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32 regularly exceed the posted speed limit, in which case the free- Quick-Estimation Method flow speed can be approximated by the average operating for Signalized Roadways speed of the vehicles on the roadway. These adjusted flow rates also are based on the following The 2000 HCM (Appendix A, Chapter 10) presents a quick- assumptions: estimation method for roadways and signalized intersection operations that is considered applicable for analysis of airport Travel lanes are at least 12 feet wide. roadways. An alternative quick-estimation method--the plan- Lateral clearances (e.g., distance from walls, abutments, or ning application of the critical movement analysis or Inter- other physical obstacles) are at least 6 feet on both the left section Capacity Utilization (ICU) method--also is applicable and right sides of the roadway. to airport roadways with signalized intersections. The ICU Any vertical grades are less than 0.25-mile in length or less method involves the following steps: than 3% (i.e., rises less than 3 feet per every 100 feet of length). The roadways operate in one direction only, or for two- 1. Identify the lane geometry. way roadways, at least two travel lanes are provided in each 2. Identify the hourly volumes, including left-turn, through, direction, separated by a median. and right-turn volumes for each intersection approach. 3. Identify the signal phasing (i.e., which movements oper- The 2010 HCM includes tables that can be used to modify ate concurrently). travel speeds and flow rates for conditions other than those 4. Perform left-turn check to determine the probability of each described above. critical approach volume clearing the identified opposing If the roadway being evaluated falls significantly outside the or conflicting left-turn volume. lane width, lateral clearance, percent of truck use, and varies 5. Assign lane volumes. from the other factors listed, then the traffic volume thresh- 6. Identify critical volumes by identifying the conflicting or olds presented in Table 4-1 may not be accurate. A more opposing traffic volumes (on a per lane basis) having the detailed macroscopic analysis using procedures described in highest total volumes for each signal phase. the 2000 HCM (or the 2010 update) may be necessary to 7. Sum the critical volumes. determine the maximum service volume for the facility. 8. Determine the intersection level of service. If the lane width, lateral clearance, percent of truck use, and other factors described are applicable to the roadway being Appendix F of this Guide presents an explanation of the analyzed, then the information in Table 4-1 should be applied use of the planning application of the critical movement as follows: analysis method and a worksheet to guide users. 1. Determine the free-flow speed for the roadway. The free- Quick-Estimation Method for Airport flow speed is usually determined by measuring the mean Roadway Weaving Sections speed of traffic under very light flow conditions. However, the posted speed limit can be used as an approximation of Table 4-2 provides example data for a procedure for quickly the free-flow speed. estimating the maximum service volumes on airport roadway 2. Determine the target level of service. The target is deter- weaving sections for one-sided and two-sided weaving areas. mined by individual airport operators (or local agencies) These service volumes were developed using the macroscopic and reflects their individual policies and standards. If such a method described in the next section. standard or policy is lacking, LOS D is a common standard for urban roadways, although many urban agencies have Macroscopic Method for Analyzing adopted LOS E as a standard. LOS C is considered the com- Airport Roadway Weaving Areas mon standard for planning new airport facilities, although at large-hub airports, LOS D is sometimes considered to be The 2000 HCM and the draft 2010 HCM provide method- acceptable on existing roadways during peak periods. ologies for evaluating traffic operations on airport roadways. 3. Using Table 4-1, select the appropriate free-flow speed However, neither edition of the HCM is designed to evaluate and the column with the desired level of service. The max- weaving conditions for low-speed airport roadways (speed imum flow provides the maximum traffic per hour per limits of 30 mph or slower). In fact, commercially available lane that the roadway can serve in one direction. software for applying the HCM methods generally prohibit For example, if the free-flow speed is 50 mph and the tar- the user from applying the software to weaving sections with get level of service is LOS D, then the maximum desirable free-flow speeds lower than 35 mph. flow rate for a two-lane one-way road would be 2,760 vehi- Consequently, a separate weaving analysis without the lim- cles per hour (twice 1,380). itation on low free-flow speeds was developed and incorpo-

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33 Table 4-2. Example service volumes for airport roadway weaving segments. One Sided Ramp Weave (single lane ramp) Number of lanes in Service Volumes (vehicles/hour) for LOS weaving section A B C D E (3 Lanes in this image) 3 1,300 1,800 2,200 2,600 4,200 4 1,650 2,250 2,800 3,200 5,600 5 2,000 2,700 3,300 3,800 6,200 One Sided Ramp Weave (two lane ramp) Number of lanes in Service Volumes (vehicles/hour) for LOS weaving section A B C D E (3 Lanes in this image) 3 1,450 2,100 2,700 3,250 4,200 4 1,950 2,800 3,600 4,300 5,600 5 2,400 3,500 4,450 5,350 6,200 Two Sided Ramp Weave Number of lanes in Service Volumes (vehicles/hour) for LOS weaving section A B C D E (3 Lanes in this image) 3 1,400 1,950 2,500 2,950 4,150 4 1,800 2,500 3,150 3,700 5,550 5 2,150 3,000 3,700 4,300 6,950 Notes: Table uses arbitrarily selected volume combination with free flow speed of 35 mph, level terrain, weaving segment length of 500 feet, 5% heavy vehicles, and approximately 20% of traffic weaving. This table is an example of what service flows could be for one volume pattern and is not intended to function as a look-up table for a quick estimation method. rated into a macroscopic model--the Quick Analysis Tool for the weaving segment, maximum service flow rates for basic Airport Roadways (QATAR). QATAR includes components