Andy Fish, a technical specialist at science-based technology company 3M, outlines the key points to consider when specifying a sign’s retroreflectivity.

Traffic signs are used to communicate everything from speed limits to school crossings, making it critical that motorists can easily spot and read them.

Conspicuity and legibility are the first two qualities a sign needs in order to be effective, followed by comprehensibility and credibility.

One of the key variables influencing both conspicuity and legibility is the sign’s level of retroreflectivity – its ability to return light back to the source.

Nowadays, traffic signs need to be able to return more light back to the vehicle than ever before, for several reasons.

First, as the population ages, so does the average age of drivers. As people grow older, they require more light to read any given piece of information.

In addition, most modern cars’ headlamps create beams with sharply-defined edges. This is ideal for illuminating the road in front of drivers, but does not allow much light to ‘spill’ upwards. Any sign higher than about six metres off the ground can receive less than half the light emitted by the headlamps.

Increasingly, traffic signs are also having to compete with other light sources for attention. For example, in urban areas, drivers’ eyes are drawn to lit advertising boards and business signs.

There are various schools of thought about how much light traffic signs need to reflect in order to be effective.

A study by Dr Helmut Frank concluded that, in order to work effectively in a dark area, a traffic sign needs a minimum of three candelas of luminance per square metre (3cd/m2) – i.e. the amount of light given off by three candles in a square metre.

The same study found that, in an illuminated area, such as an urban environment, a traffic sign needs a minimum of 10cd/m2 to work effectively. However, other studies have suggested that this figure should be as high as 100cd/m2.

The Dr Frank study also found that the maximum luminance should be 10 times the optimum luminance. This point is widely agreed on.

The level of retroreflectivity needed will depend on several factors.

For example, the sign’s position on the road is important because both overhead signs and those on the right-hand side of the road receive less light from vehicle headlamps.

Another variable to consider is whether or not the area is urban or rural, as this affects the number and intensity of competing light sources.

The volume of HGV traffic is also important, as lorry drivers sit further away from the original light source – the vehicle’s headlamps – and, as such, receive less luminance from the traffic sign.

All permanent traffic signs in the UK should be specified using BS EN 12899-1:2007. This is the standard used to show compliance with retroreflectivity, among other qualities, ranging from wind loading to temporary deflection. This standard also recommends that high-performance microprismatic retroreflective materials are used wherever signs are not directly lit.

There are three main classes of material used for traffic signs – RA1, R2/RA2 and R3B. These offer low, medium and high levels of retroreflectivity, respectively.

RA1 materials – such as Advanced Engineer Grade Prismatic reflective sheeting from 3M – can return a maximum of 16 percent of light to the driver.

R2/RA2 materials – such as High-Intensity Prismatic reflective sheeting from 3M – can return between 22 and 36 percent of light to the source.

R3B materials – such as Diamond Grade DG3 reflective sheeting from 3M – is designed for maximum optical efficiency and can provide up to 58 percent light return.

The same specification should not be used for all roads. Specs should be tailored, yet also simple, as too many choices can lead to errors. Specifiers should also document the rationale for their specification choices.