《交通工程学》课程教学资源（书籍文献）HCM 2010（全）.pdf_P36-P40

Highway Capacity Manual 2010 3. ROADWAY SYSTEM ELEMENTS TYPES OF ROADWAY SYSTEM ELEMENTS The HCM defines six main types of roadway system elements. From smallest to largest, these are points, segments, facilities, corridors, areas, and systems. The focus of the HCM is on the first three types of elements: points, segments, and facilities. Exhibit 2‐1 illustrates the spatial relationships of these elements, and the following sections provide details about each system element type. Basic Freeway Segment Urban Street Segment Urban Street Segment Urban Street Segment Urban Street Segment Freeway Weaving Segment All-Way STOP-Controlled Intersection Roundabout Two-Way STOP-Controlled Intersection Signalized Intersection Ramp Interchange Ramp Terminal Freeway Diverge Segment Freeway Facility Urban Street Facility Corridor Basic Freeway Segment Freeway Merge Segment Freeway Diverge Segment Basic Freeway Segment Interchange Ramp Terminal Urban Street Segment Freeway Merge Segment Basic Freeway Segment Urban Street Segment Urban Street Segment Urban Street Segment Urban Street Segment Freeway Weaving Segment All-Way STOP-Controlled Intersection Roundabout Two-Way STOP-Controlled Intersection Signalized Intersection Ramp Interchange Ramp Terminal Freeway Diverge Segment Freeway Facility Urban Street Facility Corridor Basic Freeway Segment Freeway Merge Segment Freeway Diverge Segment Basic Freeway Segment Interchange Ramp Terminal Urban Street Segment Freeway Merge Segment Exhibit 2-1 Illustrative Roadway System Elements Note that a two-way STOPcontrolled intersection does not normally divide the uncontrolled urban street into two segments. (a) Points, Segments, Facilities, and Corridors System Corridor Area Area Area Area Area System Corridor Area Area Area Area Area (b) Corridors, Areas, and Systems Points Points are places along a facility where (a) conflicting traffic streams cross, merge, or diverge; (b) a single traffic stream is regulated by a traffic control device; or (c) there is a significant change in the segment capacity (e.g., lane drop, lane addition, narrow bridge, significant upgrade, start or end of a ramp influence area). Roadway System Elements Page 2-4 Chapter 2/Applications December 2010

Highway Capacity Manual 2010 Some points,such as interchange ramp terminals,may actually have a significant physical length associated with them,as suggested by Exhibit 2-1(a). ated as having zero nts are us ed t efine the en r capacity, segment Segments A segment is the length of roadway between two points.Traffic volumes and although sma nal The Hc volume driveway).Segments may or may not be aay四 street segments Facilities Facilities are lengths of roadways,bicycle paths,and pedestrian walkways composed of a connected series of points and segments.Facilities may or may not be directional and are defined by two endpoints.The HCM defines freeway facilities,multilane highway facilities,two-lane highway facilities,urban stree facilities,and pedestrian and bicycle facilities Corridors Corridors are generally a set of parallel transportation facilities designed to move people between two locations.for example.a corridor may consist of a freeway facility and one or more parallel urban street facilities.There may also be rail or bus transit service on the freeway,the urban streets,or both,and transit service could be provided within a separate,parallel right-of-way. Pedestrian or bicyce facilities may also be present within the corridor,as designated portions of roadways and as exclusive,parallel facilities Areas Areas consist of an interconnected set of transportation facilities servins movements within a specified geographic space,as well as movements to and from adjoining areas.The primary factor distinguishing areas from corridors is that the facilities within an area need not be parallel to each other.Area boundaries can be set by significant transportation facilities,political boundaries or topographic features such as ridgelines or major bodies of water. Systems Systems are composed of all the transportation facilities and modes within a particular region.A large metropolitan area typically has multiple corridors passing through it,which divide the system into a number of smaller areas.Each area contains a number of facilities,which,in turn,are composed of a series of points and segments.Systems can also be divided into modal subsystems(e.g the roadway subsystem,the transit subsystem)as well as subsystems composed of specific roadway elements(e.g,the freeway subsystem,the urban street subsystem)】 ons Page 2-5 Roadway System Elements

