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Glossary Page 1 Roundabouts: An Informational Guide GLOSSARY 85th-percentile speedâa speed value obtained from a set of field-measured speeds where only 15% of the observed speeds are greater (source: HCM). A AADTâsee average annual daily traffic. AASHOâAmerican Association of State Highway Officials. Predecessor to AASHTO. AASHTOâAmerican Association of State Highway and Transportation Officials. accessibleâdescribes a site, building, facility, or portion thereof that complies with the Americans with Disabilities Act Accessibility Guidelines (source: ADAAG). accessible routeâa 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). accidentâsee crash. ADAâAmericans with Disabilities Act. ADAAGâAmericans with Disabilities Act Accessibility Guidelines. all-way stop controlâall 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, entryâsee entry angle. approachâthe portion of a roadway leading into a roundabout. approach capacityâthe capacity provided at the yield line during a specified period of time. approach curvatureâa 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 widthâterm 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 speedâthe 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 widthâthe 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. apronâthe 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 trafficâthe 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 lengthâterm 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) AWSCâsee all-way stop control. B back of queueâthe 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). benefitâcost analysisâa method of economic evaluation that uses the benefitâcost ratio as the measure of effectiveness. benefitâcost ratioâthe 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 benefitâcost ratio. bulb-outâsee curb extension. C capacityâthe 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, approachâsee approach capacity. capacity, roundaboutâsee roundabout capacity. capital recovery factorâa 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 islandâthe raised area in the center of a roundabout around which traffic circulates. CFRâCode of Federal Regulations. channelizationâthe 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, inscribedâsee inscribed circle. circular intersectionâan intersection that vehicles traverse by circulating around a central island. circulating flow rateâthe 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 radiusâthe minimum radius on the fastest through path around the central island. circulating trafficâvehicles located on the circulatory roadway. circulating volumeâthe total volume in a given period of time on the circula- tory roadway immediately prior to an entrance. circulatory roadwayâthe curved path used by vehicles to travel in a counter- clockwise fashion around the central island. circulatory roadway widthâthe width between the outer edge of the circula- tory roadway and the central island, not including the width of any apron. circulating speedâthe speed vehicles travel at while on the circulatory roadway. conflict pointâa location where the paths of two vehicles, or a vehicle and a bicycle or pedestrian, merge, diverge, cross, or queue behind each other. conflict, crossingâsee crossing conflict. conflict, divergeâsee diverge conflict. conflict, mergeâsee merge conflict. conflict, queuingâsee queuing conflict. conflicting flowsâthe two paths that merge, diverge, cross, or queue behind each other at a conflict point. control delayâdelay experienced by vehicles at an intersection due to move- ments at slower speeds and stops on approaches as vehicles move up in the queue. crashâa collision between a vehicle and another vehicle, a pedestrian, a bicycle, or a fixed object. crash frequencyâthe average number of crashes at a location per period of time. crash rateâthe 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. CRFâsee capital recovery factor. crossing conflictâthe 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 extensionâthe 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 rampâa short ramp cutting through a curb or built up to it (source: ADAAG). curvature, approachâsee approach curvature. D D factorâthe proportion of the two-way traffic assigned to the peak direction. deflectionâthe change in trajectory of a vehicle imposed by geometric features of the roadway. degree of saturationâsee volume-to-capacity ratio. delayâadditional travel time experienced by a driver, passenger, or pedes- trian beyond what would reasonably be desired for a given trip. delay, controlâsee control delay. delay, geometricâsee geometric delay. demand flowâthe number of vehicles or persons that would like to use a roadway facility during a specified period of time. departure widthâthe 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 userâany user (motorized or non-motorized) that can reasonably be anticipated to use a facility. design vehicleâthe largest vehicle that can reasonably be anticipated to use a facility. detectable warning surfaceâa 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 circleâsee inscribed circle diameter. distance, set-backâsee set-back distance. diverge conflictâthe separation of two traffic streams, typically the least severe of all conflicts. divisional islandâsee splitter island double-lane roundaboutâa 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. downstreamâthe direction toward which traffic is flowing (source: HCM).
