hcm 手册第16章.pdf

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Highway Capacity Manual 2000 Background and underlying concepts for this chapter are in Chapter 10 A lane group is indicated in formulas by the subscript i See Chapter 10 for description of quick estimation method I. INTRODUCTION SCOPE OF THE METHODOLOGY This chapter contains a methodology for analyzing the capacity and level of service (LOS) of signalized intersections. The analysis must consider a wide variety of prevailing conditions, including the amount and distribution of traffic movements, traffic composition, geometric characteristics, and details of intersection signalization. The methodology focuses on the determination of LOS for known or projected conditions. The methodology addresses the capacity, LOS, and other performance measures for lane groups and intersection approaches and the LOS for the intersection as a whole. Capacity is evaluated in terms of the ratio of demand flow rate to capacity (v/c ratio), whereas LOS is evaluated on the basis of control delay per vehicle (in seconds per vehicle). Control delay is the portion of the total delay attributed to traffic signal operation for signalized intersections. Control delay includes initial deceleration delay, queue move-up time, stopped delay, and final acceleration delay. Appendix A presents a method for observing intersection control delay in the field. Exhibit 10-9 provides definitions of the basic terms used in this chapter. Each lane group is analyzed separately. Equations in this chapter use the subscript i to indicate each lane group. The capacity of the intersection as a whole is not addressed because both the design and the signalization of intersections focus on the accommodation of traffic movement on approaches to the intersection. The capacity analysis methodology for signalized intersections is based on known or projected signalization plans. Two procedures are available to assist the analyst in establishing signalization plans. The first is the quick estimation method, which produces estimates of the cycle length and green times that can be considered to constitute a reasonable and effective signal timing plan. The quick estimation method requires minimal field data and relies instead on default values for the required traffic and control parameters. It is described and documented in Chapter 10. A more detailed procedure is provided in Appendix B of this chapter for estimating the timing plan at both pretimed and traffic-actuated signals. The procedure for pretimed signals provides the basis for the design of signal timing plans that equalize the degree of saturation on the critical approaches for each phase of the signal sequence. This procedure does not, however, provide for optimal operation. The methodology in this chapter is based in part on the results of a National Cooperative Highway Research Program (NCHRP) study (1, 2). Critical movement capacity analysis techniques have been developed in the United States (3–5), Australia (6), Great Britain (7), and Sweden (8). Background for delay estimation procedures was developed in Great Britain (7), Australia (9, 10), and the United States (11). Updates to the original methodology were developed subsequently (12–24). LIMITATIONS TO THE METHODOLOGY The methodology does not take into account the potential impact of downstream congestion on intersection operation. Nor does the methodology detect and adjust for the impacts of turn-pocket overflows on through traffic and intersection operation. II. METHODOLOGY Exhibit 16-1 shows the input and the basic computation order for the method. The primary output of the method is level of service (LOS). This methodology covers a wide range of operational configurations, including combinations of phase plans, lane 16-1 Chapter 16 - Signalized Intersections Introduction

Highway Capacity Manual 2000 utilization, and left-turn treatment alternatives. It is important to note that some of these configurations may be considered unacceptable by some operating agencies from a traffic safety point of view. The safety aspect of signalized intersections cannot be ignored, and the provision in this chapter of a capacity and LOS analysis methodology for a specific operational configuration does not imply an endorsement of the suitability for application of such a configuration. EXHIBIT 16-1. SIGNALIZED INTERSECTION METHODOLOGY Input Parameters - Geometric - Traffic - Signal Lane Grouping and Demand Flow Rate - Lane grouping - PHF - RTOR Saturation Flow Rate - Basic equation - Adjustment factors Capacity and v/c - Capacity - v/c Performance Measures - Delay - Progression adjustment - LOS - Back of queue LOS criteria LOS The average control delay per vehicle is estimated for each lane group and aggregated for each approach and for the intersection as a whole. LOS is directly related to the control delay value. The criteria are listed in Exhibit 16-2. EXHIBIT 16-2. LOS CRITERIA FOR SIGNALIZED INTERSECTIONS LOS A B C D E F Control Delay per Vehicle (s/veh) ≤ 10 > 10–20 > 20–35 > 35–55 > 55–80 > 80 Chapter 16 - Signalized Intersections Methodology 16-2

