Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Road Lighting

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_142



Road lighting is the application of illumination systems along roadways, primarily for the purpose of improving safety by increasing visibility of roadside hazards and by reducing the effects of glare from other light sources in the visual environment, such as vehicle headlamps.


Road lighting systems are an important part of the highway safety infrastructure across the world. Two important purposes of road lighting are to allow drivers to see the roadway further ahead than their own vehicle’s headlamps allow and to reduce glare from other vehicles’ headlamps [1]. Indeed, road lighting is generally associated with reduced nighttime crash rates [2, 3], and the presence of illumination that results in increased luminances of potential road hazards can reduce the impact veiling luminances produced by bright sources of light in the driver’s field of view. In the present article, existing practices and standards for road lighting are described, as well as the light source and luminaire technologies used to provide road lighting. For further details on road lighting and interactions with other visual information systems, consult references [4, 5].

Practices and Standards

CIE 115

In most of the world, the primary road lighting standard is the Commission Internationale de l’Éclairage (CIE) Lighting of Roads for Motor and Pedestrian Traffic, CIE 115 [6]. In CIE 115, three main lighting classes are specified: for motorized traffic (M), for conflict areas between vehicles or between vehicles and pedestrians (C), and for pedestrian traffic (P). Each main class is subdivided into numerical classes based upon factors such as traffic volume and speed, road access control, ambient environment, and the mix of motorized and pedestrian traffic. Lower-numbered classes refer to more complex road situations requiring higher light levels.

Specifications for M classes M1 through M6 are given in average road surface luminance and range from 2 to 0.3 cd/m2. For C classes C0 through C5, illuminance specifications range from 50 to 7.5 lx. For P classes P1 through P6, illuminance specifications range from 15 to 2 lx. CIE 115 specifications also include limits for characteristics such as uniformity, glare, and, when facial recognition of pedestrians is important, vertical illuminance [6].

CIE 115 also provides a method for the specification of adaptive lighting to account for the possibility of reduced vehicle or pedestrian traffic at certain times throughout the night [6]. Light levels can be reduced to different classes, resulting in decreased energy use and light pollution.

Road lighting calculations are often performed using software that accepts a photometric file containing tabulated luminous intensity data for a luminaire. For a set of road geometric characteristics, pole locations and heights, the software calculates the illuminances or luminances, uniformities, and veiling luminances. Such calculations should account for lamp output reductions and dirt accumulation over time. The light output from a road lighting system could be 20–30 % lower after several years than when it was new and clean.


In North America, the primary standard for most road lighting is the Illuminating Engineering Society (IES) American National Standard Practice for Roadway Lighting, RP-8 [1]. RP-8 serves as the basis for continuous road lighting system by the American Association of State Highway and Transportation Officials [AASHTO [7]], which provides warrants for road lighting based on traffic volume and crash frequency at night. RP-8 includes three specification methods based on provision of illuminance, luminance, and small target visibility (STV), the latter of which is rarely used. Each method also includes requirements for uniformity of lighting and limits to control for glare in terms of veiling luminance. In future editions of RP-8, it is planned (as of June 2012) that only the luminance method will be recommended for most road lighting specifications.

The target light levels depend upon the road type (i.e., local, collector, major, or freeway) and the pedestrian conflict level. Specific illuminance criteria exist for different road surfaces, higher for darker than for lighter pavement. (The luminance method uses the pavement type as an input to the calculation.) For example, for a local road with asphalt pavement and a medium pedestrian conflict level, the average illuminance should be at least 7 lx, the minimum no less than 1/6 the average, and the veiling luminance should not exceed 40 % of the average road surface luminance [1]. For a major roadway with the same asphalt and pedestrian conflict level, the average illuminance should be at least 13 lx, the minimum should be no less than 1/3 the average, and the veiling luminance should not exceed 30 % of the average road surface luminance [1]. Calculations for RP-8 road lighting criteria are performed similarly as for CIE 115 criteria.

RP-8 criteria are for continuous lighting along roads excluding intersections and interchanges. Intersection lighting criteria are determined by adding the recommended levels for the intersecting roads.

