U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
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As a tool for improving visibility, there are several effects agencies should consider when evaluating their policies related to lighting. Two principal concerns are glare and uniformity.
Glare is a haze within the eye that reduces visibility. When light is scattered in the eye, the phenomenon is referred to as veiling luminance. Veiling luminance can occur with bright oncoming headlamps, significantly reducing driver vision and leading to discomfort. Glare is either classified as discomfort glare or disability glare. Discomfort glare results in a sense of pain or annoyance. Disability glare inhibits vision.(1)
In addition to uniformity, AASHTO states that glare is also an indicator of quality. Uniformity and glare both depend on factors such as height and fixture type along with placement. Â Discomfort glare and impairment to drivers should be a priority of consideration. The comfort and security of pedestrians should also be considered when designing lighting for an intersection or crosswalk. Where only intersections are lighted, and not the approach to the intersection, the lighting should taper, with a gradual transition from dark to light.(2)
Uniformity is the ratio of the maximum measured light in an area (typically illuminance) to either the average or minimum measured light in an area. Uniformity of lighting is indicative of quality and should be considered along with illumination levels.(2) The AASHTO Green Book states that lighting should be both continuous and energy saving, which may appear to be at odds.(2)
Signage policies included in the MUTCD for lighting include the notion of uniformity, which must be maintained without a decrease in visibility, legibility, or driver comprehension during either day or night conditions.(3)
In general, the MUTCD offers specific and often technical guides for lighting. For example, most regulatory, warning, and guide signs must be retroreflective or illuminated to show the same shape and similar color by both day and night. Sign illumination requirements cannot be met using lighting exterior to the sign, like highway lighting or that of a nearby business.(3)
LEDs are a relatively new technology employed with signs. LEDs cannot be used in the background area of the sign, and even the size of the LEDs is prescribed by the MUTCD. According to the guide, LEDs should not have a diameter greater than ¼ inch. For LED color, the MUTCD prohibits colors outside of white or red to indicate stop or yield. Regulatory signs other than stop or yield must use white only; guide signs must also use white only. Warning signs and school area signs must use white or yellow. Temporary traffic control must use white, yellow, or orange. The MUTCD indicates that LED units for flashing devices (that enhance conspicuity) should flash simultaneously at more than 50 and less than 60 times per minute.(3)
Changeable message signs (CMSs) have been utilized increasingly on public roadways. The MUTCD instructs that on roadways with speed limits of 55 mi/h and greater, the sign should be visible from at least a half mile under both day and night conditions. The message on the sign should be legible from a minimum of 600 ft for nighttime and 800 ft for daytime conditions. If neither of those distance recommendations can be met, the MUTCD suggests using fewer words on the sign or incorporating familiar symbols. CMSs should automatically adjust their brightness under varying light conditions to maintain legibility, while the luminance should meet certain criteria for day and night conditions. Luminance contrast specifically should measure between 8 and 12 for all conditions. Contrasts on CMSs should always be positive.(3)
The FHWA Lighting Handbook offers a bulleted list of considerations when designing for lighting and selecting light. These considerations include photometric performance, durability, aesthetics, maintenance requirements, and operation costs. Specific luminaire requirements include ingress protection rating, lens material, housing, internal electrical components, and backlight, up-light, and glare (BUG) ratings.(1)
The Luminaire Classification System (LCS) was developed to help define luminaire distribution and efficiency. The LCS allows for the evaluation and comparison of outdoor luminaires. The primary areas for LCS are forward-light, backlight, and up-light zones. Each zone is then broken down further into solid angles within the area. The sum of percentages of lamp lumens within the three primary areas is equal to the photopic luminaire efficiency. This allows designers to find the best fit for their application.(1,4)
The BUG rating system existed prior to the LCS and is similar to it. Each category (backlight, up-light, and glare) consists of areas that surround the luminaire. Each region has an upper limit that must be met to obtain the rating. All criteria must be met for a luminaire to receive a B, U, or G rating.(1)
This section highlights general policy concerning light design in terms of height, placement, and utilization.