Highway Capacity Manual 2000 INPUT PARAMETERS Exhibit 16-3 provides a summary of the input information required to conduct an operational analysis for signalized intersections. This information forms the basis for selecting computational values and procedures in the modules that follow. The data needed are detailed and varied and fall into three main categories: geometric, traffic, and signalization. Inputs needed Geometric, Traffic, and Signalization EXHIBIT 16-3. INPUT DATA NEEDS FOR EACH ANALYSIS LANE GROUP Type of Condition Parameter Geometric conditions Traffic conditions Signalization conditions Area type Number of lanes, N Average lane width, W (m) Grade, G (%) Existence of exclusive LT or RT lanes Length of storage bay, LT or RT lane, Ls (m) Parking Demand volume by movement, V (veh/h) Base saturation flow rate, so (pc/h/ln) Peak-hour factor, PHF Percent heavy vehicles, HV (%) Approach pedestrian flow rate, vped (p/h) Local buses stopping at intersection, NB (buses/h) Parking activity, Nm (maneuvers/h) Arrival type, AT Proportion of vehicles arriving on green, P Approach speed, SA (km/h) Cycle length, C (s) Green time, G (s) Yellow-plus-all-red change-and-clearance interval (intergreen), Y (s) Actuated or pretimed operation Pedestrian push-button Minimum pedestrian green, Gp (s) Phase plan Analysis period, T (h) Geometric Conditions Intersection geometry is generally presented in diagrammatic form and must include all of the relevant information, including approach grades, the number and width of lanes, and parking conditions. The existence of exclusive left- or right-turn lanes should be noted, along with the storage lengths of such lanes. When the specifics of geometry are to be designed, these features must be assumed for the analysis to continue. State or local policies and guidelines should be used in establishing the trial design. When these are not readily available, Chapter 10 contains suggestions for geometric design that may be useful in preparing an assumed preliminary design for analysis. Input Parameters - Geometric - Traffic Lane Grouping & Demand Flow Rate Saturation Flow Capacity & v/c Performance Measures Traffic Conditions Traffic volumes (for oversaturated conditions, demand must be used) for the intersection must be specified for each movement on each approach. These volumes are the flow rates in vehicles per hour for the 15-min analysis period, which is the duration of 15-min flow rates can be estimated using hourly volumes and PHFs 16-3 Chapter 16 - Signalized Intersections Methodology

