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Suggested Citation:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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:"Glossary." 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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Glossary Page 1 Roundabouts: An Informational Guide GLOSSARY 85th-percentile speed—a speed value obtained from a set of field-measured speeds where only 15% of the observed speeds are greater (source: HCM). A AADT—see average annual daily traffic. AASHO—American Association of State Highway Officials. Predecessor to AASHTO. AASHTO—American Association of State Highway and Transportation Officials. accessible—describes a site, building, facility, or portion thereof that complies with the Americans with Disabilities Act Accessibility Guidelines (source: ADAAG). accessible route—a continuous, unobstructed path connecting all accessible elements and spaces of a building or facility. Exterior accessible routes may include parking access aisles, curb ramps, crosswalks at vehicular ways, walks, ramps, and lifts (source: ADAAG). accident—see crash. ADA—Americans with Disabilities Act. ADAAG—Americans with Disabilities Act Accessibility Guidelines. all-way stop control—all approaches at the intersections have stop signs where all drivers must come to a complete stop. The decision to proceed is based in part of the rules of the road, which suggest that the driver on the right has the right-of-way, and also on the traffic conditions on the other approaches (source: HCM). angle, entry—see entry angle. approach—the portion of a roadway leading into a roundabout. approach capacity—the capacity provided at the yield line during a specified period of time. approach curvature—a series of progressively sharper curves used on an approach to slow traffic to a safe speed prior to reaching the yield line. approach road half width—term used in the United Kingdom regression models. The approach half width is measured at a point in the approach upstream from any entry flare, from the median line or median curb to the nearside curb along a line perpendicular to the curb. See also approach width. (source: UK Geometric Design of Roundabouts) approach speed—the posted or 85th-percentile speed on an approach prior to any geometric or signing treatments designed to slow speeds.

Roundabouts: An Informational Guide Page 2 Glossary approach width—the width of the roadway used by approaching traffic upstream of any changes in width associated with the roundabout. The approach width is typically no more than half the total roadway width. apron—the mountable portion of the central island adjacent to the circulatory roadway. Used in some roundabouts to accommodate the wheel tracking of large vehicles. average annual daily traffic—the total volume passing a point or segment of a highway facility in both directions for one year divided by the number of days in the year (source: HCM). average effective flare length—term used in the United Kingdom regression models. Defined by a geometric construct and is approximately equivalent to the length of flare that can be effectively used by vehicles. (source: UK Geometric Design of Roundabouts) AWSC—see all-way stop control. B back of queue—the distance between the yield line of a roundabout and the farthest reach of an upstream queue, expressed as a number of vehicles. The vehi- cles previously stopped at the front of the queue may be moving (adapted from HCM). benefit–cost analysis—a method of economic evaluation that uses the benefit–cost ratio as the measure of effectiveness. benefit–cost ratio—the difference in benefits between an alternative and the no-build scenario, divided by the difference in costs between the alternative and the no-build scenario. See also incremental benefit–cost ratio. bulb-out—see curb extension. C capacity—the maximum sustainable flow rate at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a specified time period under a given roadway and geometric, traffic, environmental, and control conditions. Usually expressed as vehicles per hour, passenger cars per hour, or persons per hour (source: HCM). capacity, approach—see approach capacity. capacity, roundabout—see roundabout capacity. capital recovery factor—a factor that converts a present value cost into an annualized cost over a period of n years using an assumed discount rate of i percent. central island—the raised area in the center of a roundabout around which traffic circulates. CFR—Code of Federal Regulations. channelization—the separation or regulation of conflicting traffic movements into definite paths of travel by traffic islands or pavement marking to facilitate the

Glossary Page 3 Roundabouts: An Informational Guide safe and orderly movements of both vehicles and pedestrians (source: AASHTO Green Book). circle, inscribed—see inscribed circle. circular intersection—an intersection that vehicles traverse by circulating around a central island. circulating flow rate—the total volume in a given period of time on the circu- latory roadway immediately prior to an entrance, expressed as vehicles per hour. circulating path radius—the minimum radius on the fastest through path around the central island. circulating traffic—vehicles located on the circulatory roadway. circulating volume—the total volume in a given period of time on the circula- tory roadway immediately prior to an entrance. circulatory roadway—the curved path used by vehicles to travel in a counter- clockwise fashion around the central island. circulatory roadway width—the width between the outer edge of the circula- tory roadway and the central island, not including the width of any apron. circulating speed—the speed vehicles travel at while on the circulatory roadway. conflict point—a location where the paths of two vehicles, or a vehicle and a bicycle or pedestrian, merge, diverge, cross, or queue behind each other. conflict, crossing—see crossing conflict. conflict, diverge—see diverge conflict. conflict, merge—see merge conflict. conflict, queuing—see queuing conflict. conflicting flows—the two paths that merge, diverge, cross, or queue behind each other at a conflict point. control delay—delay experienced by vehicles at an intersection due to move- ments at slower speeds and stops on approaches as vehicles move up in the queue. crash—a collision between a vehicle and another vehicle, a pedestrian, a bicycle, or a fixed object. crash frequency—the average number of crashes at a location per period of time. crash rate—the number of crashes at a location or on a roadway segment, divided by the number of vehicles entering the location or by the length of the segment. CRF—see capital recovery factor. crossing conflict—the intersection of two traffic streams, including pedestri- ans. Crossing conflicts are the most severe type of conflict.