freeway segments under base conditions were extrapolated to that provide information about low-speed weaving and curb- correspond to input free-flow speeds (i.e., less than 55 mph). side roadway operations given certain inputs. The low-speed The draft 2010 HCM presents macroscopic methods for weaving operations are described in this section. The curb- analyzing airport roadway operations. These methods, if side operations components are described in Chapter 5. adjusted for the factors used to develop Table 4-1 (e.g., driver QATAR uses the weaving analysis calculations and method- population, heavy vehicles, and roadway geometry), are appli- ology presented in Chapter 12 of the draft 2010 HCM for one- cable to analysis of airport roadways with uninterrupted traf- sided and two-sided weaving, and applies these calculations to fic flows and flows controlled by signals or stop signs. roadways having free-flow speeds slower than the lower bound of speeds presented in the draft 2010 HCM (free-flow speeds Use of Draft 2010 Weaving less than 35 mph). Analysis Procedures The draft 2010 HCM weaving methodology is described below so that analysts can follow its implementation within The draft 2010 HCM weaving analysis procedure involves QATAR. Two modifications were made to the draft 2010 HCM the following steps, which are described in this section: weaving method to extend its application to lower speed road- way sections. First, the minimum speed for weaving traffic 1. Collect and input roadway weaving section lane geometry, was reduced from 15 mph in the draft 2010 HCM materials to lane designations, free-flow speed, and peak hour volumes. 10 mph. Second, special LOS threshold traffic densities were 2. Adjust the mixed-flow traffic volumes to equivalent pas- developed for application to weaving sections on low-speed senger car volumes (adjust for percent of heavy vehicles, airport roadways. As an input in determining the capacity of driver familiarity, and peak-hour factor).

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34 3. Determine configuration characteristic, which is based on where PT, ET, PR, ER, are percentage and equivalence of lane changes of weaving movements. trucks/buses and recreational vehicles in the traffic stream, 4. Determine the maximum weaving length, if weaving analy- respectively. sis is appropriate. The presence of recreational vehicles is typically negligible 5. Determine the weaving section capacity. for airport facilities. Suggested truck equivalence is 1.5 for 6. Determine lane-changing rates. level terrain, which is typical for airport roadways. A peak- 7. Determine the average speed of weaving and nonweaving hour factor of 0.9 is suggested in absence of field-collected vehicles. data. For airport roadways where arriving and departing pas- 8. Determine the level of service. sengers constitute the predominant users, a value of 0.85 should be used for the driver familiarity adjustment factor The rest of this section describes these steps in more detail (the full range should be between 0.85 and 1.0, with 0.85 rep- with the recommended modifications for applying this analy- resenting unfamiliar drivers, and 1.0 representing regular sis to weaving sections of low-speed airport roadways. Addi- commuters). tional detail on these steps is provided in the draft 2010 HCM. The user has two options for entering traffic volumes through the weaving segment. The first option is to enter actual O&D counts (or volumes) on the weaving section, and Collect and Input Data the second option is to enter approach and departure vol- The analyst must collect data on existing and/or forecast umes, and then use QATAR to estimate the weaving volumes peak-hour traffic volumes for each leg of the weaving section. in the segment. The traffic data should include a peak-hour factor and per- cent of heavy vehicles. The peak-hour factor is the ratio of the Determine Weaving Configuration total peak-hour flow rate in vehicles per hour (vph) divided by the peak 15-minute flow rate within the peak hour (con- Several key parameters characterize the configuration of a verted to vph). weaving segment. The first step is to determine whether the The free-flow speed or posted speed limit should be observed roadway being analyzed is a one-sided ramp weave or a two- (or estimated in the case of a new design or planning study). sided weave (illustrations are provided in QATAR as well as The proposed (or existing) lane geometry must be identi- in Figures 4-1 and 4-2). The key variables in subsequent steps of fied (number of lanes on each leg, number of lanes in the the methodology for both types of weaving configurations are weaving section, lane striping showing how the lanes on each LCMIN = minimum rate at which weaving vehicles must leg transition to and from the lanes in the weaving section, change lanes to successfully complete all weaving and the length of the weaving section). maneuvers (lc/hr). NWL = number of lanes from which weaving maneuvers Adjust Flow Rates may be made with either one lane change or no lane changes. For one-sided weaving, this value is The mixed (passenger cars, trucks, buses, etc.) flow rates either 2 or 3, and for two-sided weaving, this value should be converted to the equivalent passenger car rates is always 0 by definition. using the following formula: For a one-sided weaving segment, the two weaving move- Vi ments are the ramp-to-freeway and freeway-to-ramp flows; vi = (PHF )( f HV )( f p ) the following values are established: LCRF = minimum number of lane changes that must be where made by one ramp-to-freeway vehicle to success- vi = equivalent passenger car flow rate (passenger cars fully execute the desired maneuver. per hour, or pc/hr) LCFR = minimum number of lane changes that must be Vi = the mixed flow rate (vph) made by one freeway-to-ramp vehicle to success- PHF = peak-hour factor fully execute the desired maneuver. fHV = the heavy vehicle adjustment factor LCMIN = minimum rate of lane changing that must exist for fp = driver familiarity adjustment factor all weaving vehicles to successfully complete their The heavy vehicle adjustment factor is computed as follows: weaving maneuvers, lc/hr = (LCRF vRF) + (LCFR vFR). 1 vRF = ramp-to-freeway demand flow rate in weaving f HV = 1 + PT ( E T - 1) + PR ( E R - 1) segment, pc/hr.

