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Roundabouts: An Informational Guide – Second Edition (2010)

Chapter: Chapter 8 - Illumination

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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Suggested Citation:"Chapter 8 - Illumination." National Academies of Sciences, Engineering, and Medicine. 2010. Roundabouts: An Informational Guide – Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22914.
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Chapter 8/Illumination Page 8-1 Roundabouts: An Informational Guide CHAPTER 8 ILLUMINATION CONTENTS 8.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 8.2 GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3 8.3 LIGHTING LEVELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-5 8.4 EQUIPMENT TYPE AND LOCATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.4.1 Equipment Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.4.2 Pole Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-6 8.4.3 Example Illumination Layouts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9 8.5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11

Roundabouts: An Informational Guide Page 8-2 Chapter 8/Illumination LIST OF EXHIBITS Exhibit 8-1 Recommended Illuminance Levels for Roundabouts . . . . . . . . . . . . 8-5 Exhibit 8-2 Common Types of Illumination Equipment Used at Roundabouts . . 8-6 Exhibit 8-3 Styles of Lighting Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7 Exhibit 8-4 Summary of Advantages and Disadvantages of Perimeter and Central Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8 Exhibit 8-5 Photometric Illustration of Central and Perimeter Illumination Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-8 Exhibit 8-6 Example of Illumination Using Cobra-Style Luminaires . . . . . . . . . 8-9 Exhibit 8-7 Example of Illumination Using Pedestrian-Level Luminaires . . . . 8-10 Exhibit 8-8 Example of Illumination Using a Mix of Cobra-Style and Pedestrian-Level Luminaires . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-10

Chapter 8/Illumination Page 8-3 Roundabouts: An Informational Guide 8.1 INTRODUCTION For a roundabout to operate satisfactorily, a driver must be able to enter the roundabout, move through the circulating traffic, and separate from the circulat- ing stream in a safe and efficient manner. Pedestrians must also be able to safely use the crosswalks. To accomplish this, a driver must be able to perceive the gen- eral layout and operation of the intersection in time to make the appropriate maneuvers. Adequate lighting should therefore be provided at all roundabouts. The Design Guide for Roundabout Lighting, published by the Illuminating Engineering Society (IES), is the primary resource that should be consulted in completing a lighting plan for a roundabout (1). The IES design guide provides recommendations for lighting of roundabouts, including light levels within the roundabout and vertical light levels at locations where pedestrians and bicycles are present. Other documents that could also provide assistance when complet- ing a design include: • An Information Guide for Roadway Lighting by AASHTO (2); • Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals by AASHTO (3); • Roadway Lighting Design Guide by AASHTO (4); and • American National Standard Practice for Roadway Lighting by IES (5). International references on lighting at roundabouts include the following: • Guide for the Design of Roadway Lighting by the Transportation Association of Canada (6); • L’Éclairage des Carrefours à Sens Giratoire (The Illumination of Roundabout Intersections) by CERTU (France) (7), and • Australian/New Zealand Standard AS/NZS 1158.1.3:1997 (8). 8.2 GENERAL CONSIDERATIONS Lighting of roundabouts serves two main purposes: 1. It provides visibility from a distance for users approaching the round- about; and 2. It provides visibility of the key conflict areas to improve users’ perception of the layout and visibility of other users within the roundabout. (1) An important lighting consideration at roundabouts is that the roundabout introduces geometry and channelization that a driver may not expect unless it is visible at all times. In addition, the effectiveness of auto headlights is limited in a roundabout due to the constrained curve radius, making the roadway lighting system very important for nighttime visibility of obstructions and hazards (1). Adequate lighting should be provided at all roundabouts.

Roundabouts: An Informational Guide Page 8-4 Chapter 8/Illumination To improve the users’ understanding of the roundabout’s operations, the illumination should be designed to create a break in the linear path of the approaching roadway and emphasize the circular aspect of the roundabout (1). To achieve this, the following features are recommended: • The overall illumination of the roundabout should be approximately equal to the sum of the illumination levels of the intersecting roadways. Local illumination standards should also be considered when establish- ing the illumination at the roundabout to ensure that the lighting is consistent. • If continuous roadway lighting is not present, transition lighting should be provided for driver adaptation and should be extended along each approach to the roundabout. • Adequate illumination should be provided on the approach nose of the splitter islands, at all conflict areas where traffic is entering the circulating stream, and at all places where the traffic streams separate to exit the roundabout. • Adequate illumination should be provided for pedestrian crossing and bicycle merging areas. • Consideration should be given to the impact of the lighting system in var- ious ambient lighting zones and on adjacent properties. In addition, care should be taken to minimize glare and light trespass. The IES Design Guide for Roundabout Lighting provides more detail on these topics (1). Illumination of a roundabout is particularly beneficial when: • One or more approaches are illuminated; • An illuminated area in the vicinity can distract the driver’s view; and/or • Heavy nighttime traffic, including pedestrians and bicycles, is anticipated. Continuity of illumination is desirable between illuminated areas and the roundabout itself (9). A driver may not see a roundabout located in an unlit area immediately beyond the illuminated area due to the time it takes for the human eye to adjust to differing light levels. Illumination is recommended for all roundabouts, including those in rural environments. However, it can be costly to provide if there is no power supply in the vicinity of the intersection. Where lighting is not provided, the intersection should be well signed and marked (including the possible use of reflective pave- ment markers) so that it can be correctly perceived by day and night, recognizing that signing and markings alone cannot correct for the limited view of headlights when circulating. In areas where only the roundabout is illuminated (no lighting is provided on the approach roadways), the scope of illumination needs to be carefully con- sidered. Any raised channelization or curbing should be illuminated. A gradual illumination transition zone should be provided beyond the final trajectory changes at each exit (9). This helps drivers adapt their vision from the illumi- nated environment of the roundabout back into the dark environment of the Lighting from the central island causes vehicles to be backlit and thus less visible.

