Highway Lighting or Roadway Lighting [Standard Codes]. Highway lighting consists of providing illumination through lights in the vicinity of a highway/road/street.
It plays a major part in ensuring of highway safety during night driving and includes the aspect of roadside amenities for the convenience of the road-users, and preserving the aesthetics of the highway and its vicinity during the nights. In addition, highway lighting is important to provide security to the road-users during nights, especially to pedestrians.
Accidents are known to occur more during nights than during the day; this is attributed to poor visibility of potential hazards at night time and lack of proper lighting for the road or highway.
Further, provision of proper lighting in the vicinity of and approaches to different roadside amenities like petrol pumps, service stations, restaurants/motels, rest areas and truck parks makes it easy for fast traffic to identify such amenities sufficiently in advance besides enhancing the aesthetics.
Highway lighting is particularly required at road intersections, railway level crossing, bridges and other cross-drainage works, and near stretches of road-repair and temporary diversions, where restriction to free flow of traffic exists. This increases the road safety and adds to the visual appeal near such locations.
Although the vehicle headlights may be adequate for normal night driving, highway lighting is an additional and valuable facility for the road-users.
A highway lighting system primarily enables a driver to discern the presence of any object on or adjacent to the road which may be a potential hazard.
For identifying such hazards, there should be sufficient contrast of colour between the object and its background/pavement, or between different parts of the object.
An unlit object on the road, which is darker than the background, is discerned by the driver by virtue of this contrast; this is possible by the phenomenon of silhouette, meaning ‘outline’, ‘profile’, or ‘contour’; e.g., a dark object on a light-coloured pavement such as a cement concrete pavement can be easily noticed by a driver owing to this phenomenon.
When the object is brighter than the background/pavement, it can still be observed by the driver by what is called reverse silhouette effect. The objects at the side of the roadway, traffic islands, or other vehicles brighter in colour than the pavement are distinguished by the driver in this way; e.g., objects or vehicles on black-top surface of bituminous pavement are detected in this manner.
An object with variations in brightness and colour of different parts of its surface enables the driver to perceive it because of such variations. This is contrast of surface detail of the object itself without reference to the background/pavement.
In view of these observations, it is obvious that a light-coloured pavement with rough texture, which can reflect light from is surface is the best from the point of view of night visibility.
The following Indian Standard Codes of Practice are relevant to “Highway lighting”:
1. IS: 1944 – (Parts I & II) -1970: ‘Code of Practice for Lighting of Public Through fare’, Bureau of Indian Standards, New Delhi, 1970.
2. IS: 10322 (Parts-5) – 1987 (Superseding IS: 2149 – 1970); ‘Luminaries for Street lighting’, Bureau of Indian Standard, New Delhi, 1987.
A single luminaire produces a bright path on the pavement surface. The typical shape of this patch is roughly a ‘Tee’, when viewed by the road-user, as show in Fig. 11.3.
The head of the tee stretches across the road and the tail extends towards the observer. The shape of the patch changes with the condition of the pavement surface and the type of the luminaire. The head of the tee is a zone of diffuse reflection and whereas the tail is that of preferential reflection.
If the surface is highly polished approximating a mirror, the surface becomes a ‘specular’ or a ‘mirror reflector’; in such a case, the single luminaire appears at a single point on the road surface – its mirror image.
A smooth and wet surface has a long and luminous tail extending very near of the observer, but with a less pronounced head. As the surface becomes rough and dry, the head becomes more pronounced with very little tail. Other factors which affect the bright patch are the heights of the luminaire and the observer, the distance of the luminaire from the observer, and the distribution of the light from the luminaire.
When several luminaires are fixed in a lighting installation, bright patches are formed by each overlap and illuminate the pavement. The aim is to produce uniform bright areas when viewed by a road-user.
Rough surface, which is desirable from considerations of skid resistance is not suitable from lighting considerations. Rough and dark bituminous surfaces pose problems in lighting, while light-coloured concrete surfaces have good reflection characteristics.
The pattern of illumination of the pavement depends very much on the pattern of distribution of light from a luminaire. If a light source emits the luminous flux in all directions, the light that radiates upwards goes to waste because it cannot illuminate the pavement.
The flux from a source may be redistributed with a suitable device to redirect the flux in a pattern which yields the best bright patch.
One should guard against the phenomenon of glare, which is an undesirable and deleterious effect on the road-user observing the brighter portions in the visual field. Mild glare causes distraction and intense glare causes discomfort and fatigue.
With regard to highway/street lighting, glare may be distinguished as that which produces disability and that which causes discomfort. Glare causes a significant reduction in the observer’s capacity to see or discern a potential hazard, which is important for safe night driving. Such a glare is called disability glare.
Discomfort glare, on the other hand, is that which causes distraction, annoyance, and fatigue in the extreme.
One way to avoid glare is to increase the height of the luminaire mountings; another way is to locate the luminaires in such a manner that they are not in the driver’s normal line of the sight.
It is generally observed that light emitted at angles less than 70° from the downward vertical cannot fall on the driver’s eye, because the roof of the vehicle acts a shield and thus avoids glare. All rays above this angle cause glare; hence, it is desirable to limit the flux above this angle (Fig. 11.4).
Types of Vertical Distribution of Light in Highway Lighting:
The following are the types of vertical distribution of light:
(ii) Non-cut-off medium angle beam
(iii) Semi-cut-off (non-cut-off high angle beam)
The differences can be understood from Fig. 11.5.
In the cut-off system, the light is reduced as completely as possible above a 70° angle with the downward vertical, the main beam being at this angle. Practically no light is emitted above 80°. This system eliminates glare and performs very well with matt surface. The tail of the bright patch is negligible and the head is relied upon, making it more suitable for central mounting. Luminaire spacing should be smaller.
