Highway(Road) Lighting: Design factors, Benefits, Cost and Spacing

What is highway lighting?

The highway or street lighting is a means of improving the urban environment through increased comfort, convenience, and safety of night traffic operation. The lighting is frequently thought of as a safety measure. However, it is clear that lighting in itself cannot produce safety. It rather increases the efficiency of traffic operation and traffic safety is the by-product of efficiency.  

The public lighting should permit the traffic operations at night with the greatest possible safety, comfort and convenience. The driver must be able to see distinctly and locate with certainty all significant details of the driving environment.

In a similar way, the pedestrian must also be able to see distinctly his path and his relationship to vehicles and possible obstacles. Thus, the public lighting must satisfy the informational needs of both the drivers and pedestrians. But in actual practice, the driver’s requirements are considered more stringent and important.

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The main purpose of adequate highway lighting is to reduce the uncertainty of information. As long as the uncertainty exists, the possible alternative decisions cannot be fully evaluated.

The visual information requirements of the driver are a direct function of what the driver does in performance of the driving task. The requirements of fixed lighting are based upon how the driver sees and what he requires in terms of information for satisfactory accomplishment of the driving task.

DESIGN FACTORS OF HIGHWAY LIGHTING

Following are the eight important factors which are to be considered in the design of highway lighting:

•   Contrast

• Glare

• Lamps

• Lateral placement of lighting poles

• Lighting layouts

• Luminaire distribution of light

• Mounting height and overhang

• Spacing of lighting units.

(1) Contrast:

The contrast is one of the most important contributing factor for the visual performance during night driving. To a great extent, the recognition of objects is based on the discrimination of differences of brightness between an object and its background.

An obstacle may appear as a dark area against a bright background. It is known as discernment by silhouette. To enhance discernment by silhouette, the brightness of the pavement and uniformity of brightness along and transverse to the roadway are essential.

An obstacle may appear as a bright area against a dark background. It is known as discernment by reverse silhouette. It usually applies to the visibility of objects on the areas adjacent to the roadway, projections above the pavement surface such as channelizing islands and the upper portions of pedestrians or vehicles.

(2) Glare:    

 The disturbing influence when viewing a difficult visual task under low brightness conditions is known as glare. Following two types of glare have a critical influence on the visual performance of a driver:

• Disability glare or physiological glare
•  Discomfort glare or psychological glare.

(i) Disability glare or physiological glare:   

This glare occurs from stray light which reduces contrast sensitivity and thus producing a loss of visual efficiency. It can be reduced by increasing mounting height, moving the luminaire from the line of sight and increasing background brightness.

(ii) Discomfort glare or psychological glare:  

It occurs due to ocular discomfort from a bright light source. It can be minimized by reducing luminaire brightness, by increasing mounting height and by increasing the background brightness in the observer’s field of view.  

The reduction in luminaire brightness may be achieved by increasing the effective luminaire area and by decreasing the intensity of light at angles higher than that required for optimum pavement brightness.

(3) Lamps:     

The lamps commonly used for highway lighting are tungsten filament lamps, sodium vapour lamps, mercury vapour lamps and fluorescent lamps.

The filament lamps are widely used as their initial cost is low. The sodium vapour lamps give yellow orange colour and they are used at hazardous locations like bridges, rail-road crossings, sharp curves, etc. These lamps are costly. But they have a long life. The mercury vapour lamps produce better illumination and visibility. The fluorescent lamps provide good visibility and are the least dazzling to drivers.

(4) Lateral placement of lighting poles:  

The lighting poles should not be installed very close to the pavement edge because in that case, the capacity of the roadway is decreased and the free movement of traffic is obstructed. The horizontal clearance required for the lighting poles has been specified by the I.R.C. as follows:

• For roads with raised kerbs (as in urban roads):  

Minimum 0,30 m and desirable 0.60 m from the edge of the raised kerb.

• For roads without raised kerb (as in rural roads):

Minimum 1.50 m from the edge of the carriageway subject to a minimum of 5 m from the centre-line of the carriageway

The above clearance also applies to the poles carrying electric power and telecommunication lines.

(5) Lighting layouts:

The highway light can be arranged in different ways as shown in fig. 2 to fig. 5.

Fig. 2 shows the arrangement of highway lights in which they are placed just opposite to each other on either side of the road. In this arrangement, the road is well defined. But it can be adopted for wide roads only.

Fig. 3 shows the arrangement of highway lights in a staggered manner on either side of the road. Such an arrangement may however mislead the drivers on curves.

Fig. 4. shows the provision of highway lights only on any one side of the road. This arrangement is adopted for narrow streets.

