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Evaluating the Performance of Corridors with Roundabouts (2014)

Chapter: Chapter 4. Applications

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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Suggested Citation:"Chapter 4. Applications ." National Academies of Sciences, Engineering, and Medicine. 2014. Evaluating the Performance of Corridors with Roundabouts. Washington, DC: The National Academies Press. doi: 10.17226/22348.
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Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-1 CHAPTER 4. APPLICATIONS This chapter presents the two major applications developed under this project: the Corridor Comparison Document (CCD) and a predictive methodology for travel speed on a roundabout corridor. Section 4.1 presents an overview of the CCD. The entire CCD and four examples illustrating its use can be found in Appendix A. Section 4.2 is a step by step presentation of the predictive model for travel speed on a roundabout corridor, including equations used in the models and sample calculations. Details on the development of individual models used in the predictive method can be found in Chapter 3 of this report. 4.1. CORRIDOR COMPARISON DOCUMENT Data collection and modeling conducted as part of this NCHRP project were focused on traffic operational performance such as travel time and speed. However, there are many other performance measures to consider when assessing corridor alternatives and choosing intersection control. The CCD presented in Appendix A provides an overall framework for users to compare alternative corridor configurations and to objectively inform project decisions based on the unique context of each corridor. Chapter 1 of the CCD provides the purpose and scope of the document. It also identifies the document’s intended users and its relationship to other resource documents. Chapter 2 of the CCD provides information on different users of arterials. Users include passenger cars, buses, pedestrians, bicycles, trucks, and emergency vehicles. Chapter 2 is focused on differences between signals and roundabouts, and how these may affect the experience of users. Chapter 3 of the CCD discusses project planning processes, and is wrien from the perspective of a practitioner evaluating alternatives for reconstructing an existing corridor or constructing a new roadway where the alignment has already been determined. In other words, it is focused on intersection control and cross section decisions, not roadway alignment decisions. A typical project planning process is presented in Chapter 3 and shown here as Exhibit 4 1. The process shown in Exhibit 4 1 has three primary stages: project initiation, concept development, and alternatives evaluation. The CCD emphasizes the involvement of community stakeholders throughout the planning process.

Evaluating the Performance of Corridors with Roundabouts Page 4-2 Project initiation begins with gaining an understanding of context. What is the roadway location? Who will it serve? What type of roadway and place are stakeholders looking to create? Often, knowledge of a project’s catalyst will help answer these questions. Some typical project catalysts include: A new greenfield corridor; An existing signalized corridor being evaluated because of capacity or safety performance; An existing roundabout corridor; A corridor with a specific access management focus; A corridor that is explicitly focused on multimodal considerations; Exhibit 4-1: Corridor Planning Process Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-3 A corridor project driven by community enhancement objectives, speed management needs, or economic development or growth opportunities; and, A hybrid corridor containing roundabouts, traffic signals, and stop controlled intersections. The degree to which the users identifed in Chapter 2 are present also provides practitioners with an understanding of context. The CCD recommends choosing performance measures at an early stage of the project planning process, when a practitioner has gained an understanding of a project’s context but has not started development of alternatives. In the CCD, performance measures are grouped into six categories: Quality of Service Measures: Examples include delay and travel time for all modes. Safety Measures: Examples include the predicted number of fatal/injury crashes or expected relative difference in crash frequency. Environmental Measures: Examples include effects on public facilities, impacts to wetlands, and fuel consumption. Cost Measures: Examples include economic benefits associated with a project, the capital cost of a project, and the economic cost of crashes. Community Values: Examples include livability, place making, and community acceptance. Others: Examples include policy choices such as “roundabouts first,” tort and other legal issues, access management, economic development, speed management, and community acceptance. All projects are unique, and key performance measures will differ from project to project. Chapter 4 of the CCD provides additional information on performance measures. Concept development is the second primary phase of the project planning process shown in Exhibit 4 1. Developing concept alternatives should be an iterative process. Some alternatives, while found to be infeasible, may have certain feasible and desirable features that can be incorporated into other alternatives. Examples of design elements of arterials that may differ between alternatives are listed below: Control at major intersections (traffic signal, roundabout, stop control, or uncontrolled) Median type Number of lanes Presence of bike lanes Access/control at driveways and side streets Access management

