National Academies Press: OpenBook

Airport Curbside and Terminal Area Roadway Operations (2010)

Chapter: Chapter 4 - Analyzing Airport Terminal Area Roadways

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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
×
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
×
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Suggested Citation:"Chapter 4 - Analyzing Airport Terminal Area Roadways." National Academies of Sciences, Engineering, and Medicine. 2010. Airport Curbside and Terminal Area Roadway Operations. Washington, DC: The National Academies Press. doi: 10.17226/14451.
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30 This chapter presents an overview of terminal area roadway analyses. It presents level-of-service definitions applicable to airport roadways and describes methods for estimating the capacity and levels of service. Chapter 5 presents comparable methods for analyzing curbside roadways. As described earlier, a hierarchy of analytical methods— including quick-estimation, macroscopic, and microsimula- tion methods for analyzing airport terminal area roadway and weaving section operations, is proposed. The appropriate ana- lytical method will evolve as a project proceeds from concept to final design, and as more time and data become available to support the analyses. This chapter presents the suggested quick-estimation meth- ods for analysis of airport roadways with uninterrupted flows, signalized roadways, and airport roadway weaving sections; the macroscopic method for analyzing low-speed roadway weav- ing areas commonly found on airports; and an overview of the use of microsimulation methods. The macroscopic methods and performance measures presented in the 2000 HCM are considered applicable for analyses of airport roadways with uninterrupted traffic flows and unsignalized or signalized intersections, but not for analyses of low-speed roadway weaving areas. It is sug- gested that the method presented in the section on Macro- scopic Method for Analyzing Airport Roadway Weaving Areas be used when macroscopic analyses of airport weav- ing areas are required, and that the methods presented in the HCM be used for macroscopic analyses of all other air- port roadways. The methods and data presented in this chapter represent the best available information concerning airport roadway operations and the consensus of the research team, the Project Panel, and other reviewers at the time this Guide was prepared. It is suggested that additional research be conducted on low- speed weaving areas and maximum service rates for airport roadways. Level-of-Service Definitions for Airport Terminal Area Roadways The key performance measures defining the level of service of an airport terminal area roadway are as follows: • Average speed, which determines travel time; • Traffic density, which determines the ability of motorists to easily maneuver into and out of travel lanes; • Maximum volume-to-capacity (v/c) ratio, which indicates how close the roadway is to breakdown and is useful for determining other performance measures such as queue length and delays; and • Duration and length of queues. With the exception of the weaving analysis discussed in this chapter, the definitions, metrics, and procedures presented in the 2000 HCM are applicable to airport roadways with un- interrupted operations and signalized and unsignalized (i.e., stop-sign controlled) intersections. The weaving analysis methods presented in the 2000 HCM (and the 2010 update) are primarily oriented toward opera- tions on freeways or major arterial streets. At airports, weaving often takes place on roadway segments designed for speeds that are much slower than those on freeways or even on major arte- rial streets. As a result, although the weaving theory and meth- ods presented in the 2000 HCM (and subsequent updates) are applicable to airport roadways, the metrics defining levels of service are not. Consequently, subsequent portions of this chapter present alternative metrics for the low-speed weaving that occurs on airport roadways. Quick-Estimation Methods for Analyzing Airport Roadway Operations This section presents quick-estimation methods for ana- lyzing uninterrupted flows, signalized roadways, and airport roadway weaving sections. C H A P T E R 4 Analyzing Airport Terminal Area Roadways