Lighting pole layouts typically follow one of four formats: one-sided lighting, opposite lighting, staggered lighting, and median lighting. In a one-sided layout, all of the luminaires are on one side of the road. In an opposite lighting layout, the lighting is on direct opposite sides of the roadway. The staggered layout is like the opposite layout except the luminaires are offset from each other and do not directly face each other. Median lighting requires a divided roadway where the luminaires are placed in a single row along the median.(1)
In addition to typical luminaire pole spacing, the FHWA Lighting Handbook also details spacing designs for high mast poles that are typically found at freeway interchanges. These designs are more intricate and involve the consideration of a number of factors, including the height, number of luminaires for each pole, optics and orientation, wattage, source, and photometrics.(1)
High mast installations are generally regarded as being more visually comfortable. The height of the light source keeps it out of the typical viewing angle of drivers, but the amount of light provides illumination around a large corridor. Because more luminaires are attached to a single pole, there are fewer poles in the area, thus reducing the probability of one being struck in a roadway departure.(4)
Adaptive lighting was born from the idea of reducing power consumption by controlling light levels in off-peak hours. It is believed that a significant amount of power can be saved by varying the levels of lighting when traffic volumes change,(1) resulting in an energy savings of up to 50 percent. Light levels are originally established by applying criteria based on the road type, conflict levels, pedestrian levels, and traffic volume. The current design practice is to design based on the highest pedestrian conflict level for an area and to establish minimum lighting levels based around that. Once that minimum is established, lighting systems provide that level of lighting throughout all hours of darkness. While traffic levels and pedestrian levels taper at later hours, lights have traditionally provided the same output.(1)
It is not recommended to reduce lighting levels or implement adaptive lighting near signalized intersections, midblock crosswalks, roundabouts, rail crossings, or the canopy area of toll plazas.(1,4) Before designing for adaptive lighting, it is recommended that the Illuminating Engineering Society of North America’s (IESNA’s) RP-8 be reviewed.(1) The recommendations of the RP-8-14 suggest adjusting the fluctuation of adaptive lighting based on the existing classifications for pedestrian conflict. A major roadway with high pedestrian conflict would require 1.2 cd/m², but when pedestrian numbers drop in later hours to a low classification, the lighting can be allowed to reduce to 0.6 cd/m².(4)
The AASHTO Green Book provides some separate policies on lighting streets, highways, and freeways in both urban and rural areas. The functional classification of the roadway typically factors into the decision to light it. Other factors include pedestrian and cyclist presence; night-to-day crash ratios; geometry of the roadway; and number of lanes, curves, and intersections.(2)
Streets and roadways often have roadside infrastructure not pertinent to the roadway, such as power poles. Even light poles that are necessary for hoisting the luminaires that light the roadway must be carefully placed to minimize a potential hazard.(1,2) For the varying street classifications in the RP-8-14 roadway lighting guide, specifications for average luminance and uniformity as well as maximum uniformity and glare are detailed. Further specifications exist for each classification by the level of pedestrian conflict—either high, medium, or low. In general, the lower the classification of street, the less average luminance and uniformity prescribed.(4)
Freeways do not contain pedestrians or roadside entrances, so many conflict points typically found on streets and roadways are not found along freeways. However, freeways have higher speed limits and merging lanes. The RP-8-14 details the lighting design criteria for roadways. There are different specifications for average luminance, average uniformity ratio, maximum uniformity ratio, and glare depending on roadway class.(4)
Fixed-source lighting is essential at interchanges so that drivers can clearly discern the roadway ahead, the possible paths to be followed, and all other vehicles in enough time to make correct decisions. The AASHTO Green Book states that an unlit interchange decreases its usefulness because cars would inherently slow down as drivers approach with less certainty. It is important that retroreflective devices are also used so that drivers can make out grade separators such as curbs, piers, and abutments. In general, lighting becomes more useful as traffic volume increases.(2)
Lighting on arterials is assumed to reduce sudden braking and swerving. The policy mentions that older drivers specifically benefit from well-lit arterials, which perhaps welcomes the idea of lighting based on area demographics. An excerpt from the AASHTO Green Book claims, "A safely designed, adequate lighting system is more important to optimum operation of an urban arterial than for any other type of city street."(2)
Miscellaneous roadway segments such as railroad crossings, tunnels, toll plazas, overpasses and underpasses, and bridges have less mention in the AASHTO guidance.
For railroad crossings, the AASHTO guidance mentions the use of floodlighting or highway lighting, with little other documentation(2); however, RP-8-14 recommends lighting the conflict area 30 m before and after the crossing as well as providing auxiliary lighting to highlight the sides of the passing train cars.(4)
Railroad-highway grade crossings consist of either passive or active preemptive warnings for drivers. Passive signals include signing and pavement markings, whereas active signals include flashing beacons, gates, and bells. There is no quantitative information about the crash effects of illumination at railroad crossings.(5)
The AASHTO guidance brings to question the justification of cost for lighting rural bridges.(2)
Toll plazas have four distinct areas according to IESNA: approach/departure zones, queuing areas, toll collection islands, and infield.(4) There are also two recognized types of toll plaza types: in-line and off-line. In-line plazas require all vehicles to stop or slow down and pay a toll before continuing. Off-line plazas only require vehicles to stop and pay when they want to enter or exit a road. The lighting principles for each are similar, though the key difference is that the in-line plaza does not contain any ramps leading to or away from it. The RP-8-14 details specifics in terms of illumination for each of the four zones and two types of toll plazas, including luminaire placement, glare requirements, illumination level, and controls.