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35 Figure 4-1. Examples of airport roadway weaving configurations. (continued on next page) vFR = freeway-to-ramp demand flow rate in weaving Determine Maximum Weaving Length segment, pc/hr. The concept of maximum length of a weaving segment is For a two-sided weaving segment, only the ramp-to-ramp critical to the methodology. Strictly defined, the maximum movement is functionally "weaving." The following values length is the length beyond which weaving turbulence no are established: longer affects operations within the segment, or alternatively, LCRR = minimum number of lane changes that must be no longer affects the capacity of the weaving segment. made by one ramp-to-ramp vehicle to success- L MAX = 5, 728 (1 + VR ) - [1, 566N WL ] fully execute the desired maneuver. 1.6 LCMIN = LCRR vRR vRR = ramp-to-ramp demand flow rate in weaving seg- where VR is the ratio between weaving volume and total ment, pc/hr. volume.

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36 Figure 4-1. (Continued). If the length of the weaving segment is greater than or equal Chapter 11, Exhibit 11-17, and interpolated for low- to LMAX, then this weaving analysis methodology is not appro- speed airport access roadways. priate. The segment should then be analyzed as merge, diverge, and basic segments, as appropriate. Weaving segment capacity determined by weaving demand flows. This is computed by Determine Capacity of Weaving Segment c W = c IW f HV fP Weaving capacity is determined by two methods: density and weaving demand flows. The final capacity is the smaller where of the results of the two methods. cIW = 2,400/VR for NWL = 2 lanes. cIW = 3,500/VR for NWL = 3 lanes. Weaving segment capacity determined by density. This is With capacity determined, a v/c ratio for the weaving seg- computed by ment may be computed as follows: c W = c IWL N f HV fP v c = V f HV fP c W where Determine Lane-Change Rates cIWL = capacity of the weaving segment under equivalent ideal conditions, per lane (pc/hr/ln) The equivalent hourly rate at which weaving and nonweav- = cIFL - [438.2(1+VR)1.6] + [0.0765LS]+[119.8NWL]. ing vehicles make lane changes within the weaving segment is N = number of lanes within the weaving segment. a direct measure of turbulence in the flow of traffic (i.e., when LS = length of the weaving segment. vehicles exhibit irregular and apparently random fluctuations cIFL = capacity of a basic freeway segment with the same free- in speed). It is also a key determinant of speeds and densities flow speed as the weaving segment under equivalent within the segment, which ultimately determine the existing ideal conditions, per lane (pc/hr/ln), draft 2010 HCM, or anticipated level of service.