Highway Capacity Manual 2010 Urban street points have a physical length but are treated as having zero length for facility analysis purposes. Freeway points are used only to define the endpoints of segments— performance measures and capacity are not defined for them. Some points, such as interchange ramp terminals, may actually have a significant physical length associated with them, as suggested by Exhibit 2‐1(a). For urban street facility analysis, points are treated as having zero length—all of the delay occurs at the point. For freeway facility analysis, points are used to define the endpoints of segments, but they have no associated performance measures or capacity, as these are calculated at the segment level. Segments A segment is the length of roadway between two points. Traffic volumes and physical characteristics generally remain the same over the length of a segment, although small variations may occur (e.g., changes in traffic volumes on a segment resulting from a low‐volume driveway). Segments may or may not be directional. The HCM defines basic freeway segments, freeway weaving segments, freeway merge and diverge segments, and urban street segments. Facilities The types of facilities addressed by the HCM are described in Chapter 3, Modal Characteristics. Facilities are lengths of roadways, bicycle paths, and pedestrian walkways composed of a connected series of points and segments. Facilities may or may not be directional and are defined by two endpoints. The HCM defines freeway facilities, multilane highway facilities, two‐lane highway facilities, urban street facilities, and pedestrian and bicycle facilities. Corridors Corridors are generally a set of parallel transportation facilities designed to move people between two locations. For example, a corridor may consist of a freeway facility and one or more parallel urban street facilities. There may also be rail or bus transit service on the freeway, the urban streets, or both, and transit service could be provided within a separate, parallel right‐of‐way. Pedestrian or bicycle facilities may also be present within the corridor, as designated portions of roadways and as exclusive, parallel facilities. Areas Areas consist of an interconnected set of transportation facilities serving movements within a specified geographic space, as well as movements to and from adjoining areas. The primary factor distinguishing areas from corridors is that the facilities within an area need not be parallel to each other. Area boundaries can be set by significant transportation facilities, political boundaries, or topographic features such as ridgelines or major bodies of water. Systems Systems are composed of all the transportation facilities and modes within a particular region. A large metropolitan area typically has multiple corridors passing through it, which divide the system into a number of smaller areas. Each area contains a number of facilities, which, in turn, are composed of a series of points and segments. Systems can also be divided into modal subsystems (e.g., the roadway subsystem, the transit subsystem) as well as subsystems composed of specific roadway elements (e.g., the freeway subsystem, the urban street subsystem). Chapter 2/Applications Page 2-5 Roadway System Elements December 2010

Highway Capacity Manual 2010 ANALYSIS OF INDIVIDUAL SYSTEM ELEMENTS The HCM provides tools to help analysts estimate performance measures for individual elements of a multimodal transportation system, as well as guidance on combining those elements to evaluate larger portions of the system. Exhibit 2‐ 2 tabulates the various system elements for which the HCM provides analysis methodologies in Volumes 2 and 3, the service measure(s) used to determine LOS for each mode operating on each system element, and the HCM performance measure that can be used to aggregate results to a system level. Some combinations of system elements and travel modes combine several performance measures into a single traveler‐perception model that is used to generate a LOS score; the components of each model are listed in Exhibit 2‐3. Exhibit 2-2 HCM Service Measures by System Element and Mode Service Measure(s) System Element HCM Chapter Automobile Pedestrian Bicycle Transit Systems Analysis Measure Freeway facility 10 Density - - - Speed Basic freeway segment 11 Density - - - Speed Freeway weaving segment 12 Density - - - Speed Freeway merge and diverge segments 13 Density - - - Speed Multilane highway 14 Density - LOS scorea - Speed Two-lane highway 15 Percent timespent-following, speed - LOS scorea - Speed Urban street facility 16 Speed LOS scorea LOS scorea LOS scorea Speed Urban street segment 17 Speed LOS scorea LOS scorea LOS scorea Speed Signalized intersection 18 Delay LOS scorea LOS scorea - Delay Two-way stop 19 Delay Delay - - Delay All-way stop 20 Delay - - - Delay Roundabout 21 Delay - - - Delay Interchange ramp terminal 22 Delay - - - Delay Off-street pedestrian– bicycle facility 23 - Space, eventsb LOS scorea - Speed Notes: a See Exhibit 2-3 for the LOS score components. b Events are situations where pedestrians meet bicyclists. Exhibit 2-3 Components of TravelerPerception Models Used in the HCM System Element HCM Chapter Mode Model Components Multilane and two-lane highways 14, 15 Bicycle Pavement quality, perceived separation from motor vehicles, motor vehicle volume and speed Automobile Weighted average of segment automobile LOS scores Pedestrian Urban street segment and signalized intersection pedestrian LOS scores, midblock crossing difficulty Bicycle Urban street segment and signalized intersection bicycle LOS scores, driveway conflicts Urban street facility 16 Transit Weighted average of segment transit LOS scores Automobile Stops per mile, left-turn lane presence Pedestrian Pedestrian density, sidewalk width, perceived separation from motor vehicles, motor vehicle volume and speed Bicycle Perceived separation from motor vehicles, pavement quality, motor vehicle volume and speed Urban street segment 17 Transit Service frequency, perceived speed, pedestrian LOS Pedestrian Street crossing delay, pedestrian exposure to turning Signalized intersection 18 vehicle conflicts, crossing distance Bicycle Perceived separation from motor vehicles, crossing distance Off-street pedestrian– bicycle facility 23 Bicycle Average meetings/minute, active passings/minute, path width, centerline presence, delayed passings The automobile travelerperception model for urban street segments and facilities is not used to determine LOS, but it is included to facilitate multimodal analyses. Roadway System Elements Page 2-6 Chapter 2/Applications December 2010