Chapter 8/Illumination Page 8-5 Roundabouts: An Informational Guide exiting roadway, which takes approximately one to two seconds. In addition, it is preferable to avoid short-distance dark areas between two consecutive illumi- nated areas (9). The AASHTO Roadway Lighting Design Guide recommends that lighting be extended a minimum of 400 ft (120 m) along each road connecting to the roundabout (4). 8.3 LIGHTING LEVELS Exhibit 8-1 summarizes the IES recommended street illumination levels for roundabouts located in continuously illuminated streets. This exhibit also pres- ents the roadway and pedestrian area classifications used for determining the appropriate illumination levels. Although some other documents, zoning bylaws, and agencies may define these roadway and pedestrian areas differently, the descriptions shown in Exhibit 8-1 should be used to determine the roundabout lighting levels. Vertical luminance recommendations have also been developed to ensure ade- quate visibility for drivers approaching crosswalks with pedestrians. Based on the IES Design Guide for Roundabout Lighting, it is recommended that the average verti- cal luminance for a series of points 5 ft (1.5 m) in height, along the centerline of the crosswalk extending to the edge of the roadway, spaced at 1.65 ft (0.5 m), for each driving direction be equal to the required horizontal illuminance and uniformity for the roundabout (1). Exhibit 8-1 Recommended Illuminance Levels for Roundabouts Maintained Average Horizontal Illuminance on the Pavement Based on Pedestrian Area ClassificationFunctional Classification High Medium Low Eavg/Emin Major/Major 3.4 fc (34.0 lux) 2.6 fc (26.0 lux) 1.8 fc (18.0 lux) 3:1 Major/Collector 2.9 fc (29.0 lux) 2.2 fc (22.0 lux) 1.5 fc (15.0 lux) 3:1 Major/Local 2.6 fc (26.0 lux) 2.0 fc (20.0 lux) 1.3 fc (13.0 lux) 3:1 Collector/Collector 2.4 fc (24.0 lux) 1.8 fc (18.0 lux) 1.2 fc (12.0 lux) 4:1 Collector/Local 2.1 fc (21.0 lux) 1.6 fc (16.0 lux) 1.0 fc (10.0 lux) 4:1 Local/Local* 1.8 fc (18.0 lux) 1.4 fc (14.0 lux) 0.8 fc (8.0 lux) 6:1 Major = Roadway system that serves as the principal network for through traffic flow. Collector = Roadway servicing traffic between major and local streets. Local = Streets primarily for direct access to residential, commercial, industrial, and other abutting property. High = Areas with significant numbers of pedestrians expected to be on the sidewalks or crossing the streets during the hours of darkness. Over 100 pedestrians during the average annual peak hour of darkness, typically 18:00 to 19:00 hours. Medium = Areas where lesser numbers of pedestrians use the streets at night. Between 11 and 100 pedestrians during the average annual peak hour of darkness, typically 18:00 to 19:00 hours. Low = Areas with low volumes of nighttime pedestrian usage. Less than 11 pedestrians during the average annual peak hour of darkness, typically 18:00 to 19:00 hours. *Note: Use values for local/local functional classification if roundabout is located on roadway without continuous lighting. Source: Adapted from IES Design Guide for Roundabout Lighting (1)