For underpasses and overpasses, the consideration for day and nighttime needs must be assessed. Nighttime underpass lighting should follow the same requirements as roadway lighting for that section. Daytime lighting must follow requirements suggested by American National Standards Institute (ANSI) RP-22-11, which provides recommendations on tunnel lighting.(4)
Tunnel lighting is dependent on grade and length and how much natural light can enter the portal. Lighting expenses are typically greater near portals since special consideration must be given so that drivers do not endure optical shock from traveling between natural and artificial lighting. The finish to the walls must also be given special consideration so that the surfaces increase reflectivity to enhance brightness and uniformity. Road design should avoid the need for guide signs inside tunnels.
Steep grades and curves cause a visual problem for drivers and often require more lighting. The steeper the grade or sharper the curve is, the closer the luminaires should be spaced. Careful orientation of luminaires in these scenarios is required to maximize uniformity.(4)
Intersections often have a concentration of pedestrians and roadside interference. Fixed-source lighting in these areas tends to reduce crashes. Rural intersections require more justification in terms of traffic volume, planned geometrics, and peak-time congestion.(2,3) For the benefit of non-local highway users, the lighting of a rural intersection is desirable to aid the driver in ascertaining sign messages during non-daylight hours.
In general, the warrant for lighting an intersection must provide evidence of excessive delay, congestion, unfavorable approach conditions, or surrounding conditions that cause driver confusion.(3) The FHWA Lighting Handbook details a point scale developed by the Transportation Association of Canada that assists engineers and planners in deciding if an intersection warrants lighting and how much to provide. The scale uses a point system to decide whether to fully light, partially light, only light certain areas, or not use any lighting at the intersection. There are other warranting systems in existence, many of which simply rely on traffic volume as a determinant.(1)
Rural at-grade intersections are lit depending on the layout and traffic volumes. Intersections without channelization, or multi-roads and turning lanes, are often left unlighted. However, broad-scale, multi-road intersections are often lighted in rural areas. Sharper curves, limited ability of headlamps for wide turning radii, and presence of pedestrians often justify fixed lighting. The fixed lighting can also serve as an indication for drivers to adjust their speed on approach.(2)
IESNA details six types of intersections based on the convergence of three different road types: major, collector, and local. In general, the functional classification of the intersections ranked busiest to least busy in terms of traffic and pedestrian conflict are lit accordingly, with more light being required for major-to-major intersections and less lighting being required for local-to-local intersections. The need for uniformity also lessens with lower functional classifications.(4)
The two main purposes for lighting roundabouts are the same as those for lighting an intersection: provide visibility from afar for users of the roundabout and provide visibility in key conflict areas to improve navigation. In general, the illumination should be designed to create a break in the continuous path of the roadway and emphasize the circular aspect of the roundabout.(4,6)
Like intersections, the overall illumination of the roundabout should be approximately equal to the sum of the illumination levels of intersecting roadways. If continuous roadway lighting leading up to the roundabout is not present, transition lighting should be provided for the driver to taper in and out of the lighted section of the roundabout. In general, adequate illumination needs to be provided on the approach, at all conflict areas where traffic is entering the stream of cars in the roundabout, and at all places where cars break away to exit the roundabout.(6)
Illumination of a roundabout is beneficial when one or more approaches are illuminated and heavy traffic including pedestrians and cyclists is anticipated. Unlike intersections, it is encouraged that all roundabouts, independent of traffic volume, be illuminated.(6)
There are three classifications for pedestrian conflict, each dependent upon the type of abutting land use. High conflict areas expect pedestrians to be on sidewalks or crossings during hours of darkness. Areas near theaters and stadiums are examples where pedestrian conflict can be high. Where pedestrians are expected to be fewer at night but are still a factor during the day are considered medium conflict areas. Areas and blocks adjacent to office buildings, industrial parks, and libraries fit the description. Low areas have few pedestrians during night or day and may be in residential areas or rural zones.(4)
Crosswalks are often collocated at intersections but may be considered more dangerous at midblock locations. At an intersection crosswalk, it is important to light the crosswalk to stand out from the intersection in the background. Midblock crosswalks do not always have surrounding light sources, so lights may need to be placed specifically at certain points in the midblock in order to provide sufficient illumination to the crosswalk and pedestrians.(7)
An FHWA publication regarding the design for crosswalk lighting concluded that a vertical illuminance level of 20 lx measured at 5 ft from the road service allowed drivers to detect pedestrians more easily at midblock crossings. A high level of vertical illuminance such as this may be required for crosswalks when there is a possibility of continuous glare from opposing vehicles, the crosswalk is located in an area of high ambient light levels, or the crosswalk is located in a lighted intersection.(7)
Lighting designed specifically for pedestrians is often lower to the ground. A pole height of 3 to 6 m (10 to 25 ft) is more common for pedestrian-heavy zones than are typical roadway lighting systems, which are mounted often twice that high.(4)
In addition to lighting, other safety measures include automated pedestrian detectors that allow for adjusting the countdown timer on a crosswalk to the speed and volume of pedestrians crossing. This implementation appears to reduce pedestrian-vehicle conflicts as well as reduce the number of pedestrians who choose to cross during the "Don’t Walk" phase.(5)
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