Category HIGHWAY ENGINEERING HANDBOOK

A tunnel is defined as a structure over a roadway that restricts the normal daytime illumi­nation of a roadway section so that the driver’s visibility is substantially diminished. Design of tunnel lighting requires adaptation for driver needs in the approach, and the

threshold, transition, and interior zones, as presented in Fig. 7.59. Tunnels are classified by structure length and geometric alignment (visibility through the structure). A straight tunnel having an overall length from portal to portal equal to or less than the safe stopping sight distance (SSSD; Table 7.18) is considered to be a short tunnel. A tunnel with an overall length greater than one SSSD, or having an alignment or curvature that prevents motorists from seeing through the structure to the exit end, is considered to be a long tunnel. Overpasses and underpasses are those structures in which the length does not exceed one width of the roadway over (or under) which they are constructed.

Underpasses with a length-to-height ratio of approximately 10:1 or less will not nor­mally require daytime lighting. When the length-to-height ratio exceeds 10:1, it is neces­sary to analyze the specific conditions, including vehicular and pedestrian activity, to determine the need for daytime lighting. Roadways that are not continuously lighted warrant underpass lighting in areas having frequent nighttime pedestrian traffic or where

unusual or critical roadway geometry occurs under or adjacent to the underpass area. On roadways with continuous lighting, favorable positioning of luminaires adjacent to the underpass can often provide adequate lighting without supplemental luminaires. Lighting levels and uniformities should match the values on the adjacent roadway when practical. Because of limited mounting height, when lights are placed within a tunnel, special consideration should be given to glare and uniformity. Raised lighting levels may be achieved by using closely spaced low-wattage luminaires. Such increased levels should not exceed twice that of the adjacent roadway.

Recommended values for illuminance, luminance, and small-target visibility are pub­lished as Tables 2, 3, and 4 in ANSI/IESNA RP-8-00, American National Standard Practice for Roadway Lighting, www. iesna. org. These lighting criteria for the three recognized calculation methods are typically adopted by AASHTO and published as a part of the AASHTO Guideline for Roadway Lighting. The Federal Highway Administration, in turn, historically has accepted the RP-8 criteria as adopted by AASHTO for lighting projects partially funded with federal funds. Refer to the latest edition of the AASHTO Roadway Lighting Design Guide for current recommended criteria. Each lighting designer should have access to the latest versions of these docu­ments, which sometimes contain substantial modifications.

7.17.2 Other Considerations

In using the AASHTO tables, there may be conditions for which different luminance and illuminance levels are desirable or necessary. The lighting designer should use all available pertinent information in reaching a decision regarding the level to be used for any specific street or highway.

There are many locations where very high levels of luminance or illuminance are provided for streets in the central city business district. This is usually a commercial consideration directed toward making the downtown business area more appealing to shoppers. Levels considerably higher than the levels in the table must be justified on some basis other than solely for the safe and efficient flow of traffic. If higher than recommended levels are desired, the lighting designer should consider using a white light source, such as metal halide, rather than a monochromatic source, such as high- pressure sodium (HPS). Visibility tests have shown there is a lack of contrast with high levels of HPS. The lack of contrast reduces the ability to distinguish an object from its background, the details of an object, or the color of the object.

Fixed roadway lighting systems increase night visibility, as well as improve safety, traffic movement, and general roadway use. Traffic volume, numbers of pedestrians, at-grade intersections, turning movements, signalization, and unusual geometrics are some elements that make lighting of streets and highways desirable. Lighting, in addi­tion to its safety benefits, is a crime deterrent and a valuable aid to law enforcement agencies and often contributes to the pride of a community.

7.17.1 Warranting Conditions for Roadways Other Than Freeways [3]

It is not practical to establish specific warrants for the installation of roadway lighting to satisfy all prevailing or anticipated conditions. In general, lighting may be consid­ered for those locations where the respective governmental agencies concur that light­ing contributes substantially to the efficiency, safety, and comfort of vehicular or pedestrian traffic. Lighting may be provided for all major arterials in urbanized areas. It may also be provided for locations or sections of streets and highways where the ratio of night accidents to day accidents is higher than the statewide average for similar locations and a study indicates that lighting may be expected to significantly reduce the night accident rate. Determinations to install lighting that have been made on the basis of accident experience at a particular site can be applied to other similar highway locations. The latter should include similar geometric layouts on which experience or accident data are not available and also on highway sections where anticipated increase in vehicular and pedestrian traffic will present problems within a few years. Lighting may be considered at locations where severe or unusual weather or atmospheric conditions exist. In other situations, lighting may be considered where the local govern­mental agency finds sufficient benefit in the form of convenience, safety, policing, community promotion, or public relations to pay an appreciable percentage of the cost of, or wholly finance, the installation, maintenance, and operation of the lighting facilities [3].