Glossary Page 5 Roundabouts: An Informational Guide E entering trafficâvehicles located on a roundabout entrance. entering volumeâthe total volume in a given period of time on an entrance to a roundabout. entrance lineâa 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 angleâterm 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 flareâthe widening of an approach to multiple lanes to provide addi- tional capacity at the yield line and storage. entry flowâsee entering volume. entry path curvatureâterm 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 radiusâthe minimum radius on the fastest through path prior to the yield line. entry radiusâthe minimum radius of curvature of the outside curb at the entry. entry speedâthe speed a vehicle is traveling at as it crosses the yield line. entry widthâthe 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, perpendicularâsee perpendicular entry. exit path radiusâthe minimum radius on the fastest through path into the exit. exit radiusâthe minimum radius of curvature of the outside curb at the exit. exit widthâthe 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 trafficâvehicles departing a roundabout by a particular exit. extended splitter islandâsee splitter island, extended. F FHWAâFederal Highway Administration. flareâsee entry flare. flare, entryâsee entry flare. flow, circulatingâsee circulating volume.
Roundabouts: An Informational Guide Page 6 Glossary flow, demandâsee demand flow. flow, entryâsee entering volume. flows, conflictingâsee conflicting flows. G geometric delayâthe delay caused by the alignment of the lane or the path taken by the vehicle on a roadway or through an intersection. geometric designâa term used in this document to describe the design of horizontal and vertical alignment and cross-sectional elements of a roadway. give wayâterm used in the United Kingdom and Australia for yield. give way ruleârule adopted in the United Kingdom in November 1966 that required that all vehicles entering a roundabout give way, or yield, to circulating vehicles. H HCMâHighway Capacity Manual. I IESâIlluminating Engineers Society. incremental benefitâcost ratioâthe difference in benefits between two alter- natives divided by the difference in costs between the two alternatives. See also benefitâcost ratio. inscribed circleâthe circle forming the outer edge of the circulatory roadway. inscribed circle diameterâthe 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. interchangeâa grade-separated junction of two roadways where movement from one roadway to the other is provided for. intersectionâan at-grade junction of two or more roadways. intersection sight distanceâthe distance required for a driver without the right-of-way to perceive and react to the presence of conflicting vehicles. island, centralâsee central island. island, medianâsee splitter island. island, separatorâsee splitter island. island, splitterâsee splitter island. ITEâInstitute of Transportation Engineers.
Glossary Page 7 Roundabouts: An Informational Guide K KABCOâa 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 factorâthe proportion of the AADT assigned to the design hour. L left-turn path radiusâthe minimum radius on the fastest path of the conflict- ing left-turn movement. level of serviceâa 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, entranceâsee entrance line. line, yieldâsee yield line. lockingâstoppage 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. LOSâsee level of service. M maximum service volumeâthe 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 effectivenessâa 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 islandâsee splitter island. merge conflictâthe joining of two traffic streams. mini-roundaboutâsmall roundabouts used in low-speed urban environ- ments. The central island is fully mountable, and the splitter islands are either painted or mountable. modern roundaboutâa term used to distinguish newer circular intersections conforming to the characteristics of roundabouts from older-style rotaries and traffic circles. mountableâused 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 roundaboutâa 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. MUTCDâManual on Uniform Traffic Control Devices. N NCUTCDâNational Committee on Uniform Traffic Control Devices. neighborhood traffic circleâa 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 circleâsee traffic circle. non-traversableâsee raised. O O&M costsâoperations and maintenance costs. P peak hour factorâthe 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 refugeâan at-grade opening within a median island that allows pedestrians to safely wait for an acceptable gap in traffic. perpendicular entryâan entry angle of 70 degrees or more. PHFâsee peak hour factor. platoonâa