31 Quick-Estimation Method for Uninterrupted Flows on Airport Roadways Quick-estimation methods are most appropriate for “siz- ing” a roadway in the early stages of planning and the design process when little has been decided (or is known) about the details of the required roadway. Such methods are suitable for use when preparing airport master plans or terminal area plans to size or evaluate a roadway and identify points of existing or future constraints. Table 4-1, which is adapted from Exhibits 21-2 and 21-3 of the 2000 HCM, presents the maximum service flow rate and adjusted flow rates for multilane roadways with uninter- rupted flows. The adjusted flow rates represent the maximum flow rates of typical airport access and circulation roadways and were calculated assuming that (1) heavy trucks and buses represent less than 5% of the traffic volume on the access roadways, (2) courtesy vehicles and minibuses (which are assumed to be equivalent to recreational vehicles in terms of performance) represent about 10% of the traffic volume on access roadways, and (3) a high proportion of drivers who are infrequent users of, and are, therefore, unfamiliar with, the airport roadways. The free-flow speeds can be approximated by the posted speed limits on the roadway section unless drivers Level of service Criteria A B C D E Free-flow speed = 50 mph Minimum speed (mph) 50.0 50.0 50.0 48.9 47.5 Maximum volume/capacity ratio 0.28 0.45 0.65 0.86 1.00 Maximum service flow rate (passenger cars/ hour/lane) 550 900 1,300 1,710 2,000 Maximum flow (vehicles/hour/lane) (a) 440 730 1,050 1,380 1,620 Free-flow speed = 45 mph Minimum speed (mph) 45.0 45.0 45.0 44.4 42.2 Maximum volume/capacity ratio 0.26 0.43 0.62 0.82 1.00 Maximum service flow rate (passenger cars/ hour/lane) 490 810 1,170 1,550 1,900 Maximum flow (vehicles/hour/lane) (a) 400 650 940 1,250 1,530 Free-flow speed = 40 mph Minimum speed (mph) 40.0 40.0 40.0 39.0 38.0 Maximum volume/capacity ratio 0.26 0.42 0.61 0.82 1.00 Maximum service flow rate (passenger cars/ hour/lane) 450 740 1,060 1,400 1,750 Maximum flow (vehicles/hour/lane) (a) 360 600 860 1,130 1,410 Free-flow speed = 35 mph Minimum speed (mph) 35.0 35.0 34.0 34.0 33.0 Maximum volume/capacity ratio 0.26 0.42 0.61 0.80 1.00 Maximum service flow rate (passenger cars/ hour/lane) 410 670 980 1,280 1,600 Maximum flow (vehicles/hour/lane) (a) 330 540 790 1,030 1,290 Free-flow speed = 30 mph Minimum speed (mph) 30.0 30.0 30.0 29.6 29.0 Maximum volume/capacity ratio 0.26 0.41 0.60 0.79 1.00 Maximum service flow rate (passenger cars/ hour/lane) 370 600 870 1,150 1,450 Maximum flow (vehicles/hour/lane) (a) 300 480 700 930 1,170 Free-flow speed = 25 mph Minimum speed (mph) 25.0 25.0 25.0 24.8 24.0 Maximum volume/capacity ratio 0.25 0.40 0.59 0.79 1.00 Maximum service flow rate (passenger cars/ hour/lane) 310 500 740 990 1,250 Maximum flow (vehicles/hour/lane) (a) 250 400 600 800 1,010 mph = miles per hour (a) Flow rates adjusted to account for 0.95 heavy vehicle factor and 0.85 driver population factor due to occasional or unfamiliar users. Source: LeighFisher, based on information presented in Transportation Research Board, National Research Council, Highway Capacity Manual, Exhibits 21-2 and 21-3, December 2000. Table 4-1. Levels of service for airport terminal area access and circulation roadways.

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

rated into a macroscopic model—the Quick Analysis Tool for Airport Roadways (QATAR). QATAR includes components that provide information about low-speed weaving and curb- side roadway operations given certain inputs. The low-speed weaving operations are described in this section. The curb- side operations components are described in Chapter 5. QATAR uses the weaving analysis calculations and method- ology presented in Chapter 12 of the draft 2010 HCM for one- sided and two-sided weaving, and applies these calculations to roadways having free-flow speeds slower than the lower bound of speeds presented in the draft 2010 HCM (free-flow speeds less than 35 mph). The draft 2010 HCM weaving methodology is described below so that analysts can follow its implementation within QATAR. Two modifications were made to the draft 2010 HCM weaving method to extend its application to lower speed road- way sections. First, the minimum speed for weaving traffic was reduced from 15 mph in the draft 2010 HCM materials to 10 mph. Second, special LOS threshold traffic densities were developed for application to weaving sections on low-speed airport roadways. As an input in determining the capacity of the weaving segment, maximum service flow rates for basic freeway segments under base conditions were extrapolated to correspond to input free-flow speeds (i.e., less than 55 mph). The draft 2010 HCM presents macroscopic methods for analyzing airport roadway operations. These methods, if adjusted for the factors used to develop Table 4-1 (e.g., driver population, heavy vehicles, and roadway geometry), are appli- cable to analysis of airport roadways with uninterrupted traf- fic flows and flows controlled by signals or stop signs. Use of Draft 2010 Weaving Analysis Procedures The draft 2010 HCM weaving analysis procedure involves the following steps, which are described in this section: 1. Collect and input roadway weaving section lane geometry, lane designations, free-flow speed, and peak hour volumes. 2. Adjust the mixed-flow traffic volumes to equivalent pas- senger car volumes (adjust for percent of heavy vehicles, driver familiarity, and peak-hour factor). A CB 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 A CB 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 A CB 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. Number of lanes in weaving section One Sided Ramp Weave (single lane ramp) Service Volumes (vehicles/hour) for LOS Number of lanes in weaving section One Sided Ramp Weave (two lane ramp) Service Volumes (vehicles/hour) for LOS Number of lanes in weaving section Two Sided Ramp Weave Service Volumes (vehicles/hour) for LOS Table 4-2. Example service volumes for airport roadway weaving segments. 33

