How to light Highways

Photometric requirements

The application of the new CIE publication No.115-2010, taking into consideration several parameters linked to the traffic and the road configuration, leads us to classify a highway as being generally M3 or M4.
The values of the lighting criteria to respect are the following:

Light sources

The colour rendering is of lower importance compared with lighting commercial and pedestrian areas in city centres. Therefore, high-pressure sodium lamps are extensively used for this type of road (in some countries - Belgium for instance - low-pressure sodium lamps are also used).
However, white light can also be selected when more importance is given to a better colour rendering, particularly in areas closer to the cities. Then, metal halide lamps could be an option, but the use of LED luminaires is also possible. This type of light source needs reduced maintenance due to its very long lifetime. 

Luminaires

The luminaires chosen to light highways should have a high level of tightness (at least IP 66), in order to maintain initial performances for as long as possible throughout the lifetime of the installation.
The resistance to mechanical impacts must correspond at the very least with the degree IK 08, in line the IEC standard 62262.
These luminaires will preferably comply with the Green Light Label as defined by Schréder.

Installation layout

When the width of the central reservation between the two roads does not exceed 10m, the most appropriate installation layout consists of installing the lighting poles in the axis of this central reservation. Each pole will hold 2 luminaires, one oriented towards each lane.
The height of the poles depends first of all on the total width of the roads together with the set-back of the luminaires from the border of the road. The mounting height of the luminaires is in general 12m or higher.
When the central reservation is too wide (over 10m), a more efficient solution is obtained with two rows of poles in an opposite configuration, inside the central reservation or along the external borderline of each road.
The efficiency and the functional aspect of the lighting installation is of primary importance. The aesthetic appearance can however be preserved by using well designed functional luminaires. Ease of maintenance operations (lamp replacement) is important to reduce maintenance costs.
The lighting installation shall be energy efficient, with a SLEEC-L value (Street Lighting Energy Efficiency Criterion Luminance based) being at a maximum equal to 0.8W/m²/cd/m².
An accurate knowledge of the light reflection characteristics of the road surface is of primary importance to meet the above efficiency criterion. These characteristics can be measured by our exclusive system known as Memphis.
The following drawing shows a typical installation for a highway.