11. Street Lighting

11.1 Introduction

The principal objective in lighting a street is to reveal objects on the road surface and shoulders or footpaths, for the road user. In order to achieve this objective the lighting installation must provide the necessary visual conditions to enable drivers to identify objects with certainty and in sufficient time to take any corrective action which may be required. A further objective of street lighting for the driver, is to indicate the course of the road ahead.

Benefits also accrue to the community through improved security of property, lower crime rates and enhancement of business and commercial areas. Probably the greatest benefits of street lighting are derived from greatly improved pedestrian safety and the significant reduction of night time road accidents.

The AS/NZS 1158 Road Lighting Series of Standards recommends practice for public lighting in Australia. The main section is Part 1 which deals with the performance, installation and design of lighting systems.

11.2 Lighting Terms

Lighting Installation - The complete array of luminaire, poles (or columns) erected in position, complete with lamps and electrical or other auxiliaries, ready for operation on a particular length of road.

Luminaire - A housing for one or more lamps, together with any refractor, reflector, diffuser or other enclosure which may be associated with the lamp(s) in order to modify the light distribution and brightness or other lighting characteristics of the lamp(s).

Arrangement - The pattern according to which luminaires are sited in plan, e.g. single side, staggered, opposite or central.

Mounting Height (H) - The vertical distance between the centre of a luminaire and the surface of the carriageway immediately beneath the luminaire.

Spacing (S) - The distance measured parallel to the centre-line of the carriageway, between successive luminaires in a lighting installation.

Luminous Flux - The light emitted by a light source or luminaire, or received by a surface, irrespective of the directions in which it is distributed. The unit is the lumen (lm).

Luminous Intensity (of a source in a given direction) - The luminous flux emitted by the source in an infinitesimal cone containing the given direction divided by the solid angle of that cone. The unit is the candela (cd).

Illumination (Illuminance) - The luminous flux incident on a surface per unit area. The unit is the lux (lx).

Luminance (at a point of a surface and in a given direction) - The luminous intensity per unit projected area of a surface; e.g. if a very small portion of a surface has a luminous intensity of 1 cd in a particular direction, and if the orthogonal projection (on a plane perpendicular to the given direction of that portion has an area A, the luminance in that direction is 1/A candelas per unit area. The unit is the candela per square metre (cd/m2).

Reflection:

• Specular reflection – reflection in accordance with the laws of optical reflection, as in a mirror.
• Diffuse reflection – reflection such that the light incident upon a surface from a given direction is reflected from every part of the surface in many directions.

Light Output - The luminous flux emitted by a luminaire.

(Luminous) Intensity Distribution Curve - A curve of light distribution in values of luminous intensity, in a given plane or conical surface through the effective light centre of the luminaire.

Iso-candela Diagram - An array of iso-candela curves, each of which is a curve traced on an imaginary sphere with the source at its centre and joining all the points corresponding to those directions in which the luminous intensity is the same, or a plane projection of this curve.

Initial Light Output (of an electric discharge lamp) - The total luminous flux emitted by a lamp after 100 hours operation.

Service Life (of a lamp) - The hours of life stated by the manufacturer, in relation to the switching and operating conditions pertaining to street lighting, by which the lamp lumen output depreciates to 80 per cent of the initial light output. Alternatively, in the case of lamps which exhibit a depreciation of less than 20 per cent before failure, the service life is that stated by the manufacturer as representing the useful life based on the rate of lamp mortality.

11.3 Methods of Discernment

A clear understanding of how one discerns an object on or near the roadway at night is necessary for designing an adequate lighting system.

