Road lighting strategies

It was 3 a.m. on a Saturday. My flight was delayed by an hour and I landed at Malta International Airport, checked out, grabbed my car keys and was off to drive myself back home - a regular ritual.

At that time of night, despite my long tiring day, I can't help but analyse the road lighting design. Not that I have never done this before, as it is part and parcel of my studies, but at that particular time of night the roads are pretty much deserted and one can clearly analyse road lighting design without the intrusion of light or glare from cars' head lamps.

My drive from Luqa to Mosta, a considerable drive by Maltese standards, is enough to get an idea of what road lighting is all about.

Road lighting design strategy

Roads are usually classified into groups, each having a set of basic elements that offer a basic understanding of lighting design. However as much as we like to group or section roads due to traffic density, urban or rural, likely district brightness and much more, each road has its own identity and should offer the lighting designer a new challenge and an independent insight into comprehending EN standards, which offer guidelines from start to finish.

Gone should be the philosophies of the Sixties and Seventies when road lighting design was about a scale and a pencil to point out places where masts holding lanterns were placed as neatly as possible not to intrude in any way with road layout.

Primary aim of road lighting

The main aim of road lighting is to reveal all important road and traffic features to all users, including drivers, pedestrians and police.

A secondary issue is its social role, in creating an interesting and pleasant environment both by day and night. Visual guidance, however, should not mislead the driver's attention.

Road lighting design cannot be expected to make safe a situation, which is inherently unsafe, such as a hump bridge or sharp bend. What it should do is reveal the essential details of the road and its immediate surroundings in a familiar way.

It should, for example, show clearly the run of the road, kerb lines, side roads, and junction and roundabout layouts. It should not confuse the driver by dazzle or by the disruptive camouflage of strong patchiness.

Lighting is provided as an amenity and the provision of comfort and convenience for a driver is as important as safety. We're paying a lot of attention to landscaping and the provision of good surfaces and reducing of gradients and bends to assist pleasant transport in safety. Lighting greatly improves the pleasantness of night driving and aids safer travel.

How to achieve road lighting design aims

First of all, one needs to understand the way the eye works and also nighttime vision. In driving, as in all other visual tasks, information is pouring into the brain and we must sort out the relevant from the irrelevant.

We compare the situation with similar situations stored in our memory and make deductions on the action required. We are hardly conscious of this process and frequently draw wrong conclusions. The eye is part of the brain and the whole object of sight is to feed information through the brain to motivate the body.

The action of the eye is automatic. Focusing, adjustment for intensity and co-ordination of both eyes do not call for conscious effort. We have two types of photo-receptors, but the ones used at nighttime are mainly rods - scotopic vision where peak eye sensitivity is roughly around 505nm.

The eyes can resolve fine detail, and generally this is the most important task we set ourselves. But it can discern small differences in brightness and this is particularly important to seeing by night.

Some accidents at night are attributed to carelessness; however, current research is trying to establish a phenomenon known to ophthalmic engineers as temporary short-sightedness under conditions of low brightness. This is attributed to a retina disorder, not detected by normal eye-test procedures and involves the attachment of electrical impulse detectors to the body.

Fortunately, we generally get away with small mistakes - but sometimes we don't. We may draw the wrong conclusions from correctly observed facts, but we may misread the situation or fail to observe some important element and take wrong action because of this.

Conflict areas

When planning road lighting design patterns, conflict areas is what should be sorted out first! All roundabouts (over-abundant in Malta!) and junctions can be considered conflict areas, whether large or small, always requiring a calculation process due to the varied nature of the associated lighting arrangement.

Roundabouts are truly conflict areas and very often there has to be a compromise in their design strategy. Conflict areas must be calculated in terms of illuminance unlike the rest of the roads, which are calculated in terms of luminance.

A lamp with a higher colour temperature combined with a slightly better colour rendering complemented with an increased illuminance are some of the elements which help identify and stimulate the driver that a conflict area is approaching.

Yet what helplessly irritated me during my early morning drive is that conflict areas went simply unnoticed. To add insult to injury, another roundabout that formed part of a recent installation went even more unnoticed!

Longitudinal uniformity

A road lighting luminaire forms on the road what is called a T-shaped patch. The shape and luminance of this patch is especially important. The head of the patch does not extend beyond the point on the road below the lantern. Shape and luminance depend on the distribution of light and especially on road surface properties.

The tail, formed by light leaving the lantern near the horizontal (if this light is cut off at the lantern, no tail will be formed), always points in the direction of an individual observer whether it's a vehicle, driver or pedestrian.

Longitudinal uniformity measurement is there to calculate exactly that these T-patches do coincide with each other to form uniformity. Unless there is good uniformity, no light patches and an adequate brightness, which corresponds to the likely district brightness, the object can't be seen clearly but as a silhouette.

Yet it does not take a lighting designer to clearly see the patches of dark areas between one luminaire and the other on the roads, making them look like long spotty stretches of land.

Threshold increment

Road lighting luminaires don't just give out light, but also glare - disability or discomfort glare, as it may be called, but still glare. This glare leads to another factor called threshold increment - the percentage increase in road surface luminance needed to make an object visible in the presence of glare, when it is just visible in the absence of glare.

Obtrusive lighting studies also use TI as a nominal measure of glare from off-road lighting installations. Glare is an issue of paramount importance. As much as we need light it can be invariably difficult to obtain the correct light.

What reduces glare and therefore helps keep the TI low are flat glass lanterns. Unless columns holding lanterns are higher than 12 metres, bowled glass luminaires should not be used as their percentage increase in the glare intolerance is inevitable and cannot be controlled.

Back to where I left, my drive back to Mosta was not, to my surprise, full of disappointments. Be it in longitudinal uniformity, surrounding ratios, placement of masts, TI, and so much more. Not to mention bends. It is obligatory that lighting is always fitted on the outer side of a bend, to lead the driver the way the road is heading.

Even if a road is bendy in more ways than one along its stretch (such as the Salina coastroad) masts ought to be fitted to accommodate this condition even if this leads to an irregularly staggered arrangement of luminaires - but to clearly and unconsciously show the driver the way the road takes its path, especially where accident spots are heading in due course!

Yet, despite all this, the cherry on the cake came when I offered my services to the Building & Engineering Department of the Works Division. The answer to my approaching letter was as blunt as "I thank you for your letter offering your professional services to our department. We are very much involved in lighting design projects. However we do the designs in-house as we have a Services Unit responsible for electrical and mechanical design. Thanks."

Olivia-Ann Calleja, ELDA, LET Dip. (Lond) Elect. Lic A&B., is a professional lighting designer.

oac@di-ve.com