Roundabouts: An Informational Guide Page 4 Glossary curb extension—the construction of curbing such that the width of a street is reduced. Often used to provide space for parking or a bus stop or to reduce pedestrian crossing distances. curb ramp—a short ramp cutting through a curb or built up to it (source: ADAAG). curvature, approach—see approach curvature. D D factor—the proportion of the two-way traffic assigned to the peak direction. deflection—the change in trajectory of a vehicle imposed by geometric features of the roadway. degree of saturation—see volume-to-capacity ratio. delay—additional travel time experienced by a driver, passenger, or pedes- trian beyond what would reasonably be desired for a given trip. delay, control—see control delay. delay, geometric—see geometric delay. demand flow—the number of vehicles or persons that would like to use a roadway facility during a specified period of time. departure width—the width of the roadway used by departing traffic down- stream of any changes in width associated with the roundabout. The departure width is typically no more than half the total roadway width. design user—any user (motorized or non-motorized) that can reasonably be anticipated to use a facility. design vehicle—the largest vehicle that can reasonably be anticipated to use a facility. detectable warning surface—a standardized surface feature built in or applied to walking surfaces or other elements to warn visually impaired people of hazards on a circulation path (source: ADAAG). diameter, inscribed circle—see inscribed circle diameter. distance, set-back—see set-back distance. diverge conflict—the separation of two traffic streams, typically the least severe of all conflicts. divisional island—see splitter island double-lane roundabout—a roundabout that has at least one entry with two lanes, and a circulatory roadway that can accommodate more than one vehicle traveling side-by-side. downstream—the direction toward which traffic is flowing (source: HCM).

Glossary Page 5 Roundabouts: An Informational Guide E entering traffic—vehicles located on a roundabout entrance. entering volume—the total volume in a given period of time on an entrance to a roundabout. entrance line—a pavement marking used to mark the point of entry from an approach into the circulatory roadway and generally marked along the inscribed circle. If necessary, entering traffic must yield to circulating traffic before crossing this line into the circulatory roadway. entry angle—term used in the United Kingdom regression models. It serves as a geometric proxy for the conflict angle between entering and circulating streams and is determined through a geometric construct. (source: UK Geometric Design of Roundabouts) entry flare—the widening of an approach to multiple lanes to provide addi- tional capacity at the yield line and storage. entry flow—see entering volume. entry path curvature—term used in the United Kingdom to describe a meas- ure of the amount of entry deflection to the right imposed on vehicles at the entry to a roundabout. (source: UK Geometric Design of Roundabouts) entry path radius—the minimum radius on the fastest through path prior to the yield line. entry radius—the minimum radius of curvature of the outside curb at the entry. entry speed—the speed a vehicle is traveling at as it crosses the yield line. entry width—the width of the entry where it meets the inscribed circle, mea- sured perpendicularly from the right edge of the entry to the intersection point of the left edge line and the inscribed circle. entry, perpendicular—see perpendicular entry. exit path radius—the minimum radius on the fastest through path into the exit. exit radius—the minimum radius of curvature of the outside curb at the exit. exit width—the width of the exit where it meets the inscribed circle, mea- sured perpendicularly from the right edge of the exit to the intersection point of the left edge line and the inscribed circle. exiting traffic—vehicles departing a roundabout by a particular exit. extended splitter island—see splitter island, extended. F FHWA—Federal Highway Administration. flare—see entry flare. flare, entry—see entry flare. flow, circulating—see circulating volume.