Evaluating the Performance of Corridors with Roundabouts Page 4-4 Roadway cross section Right of way Design speed Intersection spacing Alternatives analysis is the third primary stage of the project planning process shown in Exhibit 4 1. Practitioners apply selected performance measures to the developed alternatives, and identify a preferred alternative. Chapter 4 of the CCD provides guidance on assessment techniques for a variety of performance measures shown in Exhibit 4 2. The CCD lists common performance measures that are relevent to many arterial projects. In some cases, additional performance measures not listed in Exhibit 4 2 are relevent and should be considered; no list of performance measures could ever include all possible options. Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-5 Exhibit 4-2: Performance Measures

Evaluating the Performance of Corridors with Roundabouts Page 4-6 In addition to identifying assessment techniques for performance measures, Chapter 4 of the CCD also notes cost benefit analysis and scoring as two techniques for comparing the results of an alternatives analysis and for identifying a preferred alternative. Chapter 5 of the CCD presents four fictional example applications that illustrate its use: Example Application #1 is a new suburban arterial being built in a greenfield to create access to undeveloped land and to provide increased connectivity. The alternatives considered are a signalized arterial with a two way left turn lane, a signalized arterial with a median, and a roundabout arterial. The roundabout alternative is selected primarily because of predicted safety performance. Example Application #2 is a community enhancement project on an existing urban arterial. Alternatives considered are a road diet with traditional intersections (signals and two way stop control), a one way couplet with traditional intersections, and a road diet with roundabouts. The roundabout alternative is selected primarily because of its traffic calming benefits and potental to enhance the image of the corridor. Example Application #3 is an existing two lane highway in a rural, context sensitive environment that is beginning to experience suburban style development as it transforms into a vacation and second home community. Alternatives considered are addition of a two way left turn lane and signals, addition of roundabouts (cross section varies between median and two way left turn lane), and a bypass of the area where development is occurring. The roundabout alternative is selected based on place making and aesthetic improvement opportunities. Example Application #4 is an existing suburban corridor being evaluated for safety and operational improvements due to changing context and a need for pavement rehabilitation. Alternatives considered are rebuilding the existing six lane arterial, reducing the arterial to four lanes and maintaining signals at major intersections, and reducing the arterial to four lanes and replacing signals with roundabouts. The four lane signal alternative is selected primarily because it offers pedestrian and bicycle improvements at a substantially lower cost than the roundabout alternative. However, one signal was replaced with a roundabout at an intersection with unusual geometry and past crash history. 4.2. PREDICTIVE OPERATIONS METHODOLOGY Automobile travel speed is one performance measure for an arterial. On a signalized arterial, “Percent Free Flow Speed” (%FFS), which is calculated by dividing the average segment speed by the segment free flow speed, is the performance measure that determines automobile level of service per the Urban Street chapters (16 and 17) of the Highway Capacity Manual 2010 (HCM 2010). The objective of modeling conducted as part of this project is to enhance the Urban Streets methodology in the HCM 2010 to accommodate one or more roundabouts along an urban street. The Urban Streets chapter currently allows Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-7 for the analysis of roundabouts in an urban street but does not provide a complete set of roundabout specific equations for doing so. The Urban Street Segments chapter refers users to the roundabout control delay equation in Chapter 21 in place of a signal control delay equation. However, no equation is provided for geometric delay, and there is no adjustment to running speed or free flow speed to account for the area near a roundabout where drivers accelerate and decelerate because of the roundabout’s speed limiting geometry. This project developed a framework and models to fill that gap. A summary of the framework, new equations recommended for inclusion into the next edition of the HCM, and sample calculations are presented below. A full discussion of the modeling research efforts is provided in Chapter 3. 4.2.1. FRAMEWORK An urban street segment in HCM 2010 Chapter 17 is defined as a stretch of roadway between two intersections, including the downstream boundary intersection. In other words, the total delay of an urban street segment combines the control delay at the intersection with any midsegment delays resulting from queuing, driveway friction, high vehicular volumes, or other sources. Several computational steps are necessary in the Urban Street Segments chapter to arrive at the %FFS measure. These computational steps are: Step 1: Determine Traffic Demand Adjustments Step 2: Determine Running Time Step 3: Determine Proportion Arriving During Green Step 4: Determine Signal Phase Duration Step 5: Determine Through Delay Step 6: Determine Through Stop Rate Step 7: Determine Travel Speed Step 8: Determine Spatial Stop Rate Step 9: Determine Levels of Service Step 10: Determine Automobile Perception Score For the application to roundabout corridors, Step 1 is maintained. The Step 2 procedure for average running time is generally maintained, but certain components of that step, like the free flow speed estimation procedure, are updated for roundabout corridor operations. Steps 3 and 4 are not applicable to roundabouts. Step 5 is replaced with a roundabout specific delay estimation procedure. Step 6 remains as a gap in the literature, where no model for stop rate at roundabouts is available. Because this performance measure is not used in the determination of level of service (LOS) for the urban street segment, it was not a focus in this research. Step 7 is maintained from Chapter 17 and uses earlier roundabout specific interim steps. Step 8 is again a gap in the methodology for roundabouts, as no stop rate prediction procedure is available. Step 9 is maintained for roundabouts to estimate LOS. Step 10 represents another gap in the literature, as all studies to arrive at the automobile perception score were conducted at signalized intersections. In HCM 2010 Chapter 17, an urban street segment begins at the stop bar of an upstream signalized intersection, extends through the intersection and the