group of vehicles or pedestrians traveling together as a group, either voluntarily or involuntarily because of signal control, geometrics, or other factors. point, conflictâsee conflict point. priorityâthe assignment of right-of-way to a particular traffic stream or movement. progression, signalâsee signal progression. Q queueâa 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 conflictâa 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 pathâsee circulating path radius. radius, entryâsee entry radius. radius, entry pathâsee entry path radius. radius, exitâsee exit radius. radius, exit pathâsee exit path radius. radius, left-turn pathâsee left-turn path radius. radius, right-turn pathâsee right-turn path radius. raisedâused to describe geometric features with a sharp elevation change that are not intended to be driven upon by vehicles at any time. ramp, wheelchairâsee curb ramp. refuge, pedestrianâsee pedestrian refuge. right-of-wayâ(1) an intersection user that has priority over other users. (2) Land owned by a public agency for transportation uses. right-turn bypass laneâa 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 radiusâthe minimum radius on the fastest path of a right- turning vehicle. right-turn slip laneâsee right-turn bypass lane. roadway, circulatoryâsee circulatory roadway. rotaryâa 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. roundaboutâan intersection with a generally circular shape, yield control of all entering traffic, and geometric curvature and features to induce desirable vehicular speeds. roundabout capacityâthe maximum number of entering vehicles that can be reasonably expected to be served by a roundabout during a specified period of time. roundabout, modernâsee modern roundabout. roundabout, multilaneâsee multilane roundabout. roundabout, single laneâsee single-lane roundabout. S separator islandâsee splitter island. service volumeâthe 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 distanceâthe distance between the edge of the circulatory roadway and the sidewalk. sharpness of flareâa measure of the rate at which extra width is developed in the entry flare. (source: UK Geometric Design of Roundabouts) sight distance, intersectionâsee intersection sight distance. sight distance, stoppingâsee stopping sight distance. sight triangleâan area required to be free of obstructions to enable visibility between conflicting movements. signal progressionâthe use of coordinated traffic signals along a roadway in order to minimize stops and delay to through traffic on the major road. single-lane roundaboutâa roundabout that has single lanes on all entries and one circulatory lane. speed tableâan 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, approachâsee approach speed. speed, circulatingâsee circulating speed. speed, entryâsee entry speed. splitter islandâa 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, extendedâa 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 distanceâthe 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 calmingâgeometric treatments used to slow traffic speeds or to dis- courage the use of a roadway by non-local traffic. traffic circleâa circular intersection that does not have one or more of the characteristics of a roundabout. Also known as a rotary. traffic circle, neighborhoodâsee neighborhood traffic circle. traffic circle, non-conformingâsee traffic circle. traffic, circulatingâsee circulating traffic.
Glossary Page 11 Roundabouts: An Informational Guide traffic, enteringâsee entering traffic. truck apronâsee apron. two-stage crossingâa 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-controlâstop 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. TWSCâsee two-way stop control. U U-turnâa turning movement at an intersection in which a vehicle departs the intersection using the same roadway it used to enter the intersection. upstreamâthe direction from which traffic is flowing (source: HCM). UVCâUniform Vehicle Code. V vehicle, designâsee design vehicle. volume, circulatingâsee circulating volume. volume, enteringâsee entering volume. volume, serviceâsee service volume. volume-to-capacity ratioâthe ratio of flow rate to capacity for a transporta- tion facility. W wheelchair rampâsee curb ramp. width, approachâsee approach width. width, circulatory roadwayâsee circulatory roadway width. width, departureâsee departure width. width, entryâsee entry width. width, exitâsee exit width. Y yieldâan intersection control in which controlled traffic must stop only if higher priority traffic is present. yield lineâa pavement marking used to mark the point of yielding at a roundabout entry. See also entrance line. Z zebra crossingâa crossing marked by transverse white stripes where vehicles are required to yield to pedestrians.