Lighting has been successfully used on rural conventional highways at hazardous locations to reduce the number of accidents. Lighting of spot locations in rural areas

should be considered whenever the driver is required to pass through a section of road with complex geometry and/or raised channelization as well as at intersections with higher than normal accident rates [3]. Isolated lighting of railroad grade crossings has been used to help the driver identify when a train is present in the crossing.

As with any roadway or intersection, warranting conditions should also be applied at a roundabout to determine the need for lighting. If lighting is warranted, the round­about should be lit to a level that is 1.3 to 2 times the values used on the best lit approach and the uniformity should be 3:1 or better. The lighting should be extended a minimum of 400 ft (122 m) along each road connecting to the roundabout. Providing lighting that will provide good pedestrian recognition is an important issue. Placement of light poles 10 to 30 ft (3 to 9 m) before the crosswalk is recommended as a means of accomplishing this task [7].

Continuous, complete, or partial interchange lighting is considered to be justified in

the following situations:

• In general, lighting is warranted where the local government agency finds sufficient benefit in the forms of convenience, safety, policing, community promotion, public relations, etc., to pay an appreciable percentage of the cost of the installation, main­tenance, and operation of the lighting system.

• Where there is continuous freeway lighting, there should be complete interchange lighting.

• Where complete interchange lighting is warranted but not initially fully installed, a partial lighting system that exceeds the normal partial installation in number of lighting units is considered to be justified.

• Lighting of crossroad ramp terminals is warranted regardless of volumes where the design requires the use of channelizing or divisional islands, and/or where there is poor sight distance.

7.16.3 Freeway Lighting Design Values [7, 8]

The following should be satisfied for proper design when using the illuminance method:

• Continuous freeway and complete interchange lighting should be designed to provide an average maintained horizontal illuminance in the range of 0.6 to 0.9 fc (6 to 10 lx) on the traveled roadway.

• The ramps should be lighted to the same level as the main roadway.

• The point of least illuminance on the roadway should not be less than 0.2 horizontal fc (2 lx).

• An average to minimum uniformity ratio of 3:1 or 4:1 is reasonable. The more uniform design is preferred.

• The higher levels of illuminance should be at the gores and intersections.

• Situations such as high ambient brightness near the roadway or closed circuit tele­vision surveillance equipment may justify higher levels of illuminance.

The following criteria should be satisfied for proper design when using the luminance

method:

• The average maintained luminance should be in the range of 0.4 to 1.0 cd/m2 (0.12 to 0.29 foot-lambert, or ft-L).

• The ratio of average luminance to minimum luminance should not exceed 3.5 to 1.

• The ratio of maximum luminance to minimum luminance should not exceed 6.0 to 1.

• The ratio of veiling luminance to average luminance should not exceed 0.3 to 1.

7.16.4 Transition Lighting

Rapid changes in lighting levels which occur, especially when leaving a continuously lighted section of roadway, may be compensated for by using transition lighting or adaptation techniques. Lighting levels as recommended above should be reduced to approximately one-half the recommended level for about 15 s to allow the eye to adapt.

7.16.5 Bridges and Overpasses

Lighting on bridges and overpasses should be at the same level as the roadway. It may be desirable to provide lighting on long bridges in urban and suburban areas even though the approaches are not lighted, since lighting enhances both the safety and utility of bridges. Where bridges are provided with sidewalks, lighting is warranted to increase pedestrian safety and security.

Complete lighting of freeway interchanges is warranted under the following circumstances:

Case CIL-1. Where total current ADT ramp traffic entering and leaving the freeway within the interchange area exceeds 10,000 for urban conditions, 8000 for suburban conditions, or 5000 for rural conditions.

Case CIL-2. Where current ADT on the crossroad exceeds 10,000 for urban con­ditions, 8000 for suburban conditions, or 5000 for rural conditions.

Case CIL-3. Where existing substantial commercial or industrial development, which is lighted, is located in the immediate vicinity of the interchange; or where the crossroad approaches are lighted for at least L mi (0.8 km) on each side of the inter­change.

