Traffic Systems Design
Traffic Systems Design CE 474
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Date Created: 10/23/15
GLOSSARY TABLE OF CONTENTS GLOSSARY GLOSSARY The following section is a collection of terms used in the manual Sources and references are identified for those terms with multiple definitions Accident modification factors A means of quantifying crash reductions associated with safety improvements Actuated Signal Control Phase time based on detection data Adaptive Signal Control A signal control concept where vehicular traffic in a network is detected at a point upstream andor downstream and an algorithm is used to predict when and where traffic will be and to make signal adjustments at downstream intersections based on those predictions Added Initial An interval that times concurrently with the minimum green interval and increases by each vehicle actuation received during the initial period This time cannot exceed the maximum initial Analysis period A single time period during which capacity analysis is performed on a transportation facility If the demand exceeds capacity during an analysis period consecutive analysis periods can be selected to account for initial queue from the previous analysis period Also referred to as time interval Analytical Model A model that relates system components using theoretical considerations tempered validated and calibrated by field data Annual average daily traffic The total volume oftraffic passing a point or segment of a highway facility in both directions for one year divided by the number of days in the year Approach A set of lanes at an intersection that accommodates all leftturn through and rightturn movements from a given direction Approach grade The grade of an intersection approach expressed as a percentage with positive values for upgrade and negative for downgrade Area type A geographic parameter reflecting the variation of saturation flows in different areas Arrival rate The mean of a statistical distribution of vehicles arriving at a point or uniform segment of a lane or roadway Arrival type Six assigned categories for determining the quality of progression at a signalized intersection Arterial A signalized street that primarily serves through traffic and that secondarily provides access to abutting properties with signal spacings of 20 miles or less Arterial LOS An arterial and networklevel performance measure associated with the class of arterial and the travel speed of arterial under study Automatic Vehicle Location AVL System An intelligent transportation system ITS technology to track vehicle location speed and other measures within a system Most applications are found on transit vehicles and systems Average Speed The average distance a vehicle travels within a measured amount of time Average travel speed The length of the highway segment divided by the average travel time of all vehicles traversing the segment including all stopped delay times Back of queue The distance between the stop line of a signalized intersection and the farthest reach of an upstream queue expressed as a number of vehicles The vehicles previously stopped at the front of the queue are counted even if they begin moving Bandwidth The maximum amount of green time for a designated direction as it passes through a corridor at an assumed constant speed typically measured in seconds Bandwidth attainability A measure of how well the bandwidth makes use of the available green time for the coordinated movements at the most critical intersection in the corridor Bandwidth efficiency A measure that normalizes bandwidth against the cycle length for the arterial under study Barnes Dance co mon term for an exclusive pedestrian phase where pedestrians may cross all intersections legs and sometimes diagonally Barrier A separation of intersecting movements in separate rings to prevent operating conflicting phases at the same time Base condition The best possible characteristic in terms of capacity for a given type of transportation facility that is further improvements would not increase capacity a condition without hindrances or delays Base saturation flow rate The maximum steady flow rate expressed in passenger cars per hour per lane at which previously stopped passenger cars can cross the stop line of a signalized intersection under base conditions assuming that the green signal is available and no lost times are experienced Call A term used to describe the presence of vehicle bicycle or pedestrian demand in an actuated detection controller system Capacity The maximum rate at which vehicles can pass through the intersection under prevailing conditions It is also the ratio of time during which vehicles may enter the intersection Carryover A term commonly used for the extend setting in controller manuals It is another way to describe the time provided for a vehicle to traverse from one detector to the next Change interval The yellow plus red clearance interval that occurs between phases of a traffic signal to provide for clearance of the intersection before conflicting movements are released Also known as the clearance interval Clearance lost time The time in seconds between signal phases during which an intersection is not used by any traffic Clearance time The time loss at a transit stop not including passenger dwell times This parameter can be the minimum time between one transit vehicle leaving a stop and the following vehicle entering and can include any delay waiting for a sufficient gap in traffic to allow the transit vehicle to reenter the travel lane Condition Diagram An illustration used to highlight the existing characteristics ie number of lanes signs adjacent driveways turnbay