Highway Capacity Manual 2000 Study the entire period during which volumes approach and exceed capacity Heavy vehicles are those having more than four tires on the pavement the typical analysis period (T = 0.25). If the 15-min data are not known, they may be estimated using hourly volumes and peak-hour factors (PHFs). In situations where the v/c is greater than about 0.9, control delay is significantly affected by the length of the analysis period. In these cases, if the 15-min flow rate remains relatively constant for more than 15 min, the length of time the flow is constant should be used as the analysis period, T, in hours. If v/c exceeds 1.0 during the analysis period, the length of the analysis period should be extended to cover the period of oversaturation in the same fashion, as long as the average flow during that period is relatively constant. If the resulting analysis period is longer than 15 min and different flow rates can be identified during equal-length subperiods within the longer analysis period, a multiple-period analysis using the procedures in Appendix F should be performed using each of these subperiods individually. The length of the subperiods would normally be, but not be limited to, 15 min each. Vehicle type distribution is quantified as the percent of heavy vehicles (% HV) in each movement, where heavy vehicles are defined as those with more than four tires touching the pavement. The number of local buses on each approach should also be identified, including only those buses making stops to pick up or discharge passengers at the intersection (on either the approach or departure side). Buses not making such stops are considered to be heavy vehicles. Pedestrian and bicycle flows that interfere with permitted right or left turns are needed. The pedestrian and bicycle flows used to analyze a given approach are the flows in the crosswalk interfering with right turns from the approach. For example, for a westbound approach, the pedestrian and bicycle flows in the north crosswalk would be used for the analysis. An important traffic characteristic that must be quantified to complete an operational analysis of a signalized intersection is the quality of the progression. The parameter that describes this characteristic is the arrival type, AT, for each lane group. Six arrival types for the dominant arrival flow are defined in Exhibit 16-4. Arrival Type EXHIBIT 16-4. ARRIVAL TYPES Description 1 2 3 4 5 6 Dense platoon containing over 80 percent of the lane group volume, arriving at the start of the red phase. This AT is representative of network links that may experience very poor progression quality as a result of conditions such as overall network signal optimization. Moderately dense platoon arriving in the middle of the red phase or dispersed platoon containing 40 to 80 percent of the lane group volume, arriving throughout the red phase. This AT is representative of unfavorable progression on two-way streets. Random arrivals in which the main platoon contains less than 40 percent of the lane group volume. This AT is representative of operations at isolated and noninterconnected signalized intersections characterized by highly dispersed platoons. It may also be used to represent coordinated operation in which the benefits of progression are minimal. Moderately dense platoon arriving in the middle of the green phase or dispersed platoon containing 40 to 80 percent of the lane group volume, arriving throughout the green phase. This AT is representative of favorable progression on a two-way street. Dense to moderately dense platoon containing over 80 percent of the lane group volume, arriving at the start of the green phase. This AT is representative of highly favorable progression quality, which may occur on routes with low to moderate side-street entries and which receive high- priority treatment in the signal timing plan. This arrival type is reserved for exceptional progression quality on routes with near-ideal progression characteristics. It is representative of very dense platoons progressing over a number of closely spaced intersections with minimal or negligible side-street entries. Chapter 16 - Signalized Intersections Methodology 16-4

Highway Capacity Manual 2000 The arrival type is best observed in the field but can be approximated by examining time-space diagrams for the street in question. The arrival type should be determined as accurately as possible because it will have a significant impact on delay estimates and LOS determination. Although there are no definitive parameters to precisely quantify arrival type, the platoon ratio is computed by Equation 16-1. Rp = P gi C (16-1) where = platoon ratio, = proportion of all vehicles in movement arriving during green phase, Rp P C = cycle length (s), and gi = effective green time for movement or lane group (s). P may be estimated or observed in the field, whereas gi and C are computed from the signal timing. The value of P may not exceed 1.0. Signalization Conditions Complete information regarding signalization is needed to perform an analysis. This information includes a phase diagram illustrating the phase plan, cycle length, green times, and change-and-clearance intervals. Lane groups operating under actuated control must be identified, including the existence of push-button pedestrian-actuated phases. If pedestrian timing requirements exist, the minimum green time for the phase is indicated and provided for in the signal timing. The minimum green time for a phase is estimated by Equation 16-2 or local practice. Gp = 3.2 + L Sp Gp = 3.2 + L Sp  N ped + 0.81  W E ( + 0.27 N ped   ) for W E > 3.0 m for W E ≤ 3.0 m (16-2) where Gp L Sp WE 3.2 Nped = minimum green time (s), = crosswalk length (m), = average speed of pedestrians (m/s), = effective crosswalk width (m), = pedestrian start-up time (s), and = number of pedestrians crossing during an interval (p). It is assumed that the 15th-percentile walking speed of pedestrians crossing a street is 1.2 m/s in this computation. This value is intended to accommodate crossing pedestrians who walk at speeds slower than the average. Where local policy uses different criteria for estimating minimum pedestrian crossing requirements, these criteria should be used in lieu of Equation 16-2. When signal phases are actuated, the cycle length and green times will vary from cycle to cycle in response to demand. To establish values for analysis, the operation of the signal should be observed in the field during the same period that volumes are observed. Average field-measured values of cycle length and green time may then be used. When signal timing is to be established for analysis, state or local policies and procedures should be applied where appropriate. Appendix B contains suggestions for the design of a trial signal timing. These suggestions should not be construed to be standards or criteria for signal design. A trial signal timing cannot be designed until the volume adjustment and saturation flow rate modules have been completed. In some Input Parameters - Signal Lane Grouping & Demand Flow Rate Saturation Flow Capacity & v/c Performance Measures 15th-percentile pedestrian speed is assumed as 1.2 m/s. Local values can be substituted. 16-5 Chapter 16 - Signalized Intersections Methodology

