Street Lighting and Visibility for Road Users

Good lighting values provide visual comfort and visual performance for motorists

Abstract

Visual comfort is a subjective impression related to road lighting quality. If the degree of visual comfort is not good enough, a motorist’s level of fatigue will increase and as a result, his visual performance and alertness will decrease. So both visual comfort and visual performance are important for road safety.

Introduction

Darkness brings increased hazard to motorists on roads, because visual performance deteriorates and the distance they can see become reduced. The night time accident rate on unlighted roadways is about three times the daytime rate. This ratio can be reduced if fixed lighting is installed and good lighting values are achieved. Fixed lighting on the road reveals the environment beyond the range of the vehicle headlights and alleviates glare from oncoming vehicles by increasing the eye’s adaptation level. The basic purpose of fixed lighting for motorised traffic is to enhance the motorist’s visual performance, visual comfort and alertness.

The paper will first discuss the average road-surface luminance. Then the uniformities of light on the road surface and finally how glare affect visual performance.

Average road surface luminance

Average luminance (Lave) is the average brightness of the road surface as seen by a driver, and it is measured in candela per square meter (cd/m2).The surface is made visible by virtue of light being reflected from it and entering the eye of the motorist and the greater the amount of light entering the eye, the stronger will be the visual sensation experienced. The brightness of the road surface will depend on the amount of lighting reflected from it in the direction of the motorist. The photometric measure for this is called the luminance of the surface, and it is the luminance that determines the brightness.

The changes in reflection properties of the road surface will influence the changes in the surface luminance pattern and differences in brightness will be experienced. Since brightness is determined by luminance, the visual performance and the visual comfort of the motorist are directly influenced by the patterns of luminance existing in his view of the road ahead.

Wet surface and luminance

During the rainy weather, the influence that a wet road has on the visibility is a direct result of the changed nature of that surface’s reflection properties. Light coming from the fixed lighting installation and reflected specularly by the wet road toward a motorist will produce very bright patches on its surface alternating with large dark patches as indicated in figure 1.

Figure 1 shows the wet road producing very bright patches on its surface alternating with large dark patches

The bright patches can have luminance values of more than ten times the values at the same area during dry conditions. The result is that the average luminance of the surface increases while the overall and longitudinal luminance uniformities decreases. While the increase in average road-surface luminance has a positive effect on the adaptation state of the motorist’s eyes, the decrease in uniformity has a much larger negative effect on overall visual performance.

Eye adaptation

Visual performance depends on the state of adaptation that is the more light is available, the faster unimpaired visual performance can be achieved. It takes time for the eye to adapt to different brightness. The adaptation time depends on the luminance at the beginning and end of any change in brightness, meaning that adapting from dark to bright takes only seconds and adapting from bright to dark can take several minutes. Where road lighting ends or drop to a lower light level, the decrease in luminance should be gradual. The transition zone makes it easier for the eye adapt to the darker conditions.

Luminance contrast

Contrast is the difference of the luminance of the object and its direct surroundings. Objects or obstacles on the road surface are seen because of their contrast against the background. Obstacles will be seen as being lighter than their background if the level of obstacle luminance is higher than the level of background or the road luminance and contrast is positive. If, on the contrary, the obstacle is dark and the level of obstacle luminance is lower than the luminance of the background or the road, the contrast will be negative.

In order to see an obstacle on the road, contrast must either be positive of negative. The luminance contrast needed for an obstacle to be seen, depends amongst other things, on the luminance surrounding the obstacle, as it is these that determine the adaptation condition of the motorist’s eye.

Uniformities

Uniformity is a description of the smoothness of the lighting pattern or the degree of the intensity of bright and dark areas on the road. Uniformity is expressed as a ratio minimum to average for overall uniformity and minimum to maximum for the longitudinal uniformity. The lower the ratio the more uniform the road lighting design.

Longitudinal uniformity:

In design, the longitudinal uniformity (UL) is expressed as the ratio of the minimum to maximum luminance along the centre line of a lane within the calculation area. It is a measure to reduce the intensity of bright and dark banding on road lit surface.

Overall uniformity:

In design, the overall uniformity (UO) is expressed as a ratio of the minimum to the average luminance on the road surface of the carriageway within the calculation area. It is a measure of how evenly lit the road surface is.