lengths traffic control and land uses of an intersection Concurrent Phases Two or more phases in separate rings that are able to operate together without conflicting movements Congested flow A traffic flow condition caused by a downstream bottleneck Control Delay The amount of additional travel time experienced by a user attributable to a control device Controller Memory A term that refers to the controllers ability to remember ie retain a detector actuation and includes one of two modes nonlocking or locking CoordinatedActuated Signal operations in coordination with other intersections and using vehicle bicycle andor pedestrian detection to define signal timing Coordinated Phases The phase or phases that is provided a fixed minimum amount of time each cycle under a coordinated timing plan This phase is typically the major through phase on an arterial Coordination The ability to synchronize multiple intersections to enhance the operation of one or more directional movements in a system Corridor A set of essentially parallel transportation facilities designed for travel between two points A corridor contains several subsystems such as freeways rural or twolane highways arterials transit and pedestrian and bicycle facilities Critical lane group The lane groups that have the highest flow ratio for a given signal phase Critical movement analysis A simplified technique for estimating phasing needs and signal timing parameters Critical speed The speed at which capacity occurs for a facility usually expressed as miles per hour Critical volumetocapacity ratio The proportion of available intersection capacity used by vehicles in critical lane groups Crosswalk A marked area for pedestrians crossing the street at an intersection or designated midblock location ycle A complete sequence of signal indications Cycle Length The time required for a complete sequence of signal indications Cycle Failure Occasion where all queued vehicular demand cannot be served by a single green indication or signal phase Dallas Display A type of signal display that attempts to avoid yellow trap problem by using louvers on the yellow and green ball indications to restrict visibility of the leftturn display to adjacent lanes while displaying indications based on the opposing through movement Delay 1 The additional travel time experienced by a driver passenger or pedestrian 2 A detector parameter typically used with stopline presence mode detection for turn movements from exclusive lanes Densit The number of vehicles on a roadway segment averaged over space usually expressed as vehicles per mile or vehicles per mile per lane see also volumedensity sometimes referred to as density timing Demand The volume of traffic at an intersection approach or movement Detector A device used to count andor determine the presence of a vehicle bicycle or pedestrian Dilemma Zone There are two types of dilemma zones Type occurs when yellow and red clearance times are too short for a driver to either stop or clear the intersection before the beginning of a conflicting phase Type II also known as an Option Zone ore Indecision Zone This occurs as the result of different drivers making different decision on whether to go or stop upon the change from a green to yellow indication Double Cycle A cycle length that allows phases to be serviced twice as often as the other intersections in the coordinated system Downstream The direction of traffic flow Early Return to Green A term used to describe the servicing of a coordinated phase in advance of its programmed begin time as a result of unused time from noncoordinated phases Effective green time The time during which a given traffic movement or set of movements may proceed it is equal to the cycle length minus the effective red time Effective red time The time during which a given traffic movement or set of movements is directed to stop it is equal to the cycle length minus the effective green time Effective walkway width The width in feet of a walkway usable by pedestrians or the total walkway width minus the width of unusable buffer zones along the curb and building line Exclusive pedestrian phase An additional phase that is configured such that no vehicular movements are served concurrently with pedestrian traffic See also Barnes Dance Exclusive turn lane A designated left or rightturn lane or lanes used only by vehicles making those turns Extend A detector parameter that extends a detector actuation by a setable fixed amount It is typically used with detection designs that combine multiple advance detectors and stopline detection for safe phase termination of highspeed intersection approaches Field Implementation A term used to describe the installation of new signal timings in the controller and the review of traffic operations at the intersection Fixed Force Off A force off mode where force off points cannot move Under this mode noncoordinated phases can utilize unused time of previous phases Fixed Time Signal Control A preset time is given to each movement every cycle regardless of changes in traffic conditions Flashing Don t Walk An indication warning pedestrians that the walk indication has ended and the don t walk indication will begin at the end ofthe pedestrian clearance interval Flashing Yellow Arrow A type of signal head display that attempts to avoid the yellow trap problem by providing a permissive indication to the driver that operates concurrent with the opposing through movement rather than the adjacent through movement Floating Force Off A force off mode where force off points can move depending on the demand of previous phases Under this mode noncoordinated phase times are limited to their defined split amount of