Light Sources

Road lighting systems can use a wide variety of light sources, including discharge (high- and low-pressure sodium, metal halide, fluorescent, and induction lamps) and solid-state (light-emitting diodes) types [8]; several of these are illustrated in Fig. 1.
Road Lighting, Fig. 1

(a) High-pressure sodium (HPS) lamp (Courtesy of the Lighting Research Center). (b) Metal halide (MH) lamp (Courtesy of the Lighting Research Center). (c) Fluorescent lamp (Courtesy of the Lighting Research Center). (d) Two light-emitting diode (LED) packages (Courtesy of the Lighting Research Center)

High-Pressure Sodium

High-pressure sodium (HPS) lamps emit light when a current is applied to sodium vapor. The inner envelope of an HPS lamp contains sodium, and the outer glass bulb absorbs ultraviolet (UV) energy and stabilizes the arc tube temperature. HPS lamp efficacies are high, and they have long operating lives and very high lumen maintenance. HPS lamp illumination is yellowish in color. “Whiter” HPS lamps can be developed by increasing the sodium vapor pressure or operating HPS lamps at high frequencies. HPS lamps are the most common sources used in road lighting. For additional details about the performance of HPS lamps, consult the entry “ High- and Low-Pressure Sodium Lamp” in this Encyclopedia.

Metal Halide

Metal halide (MH) lamps have an arc tube containing various metallic halide compounds in addition to mercury, which emit light across the visible spectrum. MH lamps have relatively high efficacy and are available in a range of white colors ranging from “warm” to “cool” in appearance. Lamp life is typically not as high as HPS lamps but has been improving substantially in recent years. Lumen maintenance values range from fair to good. MH lamps with ceramic arc tubes and improved starting gear are available, which have much-improved life and lumen maintenance. For additional details about the performance of MH lamps, consult the entry “ Metal Halide Lamp” in this Encyclopedia.

Low-Pressure Sodium

Like HPS, low-pressure sodium (LPS) lamps use sodium vapor but at a much lower vapor pressure. LPS lamps are linear in shape. They produce nearly monochromatic yellow light near 589 nm. They have relatively long operating lives, excellent lumen maintenance, and very high luminous efficacy, although color rendering is essentially nonexistent with these lamps. LPS lamps are sometimes used near astronomical observatories because it is fairly easy to filter out the wavelengths emitted this source. LPS is more common in Europe than in North America for road lighting. For additional details about the performance of LPS lamps, consult the entry “ High- and Low-Pressure Sodium Lamp” in this Encyclopedia.

Fluorescent and Induction

Fluorescent lamps are low-pressure gas discharge sources. Light is produced by fluorescent phosphors activated by ultraviolet energy from a mercury arc. Fluorescent lamps have reasonably long operating lives, limited primarily by the life of the lamp end electrodes that generate the electrical discharge. Fluorescent lamps require ballasts to provide the starting and operating voltages and currents needed for proper lamp operation. Fluorescent lamp color of fluorescent lamps is determined by the phosphors used to coat the envelope. Fluorescent lamps have relatively higher luminous efficacies and lumen maintenance values.

A special fluorescent lamp type, the induction lamp, is being used increasingly in road lighting. Induction lamps have somewhat more compact shapes than tubular fluorescent lamps because they use a magnetic induction coil to provide the current that stimulates the mercury vapor inside the lamp. Typical operating lives are double or triple those of fluorescent lamps, with luminous efficacy comparable in value. For additional details about the performance of fluorescent and induction lamps, consult the entries “ Tubular and Compact Fluorescent Lamp” and “ Induction Lamp” in this Encyclopedia.

Light-Emitting Diodes

Light-emitting diodes (LEDs) are solid-state semiconductor junction devices that emit light when a current is passed through the junction. White light can be created by mixing light from red, green, and blue LEDs, or by using blue LEDs with phosphors that convert some of the blue light into yellow light, resulting in a mixture that is perceived as white. New LED packages exist in addition to 5-mm diameter epoxy capsule LEDs used for indicator lights. Some packages contain metal heat sinks to dissipate internal temperatures, which can reduce LED light output and shorten operating life. LED lighting technology is increasing in luminous efficacy and LEDs also have very long operating lives with gradual reductions in light output over time. For additional details about the performance of LEDs, consult the entry “ Light-Emitting Diode, LED” in this Encyclopedia.


