Chapter 6 – Roadway Operational
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
- 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 (www.iesna.org).
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
- 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
- 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.
- 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
- 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
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
- 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
- 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
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
- 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
- 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
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.
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,
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 https://www.fhwa.dot.gov/legsregs/directives/techadvs/t504035.htm)