time and all unused time is dedicated to the coordinated phase Essentially the split time is treated as a maximum amount for the noncoordinated phases Floating car method commonly employed technique for travel time runs which requires the vehicle driver to float with the traffic stream while traveling at a speed that is representative of the other vehicles on the roadway and to pass as many vehicles as pass the floating car Flow rate The equivalent hourly rate at which vehicles bicycles or persons pass a point on a lane roadway or other trafficway computed as the number of vehicles bicycles or persons passing the point divided by the time interval usually less than 1 h in which they pass expressed as vehicles bicycles or persons per hour Flow ratio The ratio of the actual flow rate to the saturation flow rate for a lane group at an intersection Force Off A point within a cycle where a phase must end regardless of continued demand These points in a coordinated cycle ensure that the coordinated phase returns in time to maintain its designated offset Free flow A flow of traffic unaffected by upstream or downstream conditions Fully actuated control A signal operation in which vehicle detectors at each approach to the intersection control the occurrence and length of every phase Gap The time in seconds for the front bumper of the second of two successive vehicles to reach the starting point ofthe front bumper ofthe first Gap Reduction This is a feature that reduces the passage time to a smallervalue while the phase is active Green time The duration in seconds of the green indication for a given movement at a signalized intersection Green time ratio The ratio ofthe effective green time of a phase to the cycle length Green Extension A signal priority treatment to extend a current green phase to give priority to a specific movement or vehicle typically transit Hardware The devices that physically operate the signal timing controls including the controller detectors signal heads and conflict monitor Headway 1 The time in seconds between two successive vehicles as they pass a point on the roadway measured from the same common feature of both vehicles for example the front axle or the front bumper 2 The time usually expressed in minutes between the passing of the front ends of successive transit units vehicles or trains moving along the same lane or track or other guideway in the same direction Hardware in the Loop HlTL A ans of providing a direct linkage between simulation models and actual signal controllers Highway Capacity Manual A National Academies of ScienceTransportation Research Board manual containing a collection of stateoftheart techniques for estimating the capacity and determining the levelofservice for transportation facilities including intersections and roadways as well as facilities for transit bicycles and pedestrians Inhibit Max A basic timing parameter that removes the Maximum Green input as a phase parameter during coordination and allows the phase to extend beyond its normal maximum green values Interval The duration of time where a traffic signal indications do not change state red yellow green flashing don t walk A traffic signal controller also has timing intervals min green passage time that determine the length ofthe green interval Intersection Delay Average The total additional travel time experienced by users as a result of control measures and interactions with other users divided by the volume departing from the intersection Intersection Level of Service A qualitative measure describing operational conditions based on average intersection delay Isolated intersection An intersection at least one mile from the nearest upstream signalized intersection Lagging pedestrian interval pedestrian timing option that starts pedestrian walk interval several seconds after the adjacent through movement phase thus allowing a waiting rightturn queue to clear before the pedestrian walk indication is presented and thereby reducing conflicts with rightturning vehicles Lane group A set of lanes established at an intersection approach for separate capacity and levelof service analysis Lane group delay The control delay for a given lane group Lane utilization The distribution of vehicles among lanes when two or more lanes are available for a movement however as demand approaches capacity uniform lane utilization develops Leading pedestrian interval A pedestrian interval option that starts a few seconds before the adjacent through movement phase thus allowing pedestrians to establish a presence in the crosswalk and thereby reducing conflicts with turning vehicles LeadLag LeftTurn Phasing A leftturn phase sequence where one leftturn movement begins with the adjacent through movement and the opposing leftturn movement begins at the end ofthe conflicting through movement This option may create a yellow trap with some permissive signal displays Level of service A qualitative measure describing operational conditions within a traffic stream based on service measures such as speed and travel time freedom to maneuver traffic interruptions comfort and convenience Local Controller he device used to operate and control the signal displays using signal timing provided by the user master controller or central signal system Locking mode A controller memory mode used to trigger a call for service for the first actuation received by the controller on a specified channel during the red interval Lost Time The portion of time at the beginning of each green period and a portion of each yellow change plus red clearance period that is not usable by vehicles Master Clock The background timing mechanism within the controller logic to which each