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

35 Figure 4-1. Examples of airport roadway weaving configurations. (continued on next page) vFR = freeway-to-ramp demand flow rate in weaving segment, pc/hr. For a two-sided weaving segment, only the ramp-to-ramp movement is functionally “weaving.” The following values are established: LCRR = minimum number of lane changes that must be made by one ramp-to-ramp vehicle to success- fully execute the desired maneuver. LCMIN = LCRR  vRR vRR = ramp-to-ramp demand flow rate in weaving seg- ment, pc/hr. Determine Maximum Weaving Length The concept of maximum length of a weaving segment is critical to the methodology. Strictly defined, the maximum length is the length beyond which weaving turbulence no longer affects operations within the segment, or alternatively, no longer affects the capacity of the weaving segment. where VR is the ratio between weaving volume and total volume. L VR NMAX WL= +( )⎡⎣ ⎤⎦ −[ ]5 728 1 1 5661 6, ,.

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

Estimating the total lane-changing rate for weaving vehicles. This is computed by where LCW = equivalent hourly rate at which weaving vehicles make lane changes within the weaving segment, lc/hr. ID = interchange density, int/mi. Estimating the total lane-changing rate for nonweaving vehicles. Two models are used to predict the rate at which nonweaving vehicles change lanes in the weaving segment: where vNW = nonweaving demand flow rate in the weaving seg- ment, pc/hr. LC v L N LC NW1 NW S NW2 = ( )+ ( )− ( ) = 0 206 0 542 192 6 2 . . . ,135 0 223 2 000+ −( ). ,vNW LC LC L N IDW MIN S 2= + −( ) +( )⎡⎣ ⎤⎦0 39 300 10 5 0 8. . . Unfortunately, these two equations are discontinuous, there- fore, a third equation is introduced to bridge the gap between the discontinuity: 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: Determine Average Speeds of Weaving and Nonweaving Vehicles in Weaving Segment The average speed of weaving vehicles in a weaving seg- ment may be computed as follows: where SW = average speed of weaving vehicles within the weav- ing segment, miles/hour. SMIN = minimum average speed of weaving vehicles expected in a weaving segment, miles/hour; the rec- ommended setting for low-speed airport roadways is 10 miles/hour. SMAX = maximum average speed of weaving vehicles expected in a weaving segment, miles/hour; the recommended setting for airport roadways is the posted speed limit (unless a speed survey or field observations by the analyst indicate that a different speed is appropriate). W = weaving intensity factor. = 0.226  [LCALL / LS]0.789 The average speed of nonweaving vehicles in a weaving segment may be computed as follows: S FFS LC v NNW MIN= − ( )− ( )0 0072 0 0048. . S S S S WW MIN MAX MIN= + −( ) +( )[ ]1 LC LC LCALL W NW= + LC LC LC LC INW3 NW1 NW2 NW1 NW= + −( ) −( )[ ]1 300 650, , 37 Type C weave. Vehicles entering from the lower left must make two lane changes to exit on the right. Vehicles entering from the lower right require no lane changes to exit on the left. Figure 4-2. Example of weaving configurations.