Evaluating the Performance of Corridors with Roundabouts Page 4-8 section of roadway that follows, and ends at the stop bar of a downstream signalized intersection. For reasons discussed in Chapter 3 of this document, this segment definition is problematic. In general, it is problematic for roundabouts because it would include a portion of the impacts (geometric delay, free flow speed decrease) associated with the upstream roundabout and the downstream roundabout. Therefore, the roundabout version of the procedure divides each segment into an upstream and downstream sub segment, and, for some steps, separate equations are used for upstream and downstream sub segments. Sub segment definitions are illustrated in Exhibit 3 4 in Chapter 3. Exhibit 4 3 illustrates the calculation framework for applying the models within a given segment. The framework should be applied separately for each upstream and downstream sub segment before eventually aggregating to the segment level. Further aggregation to the facility level can be performed using the Urban Street Facility procedure of HCM 2010 Chapter 16. The framework is divided into computational steps A through L, with reference being made to the corresponding steps in HCM 2010 Chapter 17 for Urban Street Segments at the appropriate time. Steps for the roundabout specific Urban Streets procedure are le‹ered rather than numbered to avoid confusion with the existing, signal focused Chapter 17 procedure. Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-9 Note: After Step L, segments can be aggregated to facility level per HCM 2010 Chapter 16. Exhibit 4-3: Computation Process Step A: Gather Input Data: Sub segment length, posted speed limit, ICD, CID, circulating speed, entering flow, roundabout capacity, restrictive median length, curb length. Step B: Determine FFS for both sub segments using FFS model (assuming OL=0) Step C: Determine RIA length of both sub segments using RIA length model Step D: Do RIAs overlap? Step E: Recalculate FFS of both sub segments using FFS model (assuming OL=1) Step H: Determine geometric delay of each sub segment, adjust for negative estimates Yes No Step J: Aggregate sub segment performance measures to Chapter 17 segment level Step I: Determine impeded delay of each sub segment, adjust for negative estimates Step F: Select Controlling FFS from two sub segments (minimum) Step K: Determine segment average travel speed Consistent with HCM Ch. 17 – Step 7 Step G: Determine Segment Running Time Consistent with HCM Ch. 17 – Step 2 Step L: Determine LOS Consistent with HCM Ch. 17 – Step 9