controller is referenced during coordinated operations Master Controller An optional component of a signal system that facilitates coordination of a signal system with the local controller Manual on Traffic Control Devices MUTCD The MUTCD published by the Federal Highway Administration provides the standards and guidance for installation and maintenance for traffic control devices on roadways Maximum Allowable Headway MAH I Maximum Time Separation The maximum time separation between vehicle calls on an approach without gapping out the phase typically defined by passage time or gap time Maximum allowable headway refers to spacing between common points of vehicles in a single lane but the term is commonly used to refer to maximum time separation in single or multilane approaches as well Maximum Green The maximum length oftime that a phase can be green in the presence of a conflicting call Maximum Initial The maximum period of time for which the Added lnitial can extend the initial green period This cannot be less than the Minimum Green time Maximum Recall A recall mode that places a continuous call on a phase Measure of effectiveness A quantitative parameter indicating the performance of a transportation facility or service Minimum Gap This volume density parameter that specifies the minimum green extension when gap reduction is used Minimum Green The first timed portion of the green interval which may be set in consideration driver expectany and the storage of vehicles between the detectors and the stop line when volume density or presence detection is not used Minimum Recall A recall parameter the phase is timed for its minimum green time regardless what the demand is for the movement Movement A term used to describe the user type vehicle or pedestrian and action turning movement taken at an intersection Two different types of movements include those that have the right of way and those that must yield consistent with the rules of the road or the Uniform Vehicle Code Nonlocking mode A controller memory mode that does not retain an actuation received from a detector by the controller after the actuation is dropped by the detection unit Occupancy The percent of time that a detector indicates a vehicle is present over a total time period Offset The time relationship between coordinated phases defined reference point and a defined master reference master clock or sync pulse Offset Reference Point Coordination Point The defined point that creates an association between a signalized intersection and the master clock Overflow queue Queued vehicles left over from a green phase at a signalized intersection Oversaturation A traffic condition in which the arrival flow rate exceeds capacity Passage Time Vehicle Interval Gap Passage Gap Unit Extension A phase timer that ends a phase when the time from the last detector output exceeds the timer setting Pattern Sync Reference The set start of the master clock Peakhour factor The hourly volume during the maximumvolume hour of the day divided by four times the peak 15min flow rate within the peak hour a measure of traffic demand fluctuation within the peak hour Pedestrian An individual traveling on foot Pedestrian Recall recall mode where there is a continuous call for pedestrian service resulting in the pedestrian walk and clearance phases to occur each time the phase times Pedestrian Clearance Interval Also known as Flash Don t Walk The time provided for a pedestrian to cross the entire width ofthe intersection Pedestrian Phase Time allocated to pedestrian traffic that may be concurrent with vehicular phases Pedestrian scramble See Exclusive Pedestrian Phase Pedestrian Walk Interval An indication to the pedestrian that it allows pedestrians to begin crossing the intersection Pedestrian walking speed The average walking speed of pedestrians in feet per second Percent Runs Stopped The percentage of the total number of travel time runs conducted during which a vehicle stops Performance Index An arterial and networkLevel performance measure that allows several measures of effectiveness to be mathematically combined Performance Measures Signal system related effects on stops vehicle delay arterial travel time or existence of spill back queuing between closely spaced intersections Permissive Movements A movement where it is allowed to proceed if there are available gaps in the conflicting flow Permissive Period A period of time during the coordinated cycle in which calls on conflicting phases will be result in the coordinated phase transitioning to noncoordinated phases Permitted plus protected Compound leftturn protection that displays the permitted phase before the protected phase Permitted turn Left or right turn at a signalized intersection that is made against an opposing or conflicting vehicular or pedestrian flow Phase A controller timing unit associated with the control of one or more movements The MU TCD de nes a phase as the rightof way yellow change and red clearance intervals in a cycle that are assigned to an independent traf c movement Phasing Indication The current display for a given phase green yellow red walk flashing don t walk or don t walk Phase Pair A combination of two phases allowed within the same ring and between the same barriers such as 12 56 34 and 78 Phase Recall A call is placed for a specified phase each time the controller is servicing a conflicting phase This will ensure that the specified phase will be serviced again Types of recall include soft minimum and maximum Soft recall only calls the phase back if there is an absence of conflicting calls Phase Sequence The order of a series of phases Phasing Diagram A graphical representation of a sequence of phases Platoon