Highway Capacity Manual 2000 Appendix B contains procedure for estimating average cycle lengths under actuated control Exclusive Shared The analyst should determine if there is a de facto left-turn lane cases, the computations will be iterative because left-turn adjustments for permitted turns used in the saturation flow rate module depend on signal timing. Appendix B also contains suggestions for estimating the timing of an actuated signal if field observations are unavailable. An operational analysis requires the specification of a signal timing plan for the intersection under study. The planning level application presented in Chapter 10 offers a procedure for establishing a reasonable and effective signal timing plan. This procedure is recommended only for the estimation of LOS and not for the design of an implementable signal timing plan. The signal timing design process is more complicated and involves, for example, iterative checks for minimum green-time violations. When phases are traffic actuated, the timing plan will differ for each cycle. The traffic-actuated procedure presented in Appendix B can be used to estimate the average cycle length and phase times under these conditions provided that the signal controller settings are available. The design of an implementable timing plan is a complex and iterative process that can be carried out with the assistance of computer software. Although the methodology presented here is oriented toward the estimation of delay at traffic signals, it was suggested earlier that the computations can be applied iteratively to develop a signal timing plan. Some of the available signal timing software products employ the methodology of this chapter, at least in part. There are, however, several aspects of signal timing design that are beyond the scope of this manual. One such aspect is the choice of the timing strategy itself. At intersections with traffic-actuated phases, the signal timing plan is determined on each cycle by the instantaneous traffic demand and the controller settings. When all of the phases are pretimed, a timing plan design must be developed. Timing plan design and estimation are covered in detail in Appendix B. LANE GROUPING The methodology for signalized intersections is disaggregate; that is, it is designed to consider individual intersection approaches and individual lane groups within approaches. Segmenting the intersection into lane groups is a relatively simple process that considers both the geometry of the intersection and the distribution of traffic movements. In general, the smallest number of lane groups is used that adequately describes the operation of the intersection. The following guidelines may be applied. An exclusive left-turn lane or lanes should normally be designated as a separate lane group unless there is also a shared left-through lane present, in which case the proper lane grouping will depend on the distribution of traffic volume between the movements. The same is true of an exclusive right-turn lane. On approaches with exclusive left-turn or right-turn lanes, or both, all other lanes on the approach would generally be included in a single lane group. When an approach with more than one lane includes a lane that may be used by both left-turning vehicles and through vehicles, it is necessary to determine whether equilibrium conditions exist or whether there are so many left turns that the lane essentially acts as an exclusive left-turn lane, which is referred to as a de facto left-turn lane. De facto left-turn lanes cannot be identified effectively until the proportion of left turns in the shared lane has been computed. If the computed proportion of left turns in the shared lane equals 1.0 (i.e., 100 percent), the shared lane must be considered a de facto left-turn lane. When two or more lanes are included in a lane group for analysis purposes, all subsequent computations treat these lanes as a single entity. Exhibit 16-5 shows some common lane groups used for analysis. Chapter 16 - Signalized Intersections Methodology 16-6