Roundabouts: An Informational Guide Page 6 Glossary flow, demand—see demand flow. flow, entry—see entering volume. flows, conflicting—see conflicting flows. G geometric delay—the delay caused by the alignment of the lane or the path taken by the vehicle on a roadway or through an intersection. geometric design—a term used in this document to describe the design of horizontal and vertical alignment and cross-sectional elements of a roadway. give way—term used in the United Kingdom and Australia for yield. give way rule—rule adopted in the United Kingdom in November 1966 that required that all vehicles entering a roundabout give way, or yield, to circulating vehicles. H HCM—Highway Capacity Manual. I IES—Illuminating Engineers Society. incremental benefit–cost ratio—the difference in benefits between two alter- natives divided by the difference in costs between the two alternatives. See also benefit–cost ratio. inscribed circle—the circle forming the outer edge of the circulatory roadway. inscribed circle diameter—the basic parameter used to define the size of a roundabout, measured between the outer edges of the circulatory roadway. It is the diameter of the largest circle that can be inscribed within the outline of the intersection. interchange—a grade-separated junction of two roadways where movement from one roadway to the other is provided for. intersection—an at-grade junction of two or more roadways. intersection sight distance—the distance required for a driver without the right-of-way to perceive and react to the presence of conflicting vehicles. island, central—see central island. island, median—see splitter island. island, separator—see splitter island. island, splitter—see splitter island. ITE—Institute of Transportation Engineers.

Glossary Page 7 Roundabouts: An Informational Guide K KABCO—a severity scale used by the investigating police officer on the scene to classify injury severity for occupants with five categories: K, killed; A, disabling injury; B, evident injury; C, possible injury; O, no apparent injury. These defini- tions may vary slightly for different police agencies. (Source: National Safety Council, 1990) K factor—the proportion of the AADT assigned to the design hour. L left-turn path radius—the minimum radius on the fastest path of the conflict- ing left-turn movement. level of service—a qualitative measure describing operational conditions within a traffic stream, generally described in terms of service measures such as speed and travel time, freedom to maneuver, traffic interruptions, comfort, and convenience. line, entrance—see entrance line. line, yield—see yield line. locking—stoppage of traffic on the circulatory roadway caused by queuing backing into the roundabout from one of the exits, resulting in traffic being unable to enter or circulate. LOS—see level of service. M maximum service volume—the maximum hourly rate at which vehicles, bicycles, or persons can be reasonably expected to traverse a point or uniform section of a roadway during an hour under specific assumed conditions while maintaining a designated level of service. (source: HCM) measures of effectiveness—a quantitative parameter whose value is an indi- cator of the performance of a transportation facility or service from the perspective of the users of the facility or service. median island—see splitter island. merge conflict—the joining of two traffic streams. mini-roundabout—small roundabouts used in low-speed urban environ- ments. The central island is fully mountable, and the splitter islands are either painted or mountable. modern roundabout—a term used to distinguish newer circular intersections conforming to the characteristics of roundabouts from older-style rotaries and traffic circles. mountable—used to describe geometric features that can be driven upon by vehicles without damage, but not intended to be in the normal path of traffic.

Roundabouts: An Informational Guide Page 8 Glossary multilane roundabout—a roundabout that has at least one entry with two or more lanes, and a circulatory roadway that can accommodate more than one vehi- cle traveling side-by-side. MUTCD—Manual on Uniform Traffic Control Devices. N NCUTCD—National Committee on Uniform Traffic Control Devices. neighborhood traffic circle—a circular intersection constructed at the inter- section of two local streets for traffic calming and/or aesthetic purposes. They are generally not channelized, may be uncontrolled or stop-controlled, and may allow left turns to occur left (clockwise) of the central island. non-conforming traffic circle—see traffic circle. non-traversable—see raised. O O&M costs—operations and maintenance costs. P peak hour factor—the hourly volume during the maximum volume hour of the day divided by the peak 15-minute flow rate within the peak hour; a measure of traffic demand fluctuation within the peak hour. pedestrian refuge—an at-grade opening within a median island that allows pedestrians to safely wait for an acceptable gap in traffic. perpendicular entry—an entry angle of 70 degrees or more. PHF—see peak hour factor. platoon—a group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily because of signal control, geometrics, or other factors. point, conflict—see conflict point. priority—the assignment of right-of-way to a particular traffic stream or movement. progression, signal—see signal progression. Q queue—a line of vehicles, bicycles, or persons waiting to be served by the system in which the flow rate from the front of the queue determines the average speed within the queue. Slowly moving vehicles or persons joining the rear of the queue are usually considered a part of the queue. The internal queue dynamics may involve a series of starts and stops. (source: HCM) queuing conflict—a conflict that arises within a traffic stream between a lead vehicle and a following vehicle, when the lead vehicle must come to a stop.