• Silhouette. An object is discerned by silhouette when the general level of brightness of all or a substantial part of it is lower than the brightness of its background. This method predominates in the observation of distant objects on lighted streets where the object possesses a low average brightness relative to the roadway.
• Reverse Silhouette. When the general level of brightness of an object is higher than that of its background, it is discerned by reverse silhouette. Such brightness of an object depends upon direct illumination on the side toward the observer.
• Surface Detail. When an object is seen due to variations in brightness or in its colour over its surface, without regard to its general contrast with its background, it is discerned by the surface detail. Such discernment depends upon a high direct illumination on the side toward the observer.
• Glint. When light falls on a specular surface, the reflection forms an image of the light source. Usually the reflecting surface is not a plane, and the image may be distorted. The observer, unconsciously drawing on past experiences, deduces the presence and nature of the object from which the light is reflected.
• Shadows. An object although invisible may cast a shadow due to an inequality of brightness between itself and its background. The shadow may present an interruption of the general brightness pattern, thus disclosing the presence of the object to the observer.

11.4 AS/NZS 1158 Road Lighting Standards

The requirements for the lighting of traffic routes is set out in the series of standards AS/NZS 1158 Road Lighting series. This provides comprehensive information for the lighting of urban traffic routes and covers:

• the selection of luminaires and lamps, and desired mounting heights;
• the geometric arrangement of poles on routes and at intersections;
• installation, operation and maintenance of street lighting;
• manufacturer’s lamp data; and
• lighting of channelised intersections.

11.5 General Design Considerations in Roadway Lighting

11.5.1 Weather Conditions

As a paved surface passes from the dry state through stages of being damp, wet, and finally flooded, its reflecting characteristics change. When merely damp, the effectiveness of the lighting installation may be affected very little, if at all. Thereafter reflection changes from diffuse to specular and visibility deteriorates.

Lighting above the minimum standard prescribed in the Code is especially advantageous in wet weather by reason of the improved visibility afforded by the higher illumination of objects.

11.5.2 Reflecting Properties of Road Surfaces

Because a primary objective in traffic route lighting is to produce a bright road surface, the reflecting characteristics of that surface are vital factors affecting both the lantern design and planning of layout. The minimum standard of lighting prescribed in the Code is based on the reflecting characteristics of a dense-mix (asphaltic concrete) type road surface in moderately worn condition, as being representative of Australian conditions and trends.

11.5.3 Road Layout

In designing a street lighting installation in accordance with the Code, instances will occur where, for aesthetic or practical reasons, departures from the recommended geometry will be necessary.

On occasions potentially troublesome areas are not evident from roadway plans. It is therefore recommended that full details of the road and its features (contours, signs, adjacent buildings, etc.) should be obtained by lighting designers at an early stage.

11.5.4 Roadway Features

Visibility of roadway features (kerbs, islands, etc.) depends largely on brightness contrasts produced by diffuse reflections from the relevant surfaces (i.e. upon their inherent ‘lightness’ and ‘darkness’). It follows that the effectiveness of a lighting system will be greatly enhanced if contrasting colours or textures can be introduced into the roadway features. These considerations are particularly important at channelisations and where medians with mountable kerbs are used.

11.5.5 Visibility of Road Signs

A street lighting installation should not be adjusted to light road signs but every effort should be made to avoid the installation obscuring or otherwise detracting from their visibility, including such influences as background glare. To ensure adequate visibility at night, the use of independently illuminated signs should be considered. Where the street lighting employs sources of predominantly monochromatic light (e.g. low pressure sodium vapour lamps) it is generally essential to illuminate signs independently to restore reasonable colour appearance.

11.6 Types of Light Sources.

For public lighting, particularly for the lighting of traffic routes, the primary requirements of a light source are:

• high luminous efficacy (lumens per watt);
• high light output (lumens);
• long life, with high lumen maintenance over the usable life of the lamp; and
• reliability.

The following characteristics may also be primary considerations:

• acceptable colour rendering – important in areas of public amenity; and
• suitable colour appearance – the source colour appearance may be objectionable where there is a possibility of confusion with road, rail or marine traffic signals.

The three most commonly used types of lamps for lighting of traffic routes are:

• High pressure mercury vapour (colour corrected);
• Low pressure sodium vapour; and
• High pressure sodium vapour.

The characteristics of the principal light sources available for public lighting purposes are shown in the following table.

LINKS TO SITES ON STREET LIGHTING.

Although it is an American site, Streetlights.Net provides a wealth of information on streetlighting, particularly the historical aspects.

Page last modified 7 July 1999.