Case CIL-4. Where the ratio of night accidents to day accidents within the inter­change area is at least 1.5 times higher than the statewide average for all unlighted similar interchanges and lighting may be expected to result in a significant reduction in the night accident rate.

7.16.2 Warranting Conditions for Partial Interchange Lighting

Partial lighting of freeway interchanges (Fig. 7.58) is warranted when the following circumstances exist:

Case PIL-1. Where the total current ADT ramp traffic entering and leaving the freeway within the interchange area exceeds 5000 for urban conditions, 3000 for suburban conditions, or 1000 for rural conditions.

Case PIL-2. Where the current ADT on the freeway through traffic lanes exceeds 25,000 for urban conditions, 20,000 for suburban conditions, or 10,000 for rural conditions.

■ c Safety

Clearance

Paved Shou der

Shou der Nose

Safety

Cearance

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Optional

Unit

(b) ENTRANCE RAMP

FIGURE 7.58 Typical luminaire locations for partial interchange lighting.

Case PIL-3. Where the ratio of night accidents to day accidents within the inter­change area is at least 1.25 times higher than the statewide average for all unlighted similar sections and lighting may be expected to result in a significant reduction in the night accident rate.

Freeway lighting can substantially reduce accident frequency and increase capacity. Lighting is of additional benefit in freeway operations by providing motorists addition­al warning time of stalled or disabled vehicles on the roadway. A listing of the specific conditions considered by most authorities to warrant lighting is included below.

7.16.1 Warranting Conditions for Continuous Freeway Lighting

Continuous lighting along the freeway is warranted in the following cases:

Case CFL-1. On those sections in or near cities where the current average daily traffic (ADT) is 30,000 or more.

Case CFL-2. Where three or more successive interchanges are located with an average spacing of 1!/> mi (2.4 km) or less and adjacent areas outside the right-of – way are substantially urban.

Case CFL-3. Where for a length of 2 mi (3.25 km) or more the freeway passes through a developed suburban or urban area in which at least one of these conditions exists: (1) local traffic operates on a complete street grid having some form of street lighting, parts of which are visible from the freeway; (2) the freeway passes through a series of developments that are lighted; (3) separate cross streets with or without connecting ramps occur with an average spacing of L mi (0.8 km) or less, some of which are lighted as part of the local street lighting system; (4) the freeway cross­section elements, such as median and borders, are substantially reduced in width.

Case CFL-4. Where the ratio of night accidents to day accidents is at least 2.0 or higher than the statewide average for all unlighted similar sections and lighting may be expected to result in a significant reduction in the night accidents [3].

The above classifications appropriately describe general conditions of vehicular traffic conflict in urban areas. A second type of conflict is vehicle/pedestrian interaction. The magnitude is mostly determined by the land use of the abutting properties. Three clas­sifications of pedestrian night activity levels and types of land use with which they are typically associated are used in the lighting criteria tables:

High. Areas with significant numbers of pedestrians expected to be on the side­walks or crossing the streets during darkness. Examples are downtown retail areas and areas near theaters, concert halls, stadiums, and transit terminals.

Medium. Areas where lesser numbers of pedestrians utilize the streets at night. Typical are downtown office areas; blocks with libraries, apartments, and neigh­borhood shopping; industrial areas; older city areas; and streets with transit lines.

Low. Areas with low volumes of night pedestrian usage. These can occur in any of the cited roadway classifications but may be typified by suburban single-family streets, very-low-density residential developments, and rural or semirural areas.

The choice of appropriate lighting levels is an engineering decision. If needed, 1-h pedestrian counts can be taken during the first hour of darkness. The volume of pedes­trians that warrants increased lighting levels is not fixed but is a local option. However, the following volumes might be considered as a guide:

Low 10 or fewer

Medium 11 to 100

High Over 100

These volumes represent the total number of pedestrians walking on both sides of the street plus those crossing the street at nonintersection locations in a typical block. The lighting levels at intersections are already considered by the increased illumination for the points of conflict.

The following descriptions of facility types and area classifications are used to describe the warranting conditions and design needs of roadway lighting.

7.15.1 Roadway, Pedestrian Walkway, and Bikeway Classifications [3, 6]

• Freeway. A divided major roadway with full control of access (no crossings at grade). This definition applies to toll as well as nontoll roads.

