Photos of cars on freeway, speeding sign

Freeway Management and Operations Handbook

Chapter 6 Roadway Operational Improvements
Page 2 of 2

6.4 Roadway Lighting

The modern freeway provides an alignment and profile that, together with other factors, encourages high operating speeds. Although improved design has produced significant benefits, it has also created potential problems. For example, driving at night at high speeds may lead to reduced forward vision that is, the inability of headlights to illuminate objects in the driver's path in sufficient time for some drivers to respond (4).

The addition or enhancement of roadway lighting can improve seeing conditions and visibility at night, thereby improving throughput (higher night speeds are possible) and safety. The objectives of roadway lighting are as follows:

  • Promotion of safety at night by providing quick, accurate, and comfortable seeing for drivers and pedestrians.
  • Improvement of traffic flow at night by providing light, beyond that provided by vehicle lights, which aids drivers in orienting themselves, delineating roadway geometrics and obstructions, and judging opportunities for overtaking.
  • Illumination in long underpasses and tunnels during the day to permit drivers entering such structures from the open to have adequate visibility for safe vehicle operation.
  • Reduction of crime after dark (While not an issue, per se, on a freeway itself; it is an important consideration in the design of rest areas and crash investigation sites along the freeway).

In 1989, a task force of the Illuminating Engineering Society of North America, reported on a study made to determine the benefits of roadway lighting. Essentially, this report concluded: adequate lighting that is properly designed, installed, and maintained can usually significantly reduce night accidents. AASHTO's "Informational Guide for Roadway Lighting" (4) states, "many researchers have shown roadway lighting to reduce accident occurrence and incidences of crime and vandalism." As reported in Reference 2, Caltrans has evaluated many of its safety projects to determine what has been effective. On average, lighting resulted in a 15% reduction in night accidents.

Industry development and general experience on lighting of roadways has resulted in a reasonably well-developed technique for the design of lighting systems. For a given condition to be lighted (e.g., width of roadway / interchange area) and a known level of illuminance or luminance to be provided, there are accepted methods permitting ready analysis of different alternates in lamps, luminaires, mounting height, luminaire spacing and positioning, energy consumption, etc. to determine a preferred design (4). Guidance and criteria for designing lighting systems are provided in References 4 and 5, as well as other documents published by the Illuminating Engineering society of North America ( Accordingly, the focus of this section of this Handbook is to provide an overview of definitions, criteria, and the potential issues associated with roadway lighting basic information of which the freeway manager and operations practitioner should be cognizant.

6.4.1 Background and Definitions

The purpose of roadway lighting is to attain a level of visibility that enables the motorist (and pedestrian) to see quickly, distinctly, and with certainty all significant detail, notably the alignment of the road and any obstacles on or about to enter the roadway. Most aspects of traffic safety as related to roadway lighting involve visibility. The basic factors that influence visibility include: size, shape, and texture (i.e., identifying detail) of an object; general brightness of the roadway background; contrast between an object and its surroundings, and the contrast between pavement and its surroundings as seen by the observer; time available for seeing; vision capability of the observer / driver (and is often affected by age); the condition of the windshield; and glare.

With respect to the last item, glare may be defined as any light, either direct or indirect, which reduces the ability to see or produces a sensation of ocular discomfort. Many roadway lighting factors can affect glare, including the size of the light source, displacement angle of the source from the line of sight, illuminance at the eye, adaptation level, and exposure time and motion.

General definitions of several technical terms used in roadway lighting design are provided below:

  • Luminous Flux the rate of emission of luminous energy from a light source measured in all directions. The unit of measurement is the lumen.
  • Illuminance the density of luminous flux incident on a surface. The unit of illuminance is lux.
  • Luminance (Photometric Brightness) the luminous intensity of any surface in a given direction per unit of projected area of the surface as viewed from that direction. The term brightness usually refers to the intensity of sensation resulting from viewing surfaces or spaces from which light comes to the eye.
  • Luminaire a complete lighting unit consisting of a light source together with the parts (reflector and/or refractor) used to distribute the light, a socket to support and position the light source, amps, wiring terminals, and a housing.
  • Lighting Standard the complete assembly of a lighting standard (i.e., pole), bracket(s) or mast arm(s), and luminaire(s).
  • Light Loss Factor A depreciation factor which is applied to the initial average luminance or illuminance to determine the value of depreciated average luminance or illuminance at a predetermined time in the operating cycle (e.g., just prior to relamping). It reflects the decrease in effective light output of a lamp and luminaire over its life. It is made up of several factors, including decrease of lamp lumen output with burning hours, frequency and effectiveness of luminaire cleaning, specific equipment being used, and operation of the light source at other than rated current or voltage.
  • Uniformity of Illuminance the ratio of average illuminance on the pavement area to the minimum illuminance on the pavement. It is common called the uniformity ratio.
  • Reflectance the ratio of the light flux reflected by a surface to the incident flux. In the case of roadways it is affected by the surface characteristics and the viewing angle.