Highway Capacity Manual 2010 ASSESSMENT OF MULTIPLE FACILITIES The analysis of a transportation system starts with estimates of delay at the point and segment level. Point delays arise from the effects of traffic control devices such as traffic signals and STOP signs. Segment delays combine the point delay incurred at the end of the segment with other delays incurred within the segment. Examples of the latter include delays caused by midblock turning activity into driveways, parking activity, and midblock pedestrian crossings. The HCM estimates segment speed instead of segment delay; however, segment speed can be converted into segment delay by using Equation 2‐1: ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ −×= i i i i i ii S L S L dAVOD 0 Equation 2-1 where Di = person‐hours of delay on segment i, AVOi = average vehicle occupancy on segment i (passengers/vehicle), di = vehicle demand on segment i (vehicles), Li = length of segment i (mi), Si = average vehicle speed on segment i (mi/h), and S0i = free‐flow speed of segment i (mi/h). Typically, only the segments that constitute the collector and arterial system are used to estimate system delay. Segment delays are added together to obtain facility estimates, and the sum of the facility estimates yields subsystem estimates. Mean delays for each subsystem are then computed by dividing the total person‐hours of delay by the total number of trips on the subsystem. Subsystem estimates of delay can be combined into total system estimates, but typically the results for each subsystem are reported separately. SYSTEM PERFORMANCE MEASUREMENT An increase in congestion on one system element may result in a shift of demand to other system elements. Therefore, estimating system delay is an iterative process. HCM techniques can be used to estimate the delay resulting from a given demand, but not the demand resulting from a given delay. System performance must be measured in more than one dimension. When a single intersection is analyzed, it may suffice to compute only the peak‐period delay; however, when a system is analyzed, the geographic extent, the duration of delay, and any shifts in demand among facilities and modes must also be considered (2). System performance can be measured in the following dimensions: • Quantity of service—the number of person‐miles and person‐hours provided by the system; Dimensions of system performance. • Intensity of congestion—the amount of congestion experienced by users of the system; • Duration of congestion—the number of hours that congestion persists; • Extent of congestion—the physical length of the congested system; • Variability—the day‐to‐day variation in congestion; and • Accessibility—the percentage of the populace able to complete a selected trip within a specified time. Chapter 2/Applications Page 2-7 Roadway System Elements December 2010

Highway Capacity Manual 2010 Quantity of Service Quantity of service measures the utilization of the transportation system in terms of the number of people using the system, the distance they travel (person‐ miles of travel, PMT), and the time they require to travel (person‐hours of travel, PHT). Dividing the PMT by the PHT gives the mean trip speed for the system. Intensity of Congestion The intensity of congestion can be measured by using total person‐hours of delay and mean trip speed. Other metrics, such as mean delay per person‐trip, can also be used. In planning and preliminary engineering applications, intensity of congestion is sometimes measured in terms of volume‐to‐capacity ratio or demand‐to‐capacity ratio. Duration of Congestion The duration of congestion is measured in terms of the maximum amount of time that congestion occurs anywhere in the system. A segment is congested if the demand exceeds the segment’s discharge capacity. Transit subsystem congestion can occur either when the passenger demand exceeds the capacity of the transit vehicles or when the need to move transit vehicles exceeds the vehicular capacity of the transit facility. A segment is congested if the demand exceeds the segment’s discharge capacity. Extent of Congestion The extent of congestion may be expressed in terms of the directional miles of facilities congested or—more meaningfully for the public—in terms of the maximum percentage of system miles congested at any one time. Variability Ideally, variability should be measured in terms of either (a) the probability of occurrence of, or (b) a confidence interval for, other aspects of congestion (intensity, duration, and extent). However, the state of the practice does not yet facilitate such a calculation. Instead, a measure of the sensitivity of the results to changes in the demand can be substituted until better methods for estimating variability become available. Various levels of demand are tested (such as a 5% increase or a 5% decrease), and the resulting effects on the intensity, duration, and extent of congestion are noted in terms of a percentage increase or decrease in their values. The sensitivity can be expressed in terms of elasticity by dividing the percentage change in output by the percentage change in demand. An elasticity greater than 1.0 means that the estimated congestion measure is highly sensitive to changes in demand. Accessibility Accessibility examines the effectiveness of the system from a perspective other than intensity. Accessibility can be expressed in terms of the percentage of trips (or persons) able to accomplish a certain goal—such as going from home to work—within a targeted travel time. Accessibility can also be defined in terms of a traveler’s ability to get to and use a particular modal subsystem, such as transit. This definition is closer to the Americans with Disabilities Act’s use of the term. Roadway System Elements Page 2-8 Chapter 2/Applications December 2010