Evaluating the Performance of Corridors with Roundabouts Page 4-10 First, the analyst gathers input data and FFS is calculated based on the posted speed limit, segment length, central island diameter of the roundabout, and an assumption that overlapping roundabout influence areas (RIA) are not present. On a portion of a roundabout corridor between two roundabouts (i.e., not a first or last segment), the calculation is performed for a downstream sub segment and the upstream sub segment. Using the model predicted FFS and the circulating speed within the roundabout, the RIA length is calculated for both sub segments. RIA is the length of a sub segment over which speeds are reduced due to the impact of the roundabout. The analyst must check whether the RIA lengths of the sub segments overlap. If so, this necessitates a recalculation of the FFS with the overlap (OL) term set equal to 1, which will cause the predicted FFS to decrease. With the final sub segment FFS being determined, the analyst selects the controlling FFS for that segment. Because the FFS is defined as being measured at the segment mid point, the same FFS has to be used for a downstream sub segment and the next upstream sub segment. In this procedure, the lower of the two FFS values is selected as the controlling FFS value for the entire segment. Next, this procedure uses HCM Chapter 17 Step 2 to estimate the segment running time, followed by roundabout specific models to estimate geometric delay and impeded delay. Impeded delay consists of control delay at the node and other delay associated with traffic volume (not geometry). Next, the sub segment performance measures are aggregated to the HCM Chapter 17 segment level (Step J) and Chapter 17 Step 7 is used to determine the average travel speed on the segment. From the average travel speed, the level of service is estimated for the urban street segment with roundabouts. 4.2.2. MODELS The procedure discussed in Section 4.2.1 includes four new models. They are presented here. The FFS (Sf using HCM terminology) speed models for upstream and downstream sub segments used in Step B are: OLCIDSLLS USf 73.405.043.00037.01.15, OLCIDSLLS DSf 43.402.048.00039.06.14, where Sf,US = upstream free flow speed (mph); Sf,DS = downstream free flow speed (mph); L = sub segment length (feet); SL = posted speed limit (mph); CID = central island diameter (feet); and OL = binary variable equal to one when overlapping influence areas are present on the sub segment, zero otherwise. Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-11 The RIA models for upstream and downstream sub segments used in Step C are: cfUS SSRIA 1.218.139.165 cfDS SSRIA 5.224.318.149 where RIAUS = upstream roundabout influence area length (feet); RIADS = downstream roundabout influence area length (feet); and Sc = circulating speed (mph). The geometric delay models for upstream and downstream sub segments used in Step H are: cfUSgeom SSDelay 21.011.057.1, fc fDSgeom SS ICDSDelay 1173.009.063.2, where Delaygeom, US = upstream geometric delay (seconds); and Delaygeom, DS = downstream geometric delay (seconds). The impeded delay models for upstream and downstream sub segments used in Step I are: enteringfUSimp vxSDelay 03.050.4215.035.5, curbmedianfDSimp LLLxSDelay 0014.00010.00020.010.307.065.2, where x = volume to capacity ratio; ventering = entering flow (vph); Lmedian = length of sub segment with restrictive median (feet); and Lcurb = length of sub segment with curb (feet). 4.2.3. SAMPLE PROBLEM This section presents a fictional sample problem. The same sample problem is presented in Example Application #1 of the CCD.

Evaluating the Performance of Corridors with Roundabouts Page 4-12 STEP A: GATHER INPUT DATA Beechmont Avenue is a planned arterial facility. It will have seven roundabouts and no traffic signals. Exhibit 4 4 lists basic data about the facility. Because this project is at the planning stage, some values are approximated and assumed to be the same for all roundabouts on the corridor. Variable Value Unit Notes Posted speed limit 45 mph Planned speed limit for Beechmont Avenue Intersection volume-to-capacity (v/c) ratio 0.78 none The average of the range of v/c based on preliminary traffic analysis Circulating speed 20 mph Typical 2 fastest path speed for a double-lane roundabout Peak hour directional entry flow 1,000 vph The average of the range of flow in the corridor traffic projections Inscribed circle diameter (ICD) 160 ft Typical value for a roundabout with two circulating lanes Central island diameter (CID) 100 ft Typical value for a roundabout with two circulating lanes The sub segment lengths for Beechmont Avenue are shown in Exhibit 4 5: Roundabout Sub-segment Length (ft) 1 US 800 1 DS 1,140 2 US 1,000 2 DS 940 3 US 800 3 DS 890 4 US 750 4 DS 940 5 US 800 5 DS 1,140 6 US 1,000 6 DS 1,140 7 US 1,000 7 DS 290 “US” sub-segments are upstream of a roundabout, and “DS” sub-segments are downstream of a roundabout. STEP B: DETERMINE FREE FLOW SPEED Temporarily assuming that the roundabout influence area of each roundabout does not overlap, the free flow speed over each segment can be estimated using the free flow speed models: OLCIDSLLS USf 73.405.043.00037.01.15, OLCIDSLLS DSf 43.402.048.00039.06.14, Exhibit 4-4: Data for Analysis Exhibit 4-5: Segment Lengths Chapter 4–Applications R