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

Use of Microsimulation Methods Microsimulation modeling is an analytical process that uses sophisticated computer programs to analyze traffic operations for complex roadway systems. In microsimulation modeling, individual imaginary vehicles are assigned characteristics, such as a destination, vehicle performance capabilities, and driver behavioral profiles. Each “vehicle” then travels through a computerized roadway network, and various aspects of its performance are recorded during its simulated trip based on its interactions with other vehicles and traffic controls. These performance statistics can be summarized in many ways, including performance measures commonly used by traffic engineers and transportation planners (e.g., delays, travel times, travel speeds, and queue lengths). Some aspects of roadway systems, such as intersections con- trolled by isolated or coordinated traffic signals, can be ana- lyzed using simpler techniques than microsimulation. The use of microsimulation models can be beneficial in other roadway environments, including those with complex traffic move- ments, such as weaving operations where some vehicles are entering, some are exiting, and some are traveling through the weaving sections. Many airport roadway systems are sufficiently complex to warrant the use of microsimulation. The use of microsimula- tion models should be considered if simpler analytical tools and methodologies do not yield reasonable results, provide sufficient detail, or cannot be used because the roadway con- figuration or operating conditions are outside the range of those addressed in the HCM. However, the use of micro- simulation models and analyses of traffic using these models are relatively complex tasks requiring training in the use of the specific model and experience in traffic engineering to fully understand the simulation process so that appropriate inputs are used and the outputs are interpreted correctly. Most microsimulation software packages also require significant time to learn. Suggested guidelines on when microsimulation is proba- bly not needed are as follows: 1. Signalized or unsignalized intersections can usually be analyzed using methodologies in the 2000 HCM unless exclusive left-turn lane storage area overflows are a signif- icant problem. In such cases, HCM methodologies may yield optimistic estimates of signal performance, and micro- simulation modeling may yield more accurate results. 2. Roadway segments having few or no driveways or inter- secting side roads. 3. Weaving segments with two entries and two exits and a rea- sonable distance (e.g., at least 500 feet) between the entrance to, and exit from, the segment, and free-flow speeds of 35 mph or greater. 4. If the use of a simpler technique—even if the inputs are outside of the recommended ranges—yields outputs that are consistent with observed conditions (e.g., traffic seems very congested, and use of the HCM methodology yields LOS E or F). Guidelines regarding when to consider microsimulation: 1. Signalized or unsignalized intersections that have more than four legs or are oriented in atypical ways. The ana- lyst should initially attempt to use HCM methodologies and then consider microsimulation modeling if the inputs required to evaluate the roadway segments do not corre- spond to the HCM analysis structure. 2. Airport roadway segments with the number of lanes chang- ing along the length of the segment and with multiple, and possibly unusual, orientations of driveways or intersecting side roads. 3. Weaving segments that do not fit the orientation of the weaving analysis in the HCM. This could mean more than two entries or exits, dimensions or speeds outside the bounds defined in the HCM, signals or stop signs within the weaving segment, etc. 4. If simpler techniques are used to analyze what appear to be sufficiently simple facilities, but the results indicate oper- ations that are much worse or much better than those observed. 5. When congestion on one roadway section causes queues or backups that extend back and interfere with operations on an upstream critical roadway section. 6. When congestion on one roadway section significantly restricts the volume of vehicles that can arrive at a down- stream critical location. 7. When comparing the congestion resulting from different improvement options for situations where it is not possible to design sufficient capacity to eliminate significant conges- tion. In this case, comparisons are typically made of the extent of the congestion (duration and length of queues) produced by the various improvement options. Micro- simulation modeling is the best available tool for making these comparisons. FHWA’s Traffic Analysis Toolbox III: Guidelines for Applying Traffic Microsimulation Modeling Software (FHWA-HRT-04- 040, July 2004) provides additional information on the use and application of microsimulation. This document is available at http://ops.fhwa.dot.gov/trafficanalysistools/tat_vol3/index. htm. 39

Other Performance Measures At some airports, the adequacy of a roadway or curbside area has been defined by the length of time a motorist requires to enter and exit the terminal area. Microsimulation models can be used to establish a baseline condition and compare the baseline travel time (or a predetermined acceptable travel time) with the travel times resulting from different levels of traffic demand and access and circulation roadway config- urations. However, it is difficult to accurately estimate these travel times and queues without the aid of microsimulation models because of the relative short distances being ana- lyzed and the difficulty in estimating queue lengths through other means. 40

Next: Chapter 5 - Evaluating Airport Curbside Operations »
Airport Curbside and Terminal Area Roadway Operations Get This Book
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TRB’s Airport Cooperative Research Program (ACRP) Report 40: Airport Curbside and Terminal Area Roadway Operations includes guidance on a cohesive approach to analyzing traffic operations on airport curbside and terminal area roadways.

The report examines operational performance measures for airport curbside and terminal area roadway operations and reviews methods of estimating those performance measures. The report includes a quick analysis tool for curbside operations and low-speed roadway weaving area, highlights techniques for estimating traffic volumes, and presents common ways of addressing operational problems.

Appendix A, Glossary, to ACRP Report 40 is included in the printed report. Appendices B through G, are available online and listed below:

Appendix B: Bibliography

Appendix C: Summary of Terminal Area Roadway Traffic Volume Surveys

Appendix D: Summary of Curbside Roadway Characteristic Surveys

Appendix E: Summary of Focus Group Surveys

Appendix F: A Reproduction of Portions of TRB Circular 212

Appendix G: Overview of QATAR Curbside Analysis Methodology

Link to QATAR Curbside Analysis Methodology

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