roup of vehicles or pedestrians traveling together as a group either voluntarily or involuntarily because of signal control geometrics or other factors Preemption Traffic signal preemption is the transfer of normal operation of a traffic control signal to a special control mode of operation Preempt Trap A condition that can occur when a preemption call is serviced at a signalized intersection near an atgrade trainroadway crossing where not enough clearance green time is provided to clear a queue of vehicles and a vehicle could be trapped on the tracks with the railroad crossing lights and gates come down Presence Mode A detection mode where a signal is sent to the controller for the duration of time a vehicle is inside the detection zone Pretimed contro A signal control in which the cycle length phase plan and phase times are preset to repeat continuously Priority Traffic signal priority TSP is an operational strategy communicated between transit vehicles and traffic signals to alter signal timing for the benefit or priority of transit vehicle Green extension red truncation and phase skipping are examples of signal timing alterations under TSP Progression adjustment factor A factor used to account for the effect of signal progression on traffic flow applied only to uniform delay Protected Movements A movement where it has the rightofway and there are no conflicting movements occurring Protected plus permitted Compound leftturn protection at a signalized intersection that displays the protected phase before the permitted phase Protected turn The left or right turns at a signalized intersection that are made with no opposing or conflicting vehicular or pedestrian flow allowed Pulse Mode A detection mode where vehicle detection is represented by a single on pulse to the controller 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 people joining the rear of the queue are usually considered part of the queue The internal queue dynamics can involve starts and stops A fastermoving line of vehicles is often referred to as a moving queue or a platoon Queue discharge A flow with high density and low speed in which queued vehicles start to disperse Usually denoted as Level of Service F Queue spillback A term used to describe vehicles stopped at an intersection that exceed the available storage capacity for a particular movement Queue storage ratio The parameter that uses three parameters back of queue queued vehicle spacing and available storage space to determine if blockage will occur QuickEstimation Method A method detined in Chapter 10 of the HCM 2000 that allows an analyst to identify the critical movements at an intersection estimate whether the intersection is operating below n ar at or over capacity and approximate the amount of green time needed for each critical movement Red Change Interval The period of time following a yellow period indicating the end of a phase and stopping the flow of traffic Red time The period expressed in seconds in the signal cycle during which for a given phase or lane group the signal is red Red Truncation A signal priority treatment to terminate nonpriority approach green phasing early in order to more quickly return to green for the priority approach This treatment is also known as early return to green quot9 An phases that operate in sequence Ring Barrier Diagram A graphical representation of phases within a set of rings and phases within a set of barriers Saturation Flow Rate The equivalent hourly rate at which vehicles can traverse an intersection approach under prevailing conditions assuming a constant green indication at all time and no loss time in vehicles per hour or vehicles per hour per lane Saturation headway The average headway between vehicles occurring after the fourth vehicle in the queue and continuing until the last vehicle in the initial queue clears the intersection Section A group of signalized intersections used to analyze traffic operations develop new signal timings and operate in the same control mode manual time of day or traffic responsive Segment A portion of a facility on which a capacity analysis is performed it is the basic unit for the analysis a onedirectional distance A segment is defined by two endpoints SemiActuated Control A type of signal control where detection is provided for the minor movements only and the signal timing returns to the major movement because it has no detection and is placed in recall Signal Head An assembly of one or more signal indications Signal Coordination An operational mode that synchronizes a series of traffic signals to enhance the operation of one or more directional movements Signal Warrant A threshold condition to determine whether a traffic signal is justified based on satisfaction of an engineering study There are eight warrants provided in the MUTCD Signalization condition A phase diagram illustrating the phase plan cycle length green time change interval and clearance time interval of a signalized intersection Simple left turn protection A signal phasing scheme that provides a single protected phase in each cycle for a left turn Simultaneous Gap This parameter requires all phases to concurrently gap out prior to crossing the barrier Software in the loop SITL A means of providing a direct linkage between simulation models and software emulations of controllers Speed A rate of motion expressed as distance per unit of time Split The time assigned to a phase green and the greater of the yellow plus allred or the pedestrian walk and clearance times during coordinated operations May be expressed in seconds or percent Startup lost time The additional time in seconds consumed by the first few vehicles in a queue at a signalized