• Freeway A: Roadways with greater visual complexity and high traffic volumes. Usually this type of freeway will be found in major metropolitan areas in or near the central core and will operate through some of the early evening hours of darkness at or near design capacity.

• Freeway B: All other divided roadways with full control of access.

• Expressway. A divided major arterial highway for through traffic with full or par­tial control of access and generally with interchanges at major crossroads. Parkways are expressways for noncommercial traffic within parks and parklike areas.

• Major. That part of the roadway system that serves as the principal network for through-traffic flow. These routes connect areas of principal traffic generation and important rural roadways leaving the city. These routes are often referred to as arte- rials, throughfares, or preferentials and are sometimes subdivided into primary and secondary, but these distinctions do not affect the roadway lighting.

• Collector. Roadways serving traffic between major and local streets. These are streets used mainly for traffic movements within residential, commercial, and industrial areas. They do not handle long through trips. Collector streets may be used for truck or bus movements and give direct service to abutting properties.

• Local. Roadways used primarily for direct access to residential, commercial, industrial, or other abutting property. They do not include roadways carrying through traffic. Long local roadways will generally be divided into short sections by collector roadway systems.

• Roundabout. A circular intersection with yield control of all entering traffic, chan­nelized approaches, counterclockwise circulation, and appropriate geometric curvature to ensure that travel speeds on the circulatory roadway are typically less than 30 mi/h (50 km/h) [20].

• Alleys. A narrow public way within a block, generally used for vehicular access to the rear of abutting properties.

• Sidewalks. Paved or otherwise improved areas for pedestrian use, located within public street rights-of-way that also contain roadways for vehicular traffic.

• Pedestrian ways. Public sidewalks for pedestrian traffic generally not within rights-of-way for vehicular traffic roadways. Included are skywalks (pedestrian overpasses), subwalks (pedestrian tunnels), walkways giving access to park or block interiors, and crossings near centers of long blocks.

• Bicycle lanes. Any facility that explicitly provides for bicycle travel.

Available computer programs accurately design lighting systems using photometric curves and data for various fixtures. Some programs allow the user to input items such as trees and other physical barriers that may interfere with the output from the light fixtures. These programs will calculate luminance, illuminance, and glare. Dirt and maintenance factors that depend on the environment of the fixture location and the type of fixture are also used in the calculations. Some of the programs are easy to operate and very user friendly whereas some require much more detail. Computer-generated drawing files can be imported into the program, thus allowing the use of available site and roadway plans without the necessity of compiling a new drawing. Lighting manufacturers have made photometric data available from their company websites, and this information can be imported into the software program. After the design is completed, some programs are capable of providing a three-dimensional rendering showing a pictorial view of the pro­posed area. With the ease of obtaining all required input information, a lighting designer can experiment with the use of various fixture styles, wattages, and photometric patterns. Thus, the designer can obtain maximum light levels with consideration of both initial installation costs and annual operating costs, which include energy costs and maintenance items such as cleaning and lamp and ballast replacement.

The potential traffic safety benefits of lighting are due to an increase in driver comfort and confidence resulting from enhanced vision. This reduces driving stress and tension, increases roadway capacity, and reduces the potential for traffic accidents. The eco­nomic return of roadway lighting is greatest in urban and suburban areas with high traffic volumes. Rural locations can also benefit from full, or partial, lighting of decision points such as at isolated intersections, on – or off-ramps, and ramp terminals [3]. For some lighting applications there are warranting criteria that can be used to help determine when lighting should be installed.

Warranting conditions are based on minimum conditions which signify that providing lighting would be beneficial. Satisfying the warrants does not obligate an agency to provide lighting, since warrants are not the only criteria that should be considered. Local conditions such as frequent fog, ice, snow, roadway geometry, ambient lighting, sight distance, and signing could justify modifying the warrants either positively or negatively [3]. Judgments on lighting need should include an assessment of the antici­pated benefits, traffic volume, speed, road use during the night, night accident rate, road geometrics, and general night visibility. Some agencies justify lighting based on an economic analysis. This requires placing monetary value on the expected reductions in personal injuries, fatalities, and property damage accidents, in addition to other societal benefits estimated to be realized from illumination.

Warranting conditions have been established for freeways. However, due to the wide diversity of conditions that can exist, there are no established warrants to assist the designer in determining when lighting should be provided for urban streets, high­ways, walkways, and bikeways. The justification for urban lighting is left to engineer­ing judgment, coupled with perceived user needs and user benefits.