Design Problems of Modern Road Lighting.

If you are driving along a road at night you have a visibility problem. If there are no  street or road lamps you are dependant on the relatively small amount of light reflected back from your headlights to see and identify objects in front of you. However, your eyes rapidly adjust to the low ambient light conditions and your visibility improves. However, the lights of any traffic coming towards you overrides this and your eye adjusts again to handle the new light intensity. Of the light energy entering the eye now, only a small percentage is coming from the low energy light receptors (rods) that allow us to see in low ambient conditions, and the higher energy light from oncoming headlights effectively swamps the signals to the brain. This affectively blinds us.

However, as the high energy receptors(cones) are mainly concentrated in the centre of the retina (the fovea), we can reduce their effect by moving our eyes to one side (about 10º) which reduces the efficiency of the rods and allows a greater percentage of the low energy rods to transmit their signals to the brain. This allows us some  improved visibility.

If the road or street does have lighting, you have a different set of problems. In modern street lighting your visibility is reduced by the lighting itself, which is designed to suit the pedestrians rather than the motorists, a situation that actually endangers the pedestrians.

Modern street lighting is designed to give an even spread if light across an area, which is beneficial to the pedestrian.

However, when you are driving at night along a lit road you are conscious that there is a ‘blind spot’ under each lighting unit. Oddly, these are the areas that receive the most light. A pedestrian crossing the road in these areas will be be very difficult to see by approaching motorists.

The light shining downwards from the lamp is mostly converted to heat or reflected up into the sky, which is why this area appears as a blind spot  to motorists. This also means that the energy used to illuminate this area is wasted. An airliner or helicopter has a better view of the road than the motorist.

A further problem is light intensity.  If you are walking at night and someone shines a torch at you , you are effectively blinded, because the light intensity of the torch beam is far greater than the far lower ‘ambient’ light, (you are blinded by a bright spot’).  Driving on roads that are lit by modern light units, means that your vision is affected by many ‘bright spots’ created by the road lights shining in your eyes.

The eye can adjust to dim ambient conditions, but these bright spots prevent this adjustment from taking place. No bright spots should be visible within 15º of the eye-line. In general, you should not see any direct light from more than one road lamp and this should only be visible by swivelling the eye by 15º.  On many roads you can see up to 50 ‘bright spots’ created by badly designed lighting systems.

These problems were known and understood 50 years ago before road lighting design was transferred to ‘architects’. Architects were involved in making ‘pretty’ lamp standards rather than efficient ones. As long as they looked all right in daylight that was OK. The earlier lighting units had adjustable shades that allowed control over the areas covered. This allowed the overall lighting system on a road to be adjusted for best results.

For maximum driving (and pedestrian) safety all road lights should shine in the same direction that the traffic is travelling. Although relatively simple to do on dual carriageways, (With the aid of shields that ensure that light is not visible from the opposite carriageway), it is more difficult on narrow roads. However, the cost of modifying modern lighting would be justified by the huge savings in power costs.

It would also address the problem of light pollution at night discussed in the news item below.

HID Headlights (Bi-zenon)

The problem with these is mainly with the design of the light fittings themselves. If the higher energy beam was properly controlled they would be acceptable. Unfortunately, there is a large part of the beam energy that does not satisfy the dipped beam requirements.

The main culprits are those cars with wrap round  headlight designs, which allow a large amount of light to be shine outside the dipped beam area even though the centre of the beam focus may be correct.  This presents a problem in that test stations do not have the correct equipment to check this. We are left with a dangerous situation where HID headlights are able to pass current MOT standards even though they cause greater road dangers. This presents the government with the choices of forcing the manufactures to produce expensive modifications to the cars, banning the cars from British roads, or refitting them with traditional headlights. The moral situation is that the cars should be taken off the roads until the problem is solved in a satisfactory manner. However see  comments below.

Sidelight Light-Emitting Diodes. (LEDs)

Even the LED sidelight clusters have a detrimental affect on your vision. Over the last week while on holiday I have been checking on this problem. Even in daylight conditions looking at the LED clusters for a few seconds leaves an impression on the retinal cones that can last up to 30 seconds. This reduces your visual acuity. At night it is far more dangerous because this lingering after-image reduces the effect of turning your eyes to allow the retinal ‘rods’ to handle the dark conditions.

Note: It may be that the ‘Sidelight’ LEDs are actually the cause of this problem, and not the main headlight unit. Although LEDs use less power, the light is at a greater intensity that is higher than normal daylight. The Ministry of Transport or motoring organisations should carry out simple tests to check this. Fitting a change-over switch in the lighting system so that operation of the main headlights cuts out the sidelight clusters would probably solve the problem.

This is particularly evident on many imported cars that have sidelights on at all times.

This posting was triggered by the following news item.

www.bbc.co.uk/news/science-environment-11990737 (Road lighting)

and http://www.bbc.co.uk/news/uk-13147629 (HID headlights)

Author; Brian Williams