Road lighting systems generally consist of pole-mounted luminaires, usually with individual photocell or timer control. The most common luminaire type used for road lighting is the so-called cobrahead luminaire (Fig. 2), named for its distinctive shape. Figure 3 shows typical road luminaire construction. Most luminaires are designed for discharge lamps such as HPS or MH [9], but there is increasing interest in induction and LED systems [10, 11].
Road Lighting, Fig. 2

Cobrahead luminaire for road lighting (Courtesy of the Lighting Research Center)

Road Lighting, Fig. 3

Components within a typical road lighting luminaire (Courtesy of the Lighting Research Center)

Luminaires for road lighting can be classified in several ways. A cutoff classification system [1], used by many North American road lighting specifiers, classifies luminaires according to their luminous intensity distributions in different angular regions (relative to nadir, defined as 0° directly below the luminaire) where light output could contribute to glare or light pollution. Table 1 summarizes the luminous intensity limits (in cd) for different cutoff classifications, relative to the light output (in lm) of the lamp inside the luminaire. The IES more recently adopted a system that simply uses the light output (in lm) emitted within various angular regions to serve as the basis for classification [12].
Road Lighting, Table 1

Light output limits for luminaire cutoff classifications [8], given in luminous intensity values (in cd) as a percentage of the luminaire’s lamp lumens (in lm)

Cutoff type

Light between 80° and 90°

Light above 90°

Full cutoff

100 cd per 1,000 lm



100 cd per 1,000 lm

25 cd per 1,000 lm


200 cd per 1,000 lm

50 cd per 1,000 lm


No limitation

No limitation

As mentioned previously, the most common control system for roadway lighting is a photocell mounted to an individual luminaire, which will switch the luminaire on and off at a specified ambient light level. Some systems will use centralized control via a single photocell or time clocks. Technological innovations are making centralized control systems attractive, because these systems can also monitor performance of individual luminaires and alert the system operator when a lamp or ballast failure has occurred or is close to occurring [13]. They can also be useful in controlling adaptive road lighting systems.

Color: Mesopic Vision

The color of the light source used for road lighting can influence visual perception of drivers and pedestrians. Although photometric specifications for road lighting are based on the photopic luminous efficiency function, representing the spectral sensitivity of cone visual photoreceptors to light at daytime light levels, visual spectral sensitivity differs at low levels commonly experienced under road lighting at night. At very low light levels, rod photoreceptors, whose sensitivity is represented by the scotopic luminous efficiency function, dominate vision. At many road lighting levels, both rods and cones contribute to vision and luminous efficiency can be approximated by a combination of photopic and scotopic efficiency [14].

As a consequence, light sources with relatively greater power in the short-wavelength portion of the visible spectrum can be more effective for vision at these so-called mesopic, nighttime light levels. Lamp spectra can be characterized by their scotopic/photopic (S/P) ratio [15], which indicates a light source’s ability to stimulate the rods for an equivalent amount of cone stimulation. Table 2 lists S/P ratios for several road lighting sources. The CIE has developed a unified system of photometry [15] bridging the photopic and scotopic systems, as part of CIE Publication 191 [15], to quantify mesopic luminance. Above a luminance of 5 cd/m2, the unified luminance and the photopic luminance are equivalent, and below a luminance of 0.005 cd/m2, the unified luminance and the scotopic luminance are equivalent. The CIE 191 system uses the photopic luminance and the S/P ratio of the light source under investigation to estimate the unified luminance.
Road Lighting, Table 2

Scotopic/photopic (S/P) ratios of common light sources [14, 15]

Light source

S/P ratio

HPS 250 W clear


MH 400 W clear




Fluorescent/induction (“cool white”)


LED (4,300 K)




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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Lighting Research CenterRensselaer Polytechnic InstituteTroyUSA