6.4.2 Lighting Criteria and Warrants

AASHTO (4) identifies several warranting conditions for the purpose of establishing a basis on which lighting for freeway sections may be justified. The warrants provide "minimum conditions which are to be met whenever an agency is contemplating lighting for new or existing projects. Meeting of the warrants does not obligate the highway agency to provide lighting." Moreover, the warrants are not to be construed as the only criteria for justifying lighting. Local conditions, such as frequent fog, ice, snow, roadway geometry, ambient lighting, sight distance, signing, etc. could justify modification of these warrants in either a positive or negative way.

Continuous Freeway Lighting Continuous freeway lighting is considered to be warranted for the following cases:

  • Sections in or near cities where the current ADT is 30,000 or more
  • Sections where three or more interchanges are located with an average spacing of 1 ½ mile or less, and adjacent areas outside the right-of-way are substantially urban in character
  • Sections with a length of two or more miles that pass through a substantially developed suburban or urban area in which one or more of the following conditions exist: (a) local traffic operates on a complete street grid having some form of street lighting, parts of which are visible from the freeway; (b) the freeway passes through a series of developments (e.g., residential, commercial, industrial, colleges, parks, terminals) which include roads and/or parking areas that are lighted; (c) separate cross streets, both with and without connecting ramps, occur with an average spacing of ½ mile or less, some of which are lighted; and (d) the freeway cross section elements are substantially reduced in width below desirable sections used in relatively open country.

Complete Interchange Lighting Complete interchange lighting is defined as the lighting of the freeway through traffic lanes through the interchange, the traffic lanes at all ramps, the acceleration and deceleration lanes, all ramp terminals, and the crossroad between the outermost ramp terminals (4). Such lighting is considered to be warranted for the following cases:

  • Total current ADT ramp traffic entering and leaving the freeway within the interchange area exceeds 10,000 for urban conditions, 8,000 for suburban conditions, and 5,000 for rural conditions.
  • Current ADT on the crossroads exceeds 10,000 for urban conditions, 8,000 for suburban conditions, and 5,000 for rural conditions.
  • On unlighted freeways where existing substantial commercial or industrial development is located in the immediate vicinity of the interchange, and which is lighted during hours of darkness; or where the crossroad approach legs are lighted for ½ mile or more on each side of the interchange.
  • Where the ratio of night to day accident rate within the interchange area is at least 1.5 or higher than the statewide average for all unlighted similar sections, and a study indicates that lighting may be expected to result in a significant reduction in night accident rate.
  • Wherever there is continuous freeway lighting

Underpasses An underpass is a portion of a roadway extending through or beneath some natural or man-made structure. Supplementary lighting might be required during the daytime as well as at night. Guidance provided by AASHTO (4) is summarized below:

  • Length to height ratios of 10:1 or lower will not, under normal conditions, require underpass lighting during the daytime. When this ratio is exceeded, it is necessary to analyze the specific geometry and roadway conditions, including penetration of daylight on the roadway, to determine the need for daytime lighting. The transition from bright daylight to tunnel lighting, and back again to daylight must also be considered.
  • Underpasses that are part of a freeway section with continuous lighting warrant the use of (nighttime) illumination; with the lighting levels and uniformities duplicating, to the extent practical, the lighting values of the adjacent roadways. If continuous lighting is not provided along the adjacent freeway sections, underpass lighting may still be warranted for nighttime conditions where unusual or critical roadway geometry occurs under or adjacent to the underpass area.