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37 Unfortunately, these two equations are discontinuous, there- fore, a third equation is introduced to bridge the gap between Type C weave. Vehicles entering from the lower left must make two the discontinuity: lane changes to exit on the right. Vehicles entering from the lower right require no lane changes to exit on the left. LC NW3 = LC NW1 + ( LC NW2 - LC NW1 )[( INW - 1, 300 ) 650 ], where INW= a measure of the tendency of conditions to induce unusually high nonweaving vehicle lane-change rates. = [LS ID vNW] / 10,000, where ID is interchange spac- ing per mile. Final nonweaving vehicle lane-changing rate is defined as follows: If INW 1,300: LCNW = LCNW1 If INW 1,950: LCNW = LCNW2 If 1,300 < INW < 1,950: LCNW = LCNW3 If LCNW1 LCNW2: LCNW = LCNW2 Total Lane-Changing Rate. The total lane-changing rate LCALL of all vehicles in the weaving segment, in lane changes per hour, is computed as follows: LC ALL = LC W + LC NW Determine Average Speeds of Weaving and Nonweaving Vehicles in Weaving Segment The average speed of weaving vehicles in a weaving seg- Figure 4-2. Example of weaving configurations. ment may be computed as follows: S W = SMIN + [( SMAX - SMIN ) (1 + W )] Estimating the total lane-changing rate for weaving vehicles. This is computed by where SW = average speed of weaving vehicles within the weav- LC W = LC MIN + 0.39 ( L S - 300 ) N (1 + ID ) 0.5 2 0.8 ing segment, miles/hour. SMIN = minimum average speed of weaving vehicles where expected in a weaving segment, miles/hour; the rec- LCW = equivalent hourly rate at which weaving vehicles ommended setting for low-speed airport roadways make lane changes within the weaving segment, lc/hr. is 10 miles/hour. ID = interchange density, int/mi. SMAX = maximum average speed of weaving vehicles expected in a weaving segment, miles/hour; the Estimating the total lane-changing rate for nonweaving recommended setting for airport roadways is the vehicles. Two models are used to predict the rate at which posted speed limit (unless a speed survey or field nonweaving vehicles change lanes in the weaving segment: observations by the analyst indicate that a different speed is appropriate). LC NW1 = ( 0.206 v NW ) + ( 0.542L S ) - (192.6N ) W = weaving intensity factor. 135 + 0.223( v NW - 2, 000 ) LC NW2 = 2,1 = 0.226 [LCALL / LS]0.789 The average speed of nonweaving vehicles in a weaving where segment may be computed as follows: vNW = nonweaving demand flow rate in the weaving seg- ment, pc/hr. SNW = FFS - ( 0.0072LC MIN ) - ( 0.0048 v N )

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38 Note that usually the nonweaving speed should be mod- macroscopic model presented in this section are accurate, but estly faster than the weaving speed. However, the developers the results can provide an initial indication of whether a weav- of the draft 2010 HCM weaving methodology believe that it ing section with certain parameters might operate success- is acceptable for the nonweaving speed to be slightly slower fully or not. than the weaving speed in some cases. The results of the low-speed weaving analysis method and If the analyst finds that the nonweaving speed is more than the revised metrics appear to correlate reasonably well with 3 mph to 5 mph below that of the weaving speed, then it is the observations of airport roadway weaving operations con- recommended that the analyst recompute the weaving speed ducted as part of this research project, and produce results using a lower minimum speed of 5 mph (instead of 10 mph). suitable for planning-level analyses of low-speed airport road- The average speed of all vehicles in a weaving segment may way weaving operations. Although low speeds can be entered be computed as follows: as inputs to most microsimulation models, it is not known whether the resulting modeled traffic flows represent actual S = [ v W + v NW ] [( v W S W ) + ( v NW SNW )] traffic operation patterns under those conditions--few, if any, studies have been conducted of the low-speed weaving con- ditions typical of airport roadways to allow full verification of Determine Level of Service the suggested low-speed weaving analysis method outputs. Significantly more observations at numerous locations are The level of service in a weaving segment, as in all freeway required to provide a basis for analysis of low-speed roadway analyses, is related to the density in the segment. Density is weaving operations that is consistent with the level of analyti- computed as follows: cal precision of the Highway Capacity Manual or any similar document. D = [ v N] S The proposed low-speed weaving method is not intended to serve as a basis for any of the following: where D is measured in pc/mi/ln Density is used to look up the level of service in Table 4-3. Design of a new low-speed roadway weaving section, A special set of density thresholds has been developed for Design of modifications to an existing low-speed weaving weaving on low-speed airport roadways. Airport operators section, may choose their own thresholds based on local experience A definitive operational analysis of an existing or proposed and perceptions of quality of service. weaving section, or The assessment of the level of safety afforded by an exist- ing roadway. Caveats Without more extensive research, it is impossible to know Under the above conditions, microsimulation models may with certainty whether the results of the low-speed weaving be more appropriate for evaluating traffic operations. Table 4-3. Level-of-service criteria for weaving segments. Level Airport low-speed of Freeway weaving Collector-distributor roadways service segments (pc/mi/ln) roadways (pc/mi/ln) (pc/mi/ln) A 10 12 20 B 20 24 30 C 28 32 40 D 35 36 50 E >35 >36 60 F v/c>1.0 v/c>1.0 v/c>1.0 Notes: pc/mi/ln = passenger cars per mile per lane. If the density exceeds the LOS threshold, then the roadway is over capacity. Source: Transportation Research Board, Draft Highway Capacity Manual, Exhibit 12-10, 2010 (except for airport low-speed roadways).