Glossary Page 9 Roundabouts: An Informational Guide R radius, circulating path—see circulating path radius. radius, entry—see entry radius. radius, entry path—see entry path radius. radius, exit—see exit radius. radius, exit path—see exit path radius. radius, left-turn path—see left-turn path radius. radius, right-turn path—see right-turn path radius. raised—used to describe geometric features with a sharp elevation change that are not intended to be driven upon by vehicles at any time. ramp, wheelchair—see curb ramp. refuge, pedestrian—see pedestrian refuge. right-of-way—(1) an intersection user that has priority over other users. (2) Land owned by a public agency for transportation uses. right-turn bypass lane—a lane provided adjacent to, but separated from, the circulatory roadway, that allows right-turning movements to bypass the round- about. Also known as a right-turn slip lane. right-turn path radius—the minimum radius on the fastest path of a right- turning vehicle. right-turn slip lane—see right-turn bypass lane. roadway, circulatory—see circulatory roadway. rotary—a term used particularly in the Eastern United States to describe an older-style circular intersection that does not have one or more of the characteris- tics of a roundabout. They often have large diameters, often in excess of 300 ft (100 m), allowing high travel speeds on the circulatory roadway. Also known as a traffic circle. roundabout—an intersection with a generally circular shape, yield control of all entering traffic, and geometric curvature and features to induce desirable vehicular speeds. roundabout capacity—the maximum number of entering vehicles that can be reasonably expected to be served by a roundabout during a specified period of time. roundabout, modern—see modern roundabout. roundabout, multilane—see multilane roundabout. roundabout, single lane—see single-lane roundabout. S separator island—see splitter island. service volume—the hourly rate at which vehicles, bicycles, or persons can be reasonably expected to traverse a point or uniform section of a roadway during an

Roundabouts: An Informational Guide Page 10 Glossary hour under specific assumed conditions. See also maximum service volume. (Adapted from HCM) set-back distance—the distance between the edge of the circulatory roadway and the sidewalk. sharpness of flare—a measure of the rate at which extra width is developed in the entry flare. (source: UK Geometric Design of Roundabouts) sight distance, intersection—see intersection sight distance. sight distance, stopping—see stopping sight distance. sight triangle—an area required to be free of obstructions to enable visibility between conflicting movements. signal progression—the use of coordinated traffic signals along a roadway in order to minimize stops and delay to through traffic on the major road. single-lane roundabout—a roundabout that has single lanes on all entries and one circulatory lane. speed table—an extended, flat-top road hump sometimes used at pedestrian crossings to slow traffic and to provide a better visual indication of the crosswalk location. speed, approach—see approach speed. speed, circulating—see circulating speed. speed, entry—see entry speed. splitter island—a raised or painted area on an approach used to separate entering from exiting traffic, deflect and slow entering traffic, and provide storage space for pedestrians crossing that intersection approach in two stages. Also known as a median island or a separator island. splitter island, extended—a raised splitter island that begins some distance upstream of the pedestrian crossing to separate entering and exiting traffic. A design feature of rural single-lane roundabouts. stopping sight distance—the distance along a roadway required for a driver to perceive and react to an object in the roadway and to brake to a complete stop before reaching that object. T traffic calming—geometric treatments used to slow traffic speeds or to dis- courage the use of a roadway by non-local traffic. traffic circle—a circular intersection that does not have one or more of the characteristics of a roundabout. Also known as a rotary. traffic circle, neighborhood—see neighborhood traffic circle. traffic circle, non-conforming—see traffic circle. traffic, circulating—see circulating traffic.

Glossary Page 11 Roundabouts: An Informational Guide traffic, entering—see entering traffic. truck apron—see apron. two-stage crossing—a process in which pedestrians cross a roadway by cross- ing one direction of traffic at a time, waiting in a pedestrian refuge between the two traffic streams if necessary before completing the crossing. two-way stop-control—stop signs are present on the approach(es) of the minor street, and drivers on the minor street or a driver turning left from the major street wait for a gap in the major street traffic to complete a maneuver. TWSC—see two-way stop control. U U-turn—a turning movement at an intersection in which a vehicle departs the intersection using the same roadway it used to enter the intersection. upstream—the direction from which traffic is flowing (source: HCM). UVC—Uniform Vehicle Code. V vehicle, design—see design vehicle. volume, circulating—see circulating volume. volume, entering—see entering volume. volume, service—see service volume. volume-to-capacity ratio—the ratio of flow rate to capacity for a transporta- tion facility. W wheelchair ramp—see curb ramp. width, approach—see approach width. width, circulatory roadway—see circulatory roadway width. width, departure—see departure width. width, entry—see entry width. width, exit—see exit width. Y yield—an intersection control in which controlled traffic must stop only if higher priority traffic is present. yield line—a pavement marking used to mark the point of yielding at a roundabout entry. See also entrance line. Z zebra crossing—a crossing marked by transverse white stripes where vehicles are required to yield to pedestrians.

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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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