Special Situations

  • Long tunnels (Note: Reference 4 defines a tunnel as "long" if its portal-to-portal length is greater than the wet pavement minimum stopping distance) require the use of artificial lighting or equivalent means to provide adequate roadway and tunnel user visibility necessary for safe and efficient traffic operation.
  • Any Rest Area offering complete rest facilities (including comfort station and picnic facilities) should be lighted, including the entrance and exit, the interior roadways, parking areas, and activity areas.
  • Lighting of other specialized areas should be considered with respect to the needs of the motorist as well as the requirements of others interacting with the motorist. These other specialized areas include truck weighing stations, inspections and enforcement areas, park-and-ride lots, toll plazas, and escape ramps.

6.4.3 Roadway Lighting Design

For many years, roadway lighting was designed on the basis of pavement illuminance the amount of light or luminous flux falling on the pavement surface. In 1983, the Illuminating Society of North America introduced a new design concept, that of luminance. Pavement luminance is more realistic in that it considers the luminous flux reflected per unit of pavement surface in the direction of a standardized observer. The current American National Standard Practice for Roadway Lighting (RP-8-00: Reference 5) allows the use of both the illuminance and luminance design methods, and also introduces the new concept of small target visibility (STV). STV permits an even more realistic consideration of the driving task, because it is based on the calculation of the visibility of a field of 180 x 180 mm targets located on an area of the pavement. It even considers the contrast between the target and the immediate background, the transient adaptation characteristics of the eye, and the visual capability of the driver.

The basic goal of roadway lighting design is to provide the appropriate levels and uniformity of luminance (or illuminance) of the pavement and of objects on or near the pavement. These depend on several items, including the following:

  • Lamps Various types of lamps may be used for roadway lighting, each with varying characteristics (e.g., initial light output as measured in lumens, light loss factor, color rendition, and lamp life). Lamp alternatives include fluorescent (used only for tunnel and sign lighting), mercury (blue-white color), and high-pressure sodium (golden-white color output) has been replacing the mercury lamp.
  • Luminaire Distribution Pattern The luminaire includes a reflector and usually a glass or plastic lens or refractor. The function of the reflector and refractor is to gather the light from the source, direct it toward the roadway, and shape it into a desired pattern on the roadway. Proper distribution of the light flux fro the luminaire is an essential factor in good roadway lighting
  • Pavement Classification The texture and color of each type of pavement determines its reflectance, which affects the luminance produced by a given level of lighting.
  • Mounting Height The height of luminaires above the roadway surface varies from 5 m to more than 50 m. The lower mounting heights are used for tunnel and underpass lighting, and roadways located near or crossing aircraft approach zones. Conventional roadway lighting utilizes mounting heights of 9 to 15 m. The highest mounting heights involve groups of luminaires mounted on free standing poles or towers at 25 m (80 feet) to 55 m (180 feet) or more. This high mast lighting is used for area lighting such as freeway interchanges, roadways with wide cross sections, toll plazas, rest areas, and other complex road areas.
  • Luminaire Spacing It is usually in the interest of both good lighting and good economics to use larger lamps at reasonable spacing rather than smaller lamps at closer spacing and lower mounting heights. On wide roadways, pairs of luminaires opposite each other, mounted outside the roadway or in the median, may be required. The physical roadside conditions (e.g., sign structures, overpasses, guardrail, curvature, gore clearances) may restrict the placement and spacing of lighting poles. In general, sharp curves and steep grades require closer luminaire spacing in order to provide uniform pavement brightness. Specific location decisions must also consider access to luminaires for maintenance, visibility of traffic control devices, potential distracting shadows cast by overhead signs, and aesthetics (i.e., a pleasant daytime appearance). Safety must also be considered in determining lighting pole locations. It is desirable to place poles outside the roadside clear zone whenever practical; and if not, they should be designed with a breakaway feature.
  • Uniformity Ratios It is noted that the same average level of luminance (or illuminance) can be obtained by several different arrangements of these variables for example, a few high-output light sources mounted relatively high, or a greater number of low-output sources mounted relatively low. A major factor of concern in comparing such alternative arrangements is the uniformity of luminance (or illuminance) over the traveled way to be lighted.

Other lighting design issues include cost, power consumption, and maintenance requirements (e.g., lamp replacement, access, electrical apparatus, skill levels). Moreover, the luminance (or illuminance) values should be regularly monitored and the lighting effectiveness evaluated as part of the on-going performance monitoring effort.