In the non-cut-off system, the peak intensity is at about 80°; a certain amount of glare is inevitable; this system is not generally adopted these days.
The semi-cut-off system is in between these two, with the peak intensity at about 75°. This, of course, results in a certain amount of glare. Luminaire spacing can be more than in the cutoff system. This system is not suitable for a central arrangement of luminaires. Distribution of light in the horizontal direction is also important. Footpaths and kerbs also need lighting.
The types of horizontal distribution of light are:
(i) Symmetrical distribution
(ii) Axial distribution
(iii) Non-axial distribution
These are shown in Fig. 11.6 (a), (b) and (c).
Symmetrical distribution is useful in pedestrian sidewalks and at the end of a cul-de-sac.
Axial distribution is suitable for the central arrangement of luminaires. Non-axial distribution is preferred when most of the light has to be beam onto the carriageway; in this case, the luminaires are mounted on the far side of road kerbs. The axial and non-axial distributions throw two main beams – one upward and the other downward onto the road.
A high mounting of light makes the luminance more uniform and reduces glare while low mounting hampers discernment by silhouette by increasing the illumination of the objects.
The general heights are 7.5 to 12 m for high-traffic roads and 3 to 8 m for other minor roads.
This controls the minimum illumination on the pavement. Preferably, it can be 35 to 45 m on important routes, roughly 3 to 5 times the mounting height.
The choice of lights is governed by life, wattage, brightness, efficiency, colour and initial act.
The following are the important types available:
(i) Tungsten filament lights
(ii) Tubular fluorescent lights
(iii) Sodium-vapour lamps
(iv) High-pressure mercury-vapour lamps.
(i) Tungsten Filament Lights:
These are the familiar and inexpensive type; their use is limited to minor streets/walkways. They have very low efficiency 8 to 14 lumens per watt, and have relatively short life .When cost becomes the main criterion, this is the automatic choice, but the life is only for about 1000 hours.
(ii) Tubular Fluorescent Lights:
Fluorescent lights are long narrow tubes with fluorescent coating on their inner surface. With the excitation of the fluorescent powder by an electric discharge at low pressures, a bright white light is emitted. The wattage is low and the efficiency is high-50-75 lumens per watt .Their life extent is higher and can last for about 6000 hrs. They are preferred for business streets which require good colour-rendering properties.
(iii) Sodium Vapour Lamps:
These lamps work on the principle of electric discharge through sodium vapour. There are two types- low-pressure discharge and high-pressure discharge; the former emits monochromatic yellow light, with the absence of dazzle, while the latter gives white light.
The wattage of the low-pressure variety is 120 to 160, with an efficiency of 120 lumens per watt; the high-pressure variety has a wattage of 400, with an efficiency 100 lumens per watt.
(iv) High-Pressure Mercury-Vapour Lamps:
Sodium vapour lamps are popular for street lighting in India. Mercury vapour discharge lamps have a wattage of 400, efficiency of 23-60 lumens per watt, and a life of 7500 hours. They emit a brilliant greenish white colour.
Levels of Illumination:
The levels of illumination recommended by the I.S. Codes are given in Table 11.1.
Average illumination, Ea, is given by the formula –
lm = lamp lumens
Cu = Coefficient of utilisation
LMF = Lamp maintenance factor
S = Spacing between luminaries (m)
W = Width of roadway/street (m)
LMF takes into account decrease in efficiency with age; it is usually taken as 80% (or 0.8).
The coefficient of utilisation, Cu, is related to the ratio of width of area to mounting height of the luminaire.
The coefficient of utilisation can be determined from the graph shown in Fig. 11.7.
If the desired average illumination is specified, the spacing of luminaires may be obtained, other factors being known. Similarly, for a given spacing and other particulars, the average illumination that may be achieved may be got from Eq. 11.1.
The different types of arrangements are schematically represented in Fig. 11.8.
(v) A combination of (iii) and (ii) or (iv)
This is simple, but it can be used only for narrow roads/streets, as it leaves the other side dark.
The central arrangement is advantageous where possible, since more light reaches the carriageway. This goes well with a cut-off distribution. However, the sides are not well-lit. When luminaires are arranged on both sides, they can be staggered or opposite; these arrangements are popular on wide roads/streets. These the schematically shown in Fig. 11.8.
Typical installations of lights at a T-junction and at a rectangular intersection are shown in Figs. 11.9 and 11.10 respectively.
Illumination of Traffics Rotaries:
This requires special treatment and is covered by the IRC code of practice, “IRC: 65-1976: Recommended Practice for traffic Rotaries, Indian Roads Congress/New Delhi, 1976.” The central island is well lit by luminaires set on top of columns at its edge; further, lights are provided at the kerbs of the four arms.
Illumination on Curves:
The outside of the curves are provided with luminaires when staggered arrangement is used on the straights. Cut-off and semi-cut-off systems of vertical distribution are preferred to yield uniform standard of road brightness.
If the width of road is large in relation to the mounting height of the luminaires, the inside of the curve also may have to be provided with lights. The spacing of the luminaires is governed by the mounting height as well as by the radius of the curve, which may range from 20 to 50 m.
Illumination of Bridges:
Standard mounting of 9 m height cannot be provided on bridges. Generally, luminaires are mounted on parapets at a height of 3 to 5 m. Avoiding glare poses a problem, and usually cut-off arrangements help to overcome this.
This is advantageous in locations such as grade separations, car parks and toll plazas. The number of lighting poles can be minimal. The height of the system may range from 15 to 45 m.
Maintenance of Lighting Installations:
A high standard of maintenance is required to ensure safety. Cleaning and replacement of faulty luminaires should be carried out periodically, as also at times of need.