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-13 where Sf,US = upstream free flow speed (mph); Sf,DS = downstream free flow speed (mph); L = sub segment length (feet); SL = posted speed limit (mph); CID = central island diameter (feet); and OL = binary variable equal to one when overlapping influence areas are present on the sub segment, zero otherwise. The results are shown in Exhibit 4 6: Roundabout Sub-segment Free-Flow Speed (mph) 1 US 42.4 1 DS 42.6 2 US 43.2 2 DS 41.9 3 US 42.4 3 DS 41.7 4 US 42.2 4 DS 41.9 5 US 42.4 5 DS 42.6 6 US 43.2 6 DS 42.6 7 US 43.2 7 DS 39.3 For example, the free flow speed for sub segment 1US can be computed using the free flow speed model for an upstream sub segment: 41 mph.42)0(73.4)100(05.0)45(43.0)800(0037.01.15,USfS Using the downstream sub segment free flow speed model, the estimated FFS for sub segment 1DS follows as: 646 mph.42)0(43.4)100(02.0)45(48.0)140,1(0039.06.14,DSfS STEP C: DETERMINE ROUNDABOUT INFLUENCE AREA LENGTH The length of each roundabout influence area can be estimated using the roundabout influence area models: cfUS SSRIA 1.218.139.165 Exhibit 4-6: Free-Flow Speed Results The above values are shown rounded in Exhibit 4-6. However, unrounded values for these and other intermediate calculations should be used for subsequent calculations.

Evaluating the Performance of Corridors with Roundabouts Page 4-14 cfDS SSRIA 5.224.318.149 where RIAUS = upstream roundabout influence area length (feet); RIADS = downstream roundabout influence area length (feet); and Sc = circulating speed (mph). The resulting lengths are shown in Exhibit 4 7: Roundabout Sub- segment Roundabout Influence Area Length (feet) 1 US 329 1 DS 739 2 US 339 2 DS 715 3 US 329 3 DS 709 4 US 327 4 DS 715 5 US 329 5 DS 739 6 US 339 6 DS 739 7 US 339 7 DS 496 For example, the roundabout influence area of sub segment 1US can be calculated using the roundabout influence area model for an upstream sub segment: 42.41) 21.1 − (20) = 329 ft(8.139.165USRIA The roundabout influence area of sub segment 1DS can be calculated using the roundabout influence area model for a downstream sub segment: 646) − 22.5(20) = 739 ft.42(4.318.149DSRIA STEP D: CHECK FOR OVERLAPPING ROUNDABOUT INFLUENCE AREAS In Step B it was assumed that roundabout influence areas did not overlap. To check this assumption, the roundabout sub segment lengths listed in Exhibit 4 6 are compared to the roundabout influence areas calculated in Step C and listed in Exhibit 4 7. All sub segments, except for one (7DS), are longer than their respective roundabout influence areas and do not overlap. The one overlapping sub segment (7DS) is not a true example of two sub segments having overlapping influence areas because it lies beyond the last roundabout on the corridor. However, because the roundabout influence area is still longer than the Exhibit 4-7: Roundabout Influence Area Results Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-15 sub segment, it is considered to “overlap” and free flow speed is recalculated in the next step. STEP E: RECALCULATE FREE FLOW SPEED OF SEGMENTS WITH OVERLAPPING ROUNDABOUT INFLUENCE AREAS Treating sub segment 7DS with OL = 1, the free flow speed is now 34.9 mph. STEP F: SELECT CONTROLLING FREE FLOW SPEED FROM EACH PAIR OF SUB SEGMENTS This step takes the minimum free flow speed within each pair of sub segments for use in future calculations. For example, sub segment 1DS has a free flow speed of 42.6 mph, and sub segment 2US has a free flow speed of 43.2 mph, so the controlling free flow speed for segment 1DS/2US is 42.6 mph. STEP G: DETERMINE SEGMENT RUNNING TIME Referring to Equation 17 6 from the HCM 2010 (Step 2 of Chapter 17), Exhibit 4 8 shows the running times calculated for each sub segment: Roundabout Sub- segment Proximity Adjustment Factor Sub- segment Running Time (s) 1 US 1.027 14.6 1 DS 1.026 19.7 2 US 1.026 17.5 2 DS 1.027 16.9 3 US 1.027 14.7 3 DS 1.027 16.2 4 US 1.027 14.1 4 DS 1.027 16.9 5 US 1.027 14.7 5 DS 1.026 19.7 6 US 1.026 17.5 6 DS 1.026 19.7 7 US 1.026 17.5 7 DS 1.033 9.6 Note that this process also requires the computation of the proximity adjustment factor (HCM 2010 Equation 17 5). Due to the access management policy Exhibit 4-8: Segment Running Time Results