EXHIBIT 16-5. TYPICAL LANE GROUPS FOR ANALYSIS Number of Lanes Movements by Lanes Number of Possible Lane Groups Highway Capacity Manual 2000 1 2 2 3 LT + TH + RT EXC LT TH + RT LT + TH TH + RT EXC LT TH TH + RT (Single-lane approach) { { { OR { { { { OR 1 2 1 2 2 3 DETERMINING FLOW RATE Demand volumes are best provided as average flow rates (in vehicles per hour) for the analysis period. Although analysis periods are usually 15 min long, the procedures for this chapter allow for any length of time to be used. However, demand volumes may also be stated for a time that encompasses more than one analysis period, such as an hourly volume. In such cases, peaking factors must be provided that convert these to demand flow rates for each particular analysis period. Alternative Study Approaches Two major analytic steps are performed in the volume adjustment module. Movement volumes are adjusted to flow rates for each desired period of analysis, if necessary, and lane groups for analysis are established. Exhibit 16-6 demonstrates three alternative ways in which an analyst might proceed for a given study. Other alternatives exist. Approach A is the one that has traditionally been used in the HCM. The length of the period being analyzed is only 15 min, and the analysis period (T), therefore, is 15 min or 0.25 h. In this case, either a peak 15-min volume is available or one is derived from an hourly volume by use of a PHF. A difficulty with considering only one 15-min period is that a queue may be left at the end of the analysis period because of demand in excess of capacity. In such cases it is possible that the queue carried over to the next period will result in delay to vehicles that arrive in that period beyond that which would have resulted had there not been a queue carryover. If queue carryover occurs, a multiple-period analysis is best 16-7 Chapter 16 - Signalized Intersections Methodology

Highway Capacity Manual 2000 Approach A may involve use of PHF, but Approach C will not EXHIBIT 16-6. THREE ALTERNATIVE STUDY APPROACHES Analysis period A B C Single analysis period T = 15 min Single analysis period T = 60 min Multiple analysis periods T = 15 min l e t a R w o F d n a m e D Input Parameters Lane Grouping & Demand Flow Rate - PHF - RTOR Saturation Flow - Basic Equation Capacity & v/c Delay & LOS Study Period 1.0 h 1.0 h 1.0 h Time Approach B involves a study of an entire hour of operation at the site using an analysis period (T) of 60 min. In this case, the analyst may have included the more critical period of operation, missed under Approach A, but because the volume being used is an hourly one, it implicitly assumes that the arrival of vehicles on the approach is distributed equally across the period of study. Therefore, the effects of peaking within the hour may not be identified, especially if, by the end of the hour, any excess queuing can be dissipated. The analyst therefore runs the risk of underestimating delays during the hour. If a residual queue remains at the end of 60 min, a second 60-min period of analysis can be used (and so on) until the total period ends with no excess queue. Approach C involves a study of the entire hour but divides it into four 15-min analysis periods (T). The procedures in this chapter allow the analyst to account for queues that carry over to the next analysis period. Therefore, when demand exceeds capacity during the study period, a more accurate representation of delay experienced during the hour can be achieved using this method. A peak 15-min flow rate is derived from an hourly volume by dividing the movement volumes by an appropriate PHF, which may be defined for the intersection as a whole, for each approach, or for each movement. The flow rate is computed using Equation 16-3. V vp = PHF (16-3) Use of a single PHF assumes that all movements peak in the same period where vp V PHF = flow rate during peak 15-min period (veh/h), = hourly volume (veh/h), and = peak-hour factor. The conversion of hourly volumes to peak flow rates using the PHF assumes that all movements peak during the same 15-min period, and somewhat higher estimates of control delay will result. PHF values of 1.0 should be used if 15-min flow rates are entered directly. Because not all intersection movements may peak at the same time, it is valuable to observe 15-min flows directly and select critical periods for analysis. It is particularly conservative if different PHF values are assumed for each movement. It should be noted also that statistically valid surveys of the PHF for individual movements are difficult to obtain during a single peak hour. Chapter 16 - Signalized Intersections Methodology 16-8

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