intersection above and beyond the saturation headway because of the need to react to the initiation ofthe green phase and to accelerate Stopped Delay A measurement of the aggregate sum of stopped vehicles for a particular time interval divided by the total entering volume for that movement Stop time A portion of control delay when vehicles are at a complete stop TimeBeforeReduction This volume density timing period begins when the phase is Green and there is a serviceable call on a conflicting phase When this period is completed the linear reduction of the Passage Time begins Time of Day Plans Signal timing plans associated to specific hours of the day associated with fluctuations in demand TimeSpace Diagram A chart that plots the location of signalized intersections along the vertical axis and the signal timing along the horizontal axis This is a visual tool that illustrates coordination relationships between intersections TimeToReduce This volume density timing period begins when the TimeBeforeReduction ends and controls the linear rate of reduction until the Minimum Gap is achieved Total delay The sum of all components of delay for any lane group including control delay traffic delay geometric delay and incident delay Total lost time The time per signal cycle during which the intersection is effectively not used by any movement this occurs during the change and clearance intervals and at the beginning of most phases Track Clearance Green Time Associated with rail preemption at signalized intersection near rail crossings Track clearance green time is the signal timing provided to an approach to ensure queued vehicles can be moved off the rail crossing prior to the beginning of railroad warning lights and gate lowering Traffic Control Center Traffic Management Center An optional physical component of a signal system which contains the operational database that stores controller data allows monitoring ofthe system and allows timing and other parameters to be modified Traffic Control Devices device used to control conflicting traffic flows typically at an intersection Examples include traffic signals stop signs yield signs and roundabouts Traffic Responsive Operation A signal operational method which uses data from traffic detectors rather than time of day to automatically select the timing plan best suited to current traffic conditions Traffic Signal A device to warn control or direct at least one traffic movement at an intersection Traffic Signal Controller A device controlling indication changes at a traffic signal Traffic Signal Inventory A database related to the traffic signal including information such as location signal layout signal timing coordinated or uncoordinated signal operation communication operating agency and history of updates Transit Efficiency Performance measures for transit vehicles such as percent ontime ridership and travel time Travel Time Average The total elapsed time spent traversing a specified distance The average travel time represents an average of the runs for a particular link or corridor Travel Time and Delay Study This study is used to evaluate the quality of traffic movements along an arterial and determine the locations types and extent of traffic delays Typical measures of effectiveness include travel time delay percent runs stopped and average speed Twoway leftturn lane A lane in the median area that extends continuously along a street or highway and is marked to provide a deceleration and storage area out of the throughtraffic stream for vehicles traveling in either direction to use in making left turns at intersections and driveways Uniform dela The first term ofthe equation for lane group control delay assuming uniform arrivals Uniform Vehicle Code A set of traffic laws prepared by the National Committee on Uniform Traffic Laws and Ordinances The specific traffic laws within the code vary from state to state and within differentjurisdictions Unit extension See passage time Unmet demand The number of vehicles on a signalized lane group that have not been served at any point in time as a result of operation in which demand exceeds capacity in either the current or previous analysis period This does not include the normal cyclical queue formation on the red and discharge on the green phase Upstream The direction from which traffic is flowing Variable Initial volume density parameter that uses detector activity to determine if the initial green interval will exceed minimum green time Volume The number of persons or vehicles passing a point on a lane roadway or other trafficway during some time interval often 1 h expressed in vehicles bicycles or persons per hour VolumeDensity phase timing function that uses parameters variable initial min gap time before reduction time to reduce to provide appropriate minimum green time to clear intersection queues when stop bar detectors are not used andor it is desired to adjust the passage time VolumetoCapacity Ratio Also known as degree of saturation is a ratio of demand volume to the capacity for a subject movement Walk Interval An indication providing rightofway to pedestrians during a phase Yellow Change Interval An indication warning users that the green indication has ended and the red indication will egin Yellow Extension The portion of the yellow change interval that some vehicles use to pass through the intersection during the yellow change interval Yellow Trap A condition that leads the leftturning driver into the intersection when it is possibly unsafe to do so even though the signal displays are correct Yield Point A point in a coordinated signal operation that defines where the controller decides to terminate the coordinated phase