Roundabouts: An Informational Guide Page 8-6 Chapter 8/Illumination 8.4 EQUIPMENT TYPE AND LOCATION To determine the appropriate lighting equipment and pole location, a photo- metric analysis is required. The number of fixed objects in the public right-of-way adjacent to a roundabout should be considered when identifying optimal locations for lighting poles; fewer poles with higher intensity light fixtures minimize the number of fixed objects. The type of area should also be considered when deter- mining the equipment type and location. In an urban area with a high level of pedestrian activity, it may be more appropriate to install illumination at lower mounting heights. In these cases, the illumination at lower mounting heights may need to be supplemented with taller, cobra-style assemblies to ensure adequate lighting is provided within the key conflict areas. 8.4.1 EQUIPMENT TYPE A sample of typical illumination equipment types used at roundabouts is shown in Exhibit 8-2. The illumination equipment shown in Exhibit 8-2 can vary depending on the project and the specific jurisdiction. The appropriate agency staff and the power company for a particular jurisdiction can likely provide additional guidance on the type of illumination equipment that is recommended or even required. Exhibit 8-2 Common Types of Illumination Equipment Used at Roundabouts Type of Lighting Assembly Typical Wattage Typical Distribution Common Mounting Height Cobra-style 75 W–400 W HPS Type II or III (full or semi cutoff) 30 to 50 ft (9 to 15 m) Ornamental 75 W–200 W HPS Type V(360° spread) 14 to 20 ft (4 to 6 m) High-Mast 400 W–1,000 W HPS Type V (360° spread) 50 to 100 ft (15 to 30 m) W = watts; HPS = High Pressure Sodium Source: Kansas Roundabout Guide (9) Exhibit 8-3 illustrates example photographs of a range of equipment types used at roundabouts. Exhibit 8-3(a) shows a roundabout lit only with cobra-style assemblies (pole, arm, and light fixture), and Exhibit 8-3(b) shows a roundabout with a mix of cobra-style assemblies and shorter, ornamental assemblies. As the proportion of lights with lower mounting heights increases, the visibility of the central island from a distance decreases. 8.4.2 POLE LOCATIONS The ability to provide adequate visibility at a roundabout is highly dependent on the illumination pole locations. Roundabout lighting can be achieved by installing lighting within the central island or around the perimeter of the inter-

Chapter 8/Illumination Page 8-7 Roundabouts: An Informational Guide section. The IES Design Guide for Roundabout Lighting recommends lighting be placed around the perimeter of the roundabout and at locations on the approach side of the crosswalks. Perimeter illumination provides the most optimal visibility within the key conflict areas and visibility of circulating vehicles to vehicles approaching the roundabout. In addition, the vertical lighting level in the cross- walks cannot be achieved unless approach lighting is used (1). Therefore, roundabouts with central island illumination may require additional approach lighting or may be combined with perimeter illumination to achieve vertical light- ing levels. Exhibit 8-4 summarizes some of the key advantages and disadvantages for each type of illumination design. Exhibit 8-5 shows the distinct differences in the illumination on the central island and circulatory roadway between central and perimeter illumination. Both illumination designs include approach lighting. The position of lighting poles relative to the curbs at a roundabout should also be considered and is influenced by the speed environment and the potential Exhibit 8-3 Styles of Lighting Assemblies (a) Cobra-style (Loveland, Colorado) (b) Mixed ornamental and cobra-style (Bend, Oregon)

Roundabouts: An Informational Guide Page 8-8 Chapter 8/Illumination Illumination Type Advantages Disadvantages Perimeter illumination Illumination can be strongest around critical bicycle and pedestrian areas. Continuity of poles and luminaires is maintained for the illumination of the lanes, as well as good visual guidance on the circulatory roadway. Approach signs typically appear in positive contrast and thus are clearly visible. Maintenance of luminaires is easier due to curbside location. Illumination is weakest in central island, which may limit visibility of roundabout from a distance. More poles are required to achieve the same illumination level. Poles may need to be located in critical conflict areas to achieve illumination levels and uniformity. Central illumination Perception of the roundabout is assisted at a distance by illuminating the central island. Fewer poles are required to achieve the same illumination. Pole in central island is clear of critical conflict areas for all but the smallest of roundabouts. Exit guide signs on the periphery appear in positive contrast (front lit) and thus are clearly visible. Cannot achieve adequate vertical lighting levels without additional approach lighting. Illumination is weakest in critical pedestrian and bicycle areas. Signs on the approach are in negative contrast (back lit). A path is needed to the base of the central pole for maintenance. There is a greater risk of glare. The central pole affects central island landscaping plan. High mast lighting may be inappropriate in urban areas, especially residential areas. Source: Adapted from Kansas Roundabout Guide (9) Central Illumination Design Perimeter Illumination Design Exhibit 8-4 Summary of Advantages and Disadvantages of Perimeter and Central Illumination Exhibit 8-5 Photometric Illustration of Central and Perimeter Illumination Design