Figure 2 &3 Show the differences between longitudinal and overall uniformity

An important comfort aspect of road lighting is the lengthwise uniformity of the luminance pattern of the road in front of a motorist. A continuously – alternating sequence of bright and dark strips (known as “Zebra Effect”) on the road disappearing under the vehicle while driving may or may not have a negative effect on visual performance, but is always experienced as being uncomfortable if the brightness difference between bright and dark areas is too great (see figure 3 above). The “zebra effect” of a poorly-designed fixed road lighting installation is due to the lighting columns being spaced too far apart.

Figure 2, shows an installation with low value of overall uniformity Uo, which is the ratio of minimum luminance Lmin, to the average luminance, Lave. From the visual performance point of view this is really-bad road lighting because objects on the left lane of the road will be invisible. However, from the visual comfort point of view the installation is not so bad. This is because the lengthwise, or the longitudinal uniformity UL is very good.

The installation shown on figure 3, is slightly better from a visual performance point of view because the ratio of minimum luminance to average luminance (Uo) is somewhat better. However, this installation is very bad from a visual comfort point of view. The problem is the sequence of bright and dark areas on the road ahead of motorist, viz. the so called “zebra effect”. The longitudinal uniformity UL, defined as the ratio of minimum to maximum luminance on the line parallel to the road axis, is very poor.

Glare and Threshold Increment (TI)

Glare is the condition of vision in which there is discomfort or a reduction in the ability to see significant objects. Glare affects human vision and it is subdivided into two components, disability glare and discomfort glare.

Disability glare is the glare that results in reduced visual performance and visibility.

Discomfort glare is the glare producing discomfort. It does not necessarily interfere with visual performance or visibility.

Glare in a fixed lighting installation is a function of luminaire design and road surface reflectivity. A luminaire with well controlled light output and a road with good reflectivity will provide the least amount of glare.

Threshold increment (TI) is used for the restriction of glare for motorized traffic. Threshold increment is a measure of the loss of contrast a motorist suffers because of light shining directly from the luminaire to the motorist’s eye. The effect is referred to as disability glare. In lighting design, TI is the percentage increase in the luminance level required to make an object equally visible in the absence of glare.

Lighting requirements

Table 1 (from SANS 10098-1), stipulates recommended lighting values for fast motorized traffic according to the lighting categories, type of road, volume of traffic and the cross-section of road. For the visual comfort and visual performance to be experienced, the lighting design and fixed lighting installation must conform to the lighting values specified in table 1 for the specific type of road, lighting category etc.

In the lighting design simulation, the results for the average luminance (Ln), overall uniformity (Uo) and longitudinal uniformity (UL) must not be below the minimum recommended lighting values, and threshold increment (TI) must not be greater than the recommended percentage value.

Table 1 - Recommended lighting values for group A roads

Table 1 shows the recommended lighting values for fast motorized traffic

Lighting level examples

The lighting values below are representative for “good” road lighting, because all lighting levels comply with the required values according to class A2 as per table 1. With the lighting values achieved below, both motorist visual performance and visual comfort will improve.

The lighting values below are representative for “bad” overall uniformity and too much glare (TI). The overall uniformity (Uo) is far less than required value of 0.4 and TI is more than 20%.

The motorist’s visual performance will decrease as a result of poor fixed road lighting.

Conclusion

Fixed road lighting installation with good lighting values at night time improves visual performance, visual comfort, and reduces motor vehicle accidents.

When the average luminance is increased and the contrast is large between objects and the surroundings, motorists will recognise other road users, obstacles on the road and hazards from the side of the road. Visual performance depends on the state of adaptation; the more luminance, the faster unimpaired visual performance can be achieved. The average luminance with good uniformities and limited glare will enhance motorists’ visual comfort and visual performance.

References

1. LICHT, WISSEN 03: Roads, Paths and Squars. Fördergemeinschaf. Gutes Licht
2. SANS 10098 – 1: Public Lighting Part 1: The Lighting of public thoroughfares. The council of the South African Bureau of Standards.    
3. VAN BOMMEL, W. 2015. Road Lighting: Fundamentals, Technology and Application. Springer International Publishing, Switzerland.

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