6.5 Traffic Demand Management (TDM) Considerations

Based on principles adopted through the regional planning process, the goal of traffic operations functions may include redirecting traffic demand (from the freeway) through Traffic Demand Management (TDM) methods. Several TDM actions are an integral part of freeway management and operations (e.g., managed lanes), and are discussed in subsequent chapters. Other TDM actions go well beyond the scope of this Handbook. Nevertheless, freeway management practitioners should be acquainted with the entire TDM spectrum.

In its broadest sense, demand management is any action or set of actions intended to influence the intensity, timing and spatial distribution of transportation demand for the purpose of reducing the impact of traffic or enhancing mobility options. Such actions can include offering commuters one or more alternative transportation modes and/or services, providing incentives to travel on these modes or at non-congested hours, providing opportunities to better link or "chain" trips together, and/or incorporating growth management or traffic impact policies into local development decisions (6).

The traditional perspective of TDM was characterized as getting commuters away from driving alone and into carpools, vanpools, public transit, bicycles, walking, etc. This primary mission was supported through the provision of incentives and support services to enable this transition to occur. These support services included: guaranteed ride home, alternative / flexible work hours, preferential parking, and/or transit and vanpool benefit programs. A related mission was to eliminate some commute trips entirely through the application of telecommuting, either at home or at a satellite center located near home, or implementing compressed week programs.

As discussed by Berman (7), a new, more contemporary model of TDM is emerging and needs to be recognized. TDM should now be viewed as the policies, programs and actions implemented to:

  • Increase the use of commute alternatives
  • Spread the timing of travel to less congested periods
  • Reduce the need to travel, and/or
  • Shift the routing of vehicles including trucks and single occupant vehicles to less-congested facilities or systems

This definition addresses mode choice, time choice, location choice, and route choice.

"Managing travel demand today is about providing travelers, regardless of whether they drive alone, with choices of location, route, and time, not just mode" (7).

This broader definition of TDM encompasses three key trends or enablers:

  • Information in an accessible and timely format, including construction updates, incidents, emergencies, weather, real-time conditions on all transportation modes, real-time transit schedules, and transit-carpool availability
  • Technologies that support the dissemination of this information, including navigation devices, Internet, GPS, and wireless communications.
  • Financial Incentives such as tax incentives and credits, direct subsidies, cost sharing, and variable pricing.

These elements help enable the various aspects of TDM, including mode choice (how to travel) via preferential parking, transit / vanpool benefits, guaranteed ride home, traveler Information, and electronic road and parking pricing strategies and systems; time choice (when and how fast) via flexible schedules, compressed work weeks, express bus service, HOV lanes and high occupancy toll lanes, and road pricing; location choice (where and whether to travel) as supported by telecommuting; and route choice (which way to travel).

In the broadest sense, transportation demand management (TDM) is any action or set of actions intended to influence the intensity, timing, and spatial distribution of transportation demand for the purpose of reducing the impact of traffic or enhancing mobility options (1). A variety of government- and employer-sponsored programs can be designed to reduce vehicle trips during congested periods and in congested locations. These include flexible work schedules that allow employees to travel off-peak (or work at home), amenities to improve the safety and efficiency of biking and walking, ridematching services for vanpools and carpools, community-based carsharing, employer-subsidized transit passes, guaranteed emergency rides home for transit users, and incentives to decrease employer-paid parking.

6.6 References

1. Manual on Uniform Traffic Control Devices – Millennium Edition, FHWA, 2000

2. Homburger, W.S., Hall, J.W., Reilly, W.R. and Sullivan, E.C. Fundamentals of Traffic Engineering – 15th Edition. University of California, Berkley (UCB-ITS-CN-01-1), January 2001.

3. A Policy on Geometric Design of Highways and Streets, American Association of State Highway and Transportation Officials, Washington, D.C. 2001

4. An Information Guide for Roadway Lighting, American Association of State Highway and Transportation Officials, Washington, D.C. 1984

5. American National Standard Practice for Roadway Lighting, ANSI/IES RP-8-00, Illuminating Engineering Society of North America, approved August 8, 1999

6. Meyer, M.D. A Toolbox for Alleviating Traffic Congestion and Enhancing Mobility. Institute of Transportation Engineers, Washington D.C. 1997

7. Berman, Wayne; "Travel Demand Management: Thoughts on the New Role for TDM as a Management and Operations Strategy", ITE Journal, September 2002

8. "Roadway Shoulder Rumble Strips"; FHWA Technical Advisory T 5040.35; December 20, 2001 (Available at