigger savings come from the ability to dim LEDs. Why argue about a few lm/W when you can dim down to 50% or even 25% when the roads are quiet.
It’s worth remembering that streetlighting was introduced long before the invention of the motor car. The reason was to reduce muggings, theft and break-ins. It’s no surprise that people feel safer walking down a well-lit street than a dark alleyway. Some argue that lighting streets simply displaces the crime to somewhere else but hard evidence is difficult to obtain because cities are not totally uniform – adjacent areas can differ in all sorts of ways other than just the streetlighting.
It’s worth remembering that streetlighting was introduced long before the invention of the motor car"
Circadian rhythms and human centric lighting
This is possibly the most contentious issue concerning streetlighting. There have been thousands of complaints in the UK about Cool white LED streetlights shining in bedroom windows and disturbing people’s sleep.
The first point to make is that a streetlighting layout that shines light in upper floor windows is poor design. The light is meant to be on the road and pavement; not in bedrooms. There is a case to be made that some light should illuminate front gardens in order to give a sense of spatial awareness and location – maybe even light your front door but please, no higher! Surprisingly, some manufacturers don’t offer the option of a rear cut-off visor.
Secondly, I have yet to meet an experienced streetlighting designer who would recommend a source much over 4,000K. For example, Westminster City Council specify 3,000K for their streets. Many Public Finance Initiatives which paid to upgrade the streetlighting were based on lowest cost per kilometre without much regard for the people whose houses were alongside the road.
Most research on the topic shows that high levels of illumination and Cool or blue-rich lighting can affect sleep patterns and alertness. Conversely, warm lighting at a low level helps you relax and sleep. The difficulty starts when you try to actually quantify the effect. Some research shows that lighting only starts to affect your circadian rhythms above 30 lux whereas other research has shown an effect at 10 lux or even less.
Another related issue is that of mesopic vision whereby at low levels of illumination your eye becomes more sensitive to the blue end of the spectrum. It’s why bluebell flowers suddenly become much brighter at dusk – and why roses can appear dull. This effect is exploited in BS 5489 (but not EN 13201) whereby you can design to lower levels of illumination by using a Cool light source. If you want to know more, search for S/P ratio.
One last point that isn’t often mentioned in the discussion is that most of us would welcome streetlighting that improved the alertness of all night bus and taxi drivers. Just don’t shine it in our windows.
Light pollution and the environment
This shouldn’t really be an issue because you can easily control the beam on an LED lantern so that no light is emitted above the horizontal. However, this is sometimes compromised slightly by tilting the lantern upwards a few degrees so that the beam reaches to the far side of a wide road. However, the actual quantity of light (lumens) is usually very small. An alternative is to use a taller column but that may be visually unacceptable by day.
If you look at the skyglow above our towns and cities, it is mostly yellow from the older sodium sources. Hopefully, this will disappear as it is replaced by LED lanterns with tighter beam control. Of course, poorly designed schemes can make things worse. There is some evidence to show that, lumen for lumen, white light skyglow is perceived as more of a nuisance than dull yellow.
As a huge generalisation, wildlife is often more disturbed by white light than yellow. Similarly, astronomers can filter out monochromatic sodium light but not broad spectrum white. Of course, the solution is minimise the amount of upward light in the first place, or even better, emit none at all.
My recommendation is to employ a fully competent streetlighting engineer.
Another much discussed topic. Streetlighting is ideally placed for exterior connectivity. There is a power supply at each location and line of sight view between each lantern. You can easily add low power devices such as motion sensors for footpaths, traffic flow counters, rain gauges, ground level temperature sensors and parking space monitors. All this could improve the environment of our towns and cities.
One of the big issues is that of who pays and who benefits? They are often different cost centres. For example, a local authority usually pays for, and is responsible for, the streetlighting. However, a reduction in accidents is a cash benefit to the hospitals or police forces rather than the local authority. Similarly, the cost of improving traffic flow might benefit private taxi companies and out of town commuters.