Fig. 5. shows the arrangement of highway lights on the center-line of the road. In this case, a narrow central strip or median divides the roadway longitudinally into two portions. This arrangement provides a very effective lighting system at less cost because of the saving in luminaire supports and electrical conductors, It gives the impression of a continuous well-lighted road without any obstruction. However, the defect with this arrangement is that the footpaths and cycle paths remain comparatively dark.

(6) Luminaire distribution of light:

For making the maximum use of luminaire or source of light, it is desirable to have proper distribution of light. It should be aimed to achieve the following purposes:

• It should cover the pavement between the kerbs.
• It should illuminate the pavement and the adjacent or surrounding area to the extent of about 3 m to 5 m beyond the pavement edge.
• It should make prominent the traffic signs and other objects on the road.
•  It should produce maximum uniformity of pavement brightness.

(7) Mounting height and overhang:

The mounting height plays a great role in deciding the distribution of light, shadow and glare effect from the street lamps. The usual mounting heights range from 6 m to 10 m. The higher value is preferred for important urban roads.

The overhangs on the lighting poles would keep the poles away from the pavement edges and at the same time, the lamp will be held above the kerb or towards the pavement. The one benefit of overhang is it gives better distribution of light on the pavement with less glare on eyes of road users.

A special roadway lighting technique referred to as high-mast lighting has been utilized with mounting heights of 30 m to 52.50 m.

It is used to light mainly areas such as interchanges, intersections, tall plazas, parks, etc. In high-mast lighting, the floodlight type units are arranged in combinations to distribute light over a large area.

The justifications for high-mast lighting pertain mainly to the removal of luminaire supports near the traffic area for safety reasons and to the provision of a complete view of an area similar to the daylight conditions.

(8) Spacing of lighting units:

The spacing between lamps on straight roads is about 3 to 5 times the mounting height. For sharp curves, the spacing between lamps is closer for better visibility and comfort conditions.

Formula for spacing between lighting unit:

S = (lamp lumen x coefficient of utilization x Maintenance factor) / (Average Lux X width)

• The coefficient of utilization is obtained from the appropriate chart as in fig below
• The maintenance factor is taken into account for the decrease in coefficient of lamp with age and is 80 % assumed.

Where, S= spacing between lighting Units

The spacing between lamps can be increased substantially by adopting higher mounting heights with powerful lamps. The spacing of lighting units is also sometimes influenced by various factors such as electrical distribution poles, property lines, road layout, etc.

BENEFITS OF HIGHWAY LIGHTING

A well-designed highway lighting system grants various benefits and they can be mentioned as follows:

• Appreciation by police forces:

The police forces have praised and actively pushed for improved lighting to make their own jobs easier and more manageable.

• Increase in business:

The city or downtown areas which otherwise would almost have been totally deserted after dark have increased their aesthetic appeal and business activity after the addition of the improved lighting on the streets.

• Planning:

The architects and town planners have found light to be a major source of economic stimulation and beautification for their projects.

• Recreation centres:

It is found that many cities and towns have once again opened their parks and recreation areas after dark because of the effects lighting has on the crime rate, business and personal security,

• Reduction in night-time accident rate:

 It has been shown by many studies and observations that there is a considerable reduction in accident rate during night because of proper lighting in urban, suburban and rural areas regardless of the class of highway. It is also observed that the proper application of modern lighting techniques undoubtedly results in a noticeable and significant reduction in the number of fatal and non-fatal accidents as well as a reduction in the severity of those accidents that do occur despite improved lighting techniques.

• Reduction in serious crimes:

The illegal actions and anti social activities on the roads are certainly discouraged by the improved lighting techniques.

COST CONSIDERATIONS OF HIGHWAY LIGHTING

The design and specification of a highway lighting system should be such that maximum returns are achieved at the lowest possible costs. For this purpose, it is customary to ascertain several designs which give a predetermined level of effectiveness and then to compare these designs as to cost of each. The design giving the maximum benefits with the least cost is then recommended.

The variables which influence both effectiveness and cost can be grouped in the following three categories:

(1) Design variables

(2) Roadway variables

(3) Miscellaneous variables.

(1) Design variables:

Following are the design variables which are usually considered:

•  arrangement or placing of lighting units on the roadway:

• burning hours per year;

• characteristics of lamps;

• light distribution system;

• mounting heights;

• type of lighting poles;

• type of luminaire;

• type of ownership and maintenance; etc.

(2) Roadway variables:

Following items are included among the roadway variables which are influential:

(i) number and type of intersections;

(ii) number of traffic lanes;

(iii) overall cross-section of roadway:

(iv) presence or absence of a median barrier and its type:

(v) width of median or central narrow strip

(vi) width of roadway shoulders; etc.

(3) Miscellaneous variables:

Among the other miscellaneous variables of importance are the amount and type of traffic, the type of area through which the roadway passes, and the weather conditions in the locality.

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