Chapter 8/Illumination Page 8-9 Roundabouts: An Informational Guide speeds of errant vehicles. In particular, care should be given to placement of poles along the exit leg of the roundabout to consider potential paths of errant vehicles that may not successfully navigate the exit curvature upon leaving the circulatory roadway. Single-vehicle crash rates involving out-of-control vehicles at round- abouts are high compared to other intersection types. Therefore, it is desirable to have adequate amounts of clear zone where there are no roadside hazards on each side of the roadway. The reader is encouraged to refer to the AASHTO Roadside Design Guide (10) and Policy on Geometric Design of Highways and Streets (11) for further discussion on clear-zone requirements. 8.4.3 EXAMPLE ILLUMINATION LAYOUTS Exhibit 8-6 through Exhibit 8-8 present sample illumination plans demonstrat- ing layouts using various types of lighting assemblies. Each illumination plan has been customized to the specific geometry of the roundabout, photometric require- ments, equipment options, and site constraints. Caution is advised if attempting to adapt these plans to another location. Exhibit 8-6 Example of Illumination Using Cobra-Style Luminaires Inscribed Circle Diameter: 190 ft (58 m) Equipment: Luminaires over circulatory roadway: 400 W HPS, Type M-C-III, 37 ft (11.2 m) mounting height Remainder: 200 W HPS, Type M-C-III, 35 ft (10.7 m) mounting height Photometric Requirements: Avg. illuminance: 2.6 fc (26 lux) Avg./min. uniformity: 3:1 Layout: Source: Kansas Roundabout Guide (9)

Roundabouts: An Informational Guide Page 8-10 Chapter 8/Illumination Inscribed Circle Diameter: 140 ft (43 m) Equipment: Cobras over circulatory roadway: 200 W HPS, Type M-C-III, 30 ft (9.1 m) mounting height Pedestrian-level luminaires: 200 W HPS, Type V,14 ft (4.3 m) mounting height Photometric Requirements: Avg. illuminance: 2.0 fc (20 lux) Avg./min. uniformity: 3:1 Layout: Source: Kansas Roundabout Guide (9) Inscribed Circle Diameter: 120 ft (37 m) Equipment: Pedestrian-level luminaires: 250 W HPS, Type V, 18 ft (5.5 m) mounting height Photometric Requirements: Avg. illuminance: 2.7 fc (27 lux) Avg./min. uniformity: 3:1 Layout: Source: Kansas Roundabout Guide (9) Exhibit 8-7 Example of Illumination Using Pedestrian-Level Luminaires Exhibit 8-8 Example of Illumination Using a Mix of Cobra-Style and Pedestrian-Level Luminaires

Chapter 8/Illumination Page 8-11 Roundabouts: An Informational Guide 8.5 REFERENCES 1. Design Guide for Roundabout Lighting. Publication IES DG-19-08. Illuminating Engineering Society of North America, New York, February 2008. 2. An Information Guide for Roadway Lighting. AASHTO, Washington, D.C., 1985. 3. Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals. AASHTO, Washington, D.C., 1994. 4. Roadway Lighting Design Guide. AASHTO, Washington, D.C., 2005. 5. American National Standard Practice for Roadway Lighting. Standard RP-8-00. Illuminating Engineering Society of North America, New York, 2005. 6. Guide for the Design of Roadway Lighting, 2006 ed. Transportation Association of Canada (TAC), Ottawa, Ontario, Canada, 2006. 7. L’Éclairage des Carrefours à Sens Giratoire (The Illumination of Roundabout Intersections). Centre d’Etudes sur les Réseaux les Transports, l’Urbanisme et les constructions publiques (CERTU), Lyon, France, 1991. 8. Road Lighting, Part 1.3: Vehicular Traffic (Category V) Lighting—Guide to Design, Installation, Operation and Maintenance. Australian/New Zealand Standard AS/NZS 1158.1.3:1997. Standards Australia, Homebush, Australia, and Standards New Zealand, Wellington, New Zealand, 1997. 9. Kittelson & Associates, Inc., and TranSystems Corporation. Kansas Roundabout Guide: A Supplement to FHWA’s Roundabouts: An Informational Guide. Kansas Department of Transportation, Topeka, Kansas, 2003. 10. Roadside Design Guide. AASHTO, Washington, D.C., 2006. 11. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington, D.C., 2004.

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Roundabouts: An Informational Guide – Second Edition Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 672: Roundabouts: An Informational Guide – Second Edition explores the planning, design, construction, maintenance, and operation of roundabouts. The report also addresses issues that may be useful in helping to explain the trade-offs associated with roundabouts.

This report updates the U.S. Federal Highway Administration’s Roundabouts: An Informational Guide, based on experience gained in the United States since that guide was published in 2000.

Errata

Equation 6-3 on page 6-58 incorrectly contains an addition sign (+) as an operator. The correct operator should be a subtraction sign (-).

Errata #2

Exhibit 5-23 on page 5-29 of NCHRP Report 672 contains an incorrect calculation of estimated injury crashes.

There is a summary document, Paths to Practice, available.

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