Evaluating the Performance of Corridors with Roundabouts Page 4-16 associated with the context of the site development, all midsegment access point delays on Beechmont Avenue were assumed to be zero. STEP H: DETERMINE GEOMETRIC DELAY OF EACH SUB SEGMENT Using these controlling free flow speeds, the geometric delay incurred over the roundabout influence area can be estimated for each segment using the following model: cfUSgeom SSDelay 21.011.057.1, fc fDSgeom SS ICDSDelay 1173.009.063.2, where Delaygeom,US = upstream geometric delay (seconds); and Delaygeom,DS = downstream geometric delay (seconds). The resulting geometric delays are shown in Exhibit 4 9: Roundabout Sub- segment Geometric Delay (s) 1 US 2.0 1 DS 4.2 2 US 2.1 2 DS 4.1 3 US 2.0 3 DS 4.1 4 US 2.0 4 DS 4.1 5 US 2.0 5 DS 4.2 6 US 2.1 6 DS 4.2 7 US 2.1 7 DS 2.9 For example, the geometric delay of sub segment 1US can be calculated using the geometric delay model for an upstream sub segment: s0.2)20(21.0)4.42(11.057.1,USgeomDelay Exhibit 4-9: Geometric Delay Results Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-17 The geometric delay of sub segment 1DS can be calculated using the geometric delay model for a downstream sub segment: s2.4 6.42 1 20 173.0)20(21.0)6.42(11.063.2,DSgeomDelay STEP I: DETERMINE IMPEDED DELAY OF EACH SUB SEGMENT Using the controlling free flow speeds and traffic characteristics, impeded delay (i.e., the delay incurred due to traffic conditions and not geometric constraints) of each sub segment is now calculated. The following are the impeded delay models: enteringfUSimp vxSDelay 03.050.4215.035.5, curbmedianfDSimp LLLxSDelay 0014.00010.00020.010.307.065.2, where x = volume to capacity ratio; ventering = entering flow (vph); Lmedian = length of sub segment with restrictive median (feet); and Lcurb = length of sub segment with curb (feet). The results are shown in Exhibit 4 10: Roundabout Sub- segment Impeded Delay (s) 1 US 4.2 1 DS 5.5 2 US 4.1 2 DS 5.0 3 US 4.1 3 DS 4.8 4 US 4.1 4 DS 5.0 5 US 4.2 5 DS 5.5 6 US 4.2 6 DS 5.5 7 US 3.0 7 DS 2.9 For example, the impeded delay of sub segment 1US can be calculated using the impeded delay model for an upstream sub segment: s2.4)000,1(03.0)78.0(5.42)4.42(15.035.5,USimpDelay Exhibit 4-10: Impeded Delay Results

Evaluating the Performance of Corridors with Roundabouts Page 4-18 The impeded delay of sub segment 1DS can be calculated using the impeded delay model for a downstream sub segment: s5.5)140,1(0014.0 )140,1(0010.0)140,1(0020.0)78.0(10.3)6.42(07.065.2,DSimpDelay STEP J: AGGREGATE SUB SEGMENT PERFORMANCE MEASURES TO CHAPTER 17 SEGMENT LEVEL The average travel time over each segment is calculated by adding the following elements of each (non overlapping) sub segment: 1. The sub segment running time, 2. The geometric delay, and 3. The impeded delay. Exhibit 4 11 displays the average travel time for each segment, as well as a list of the sub segments that comprise each segment: Segment Sub-segments Aggregated to Comprise Segment Average Travel Time (s) Downstream Upstream A N/A 1US 20.8 B 1DS 2US 53.2 C 2DS 3US 46.7 D 3DS 4US 45.2 E 4DS 5US 46.7 F 5DS 6US 53.2 G 6DS 7US 53.2 H 7DS N/A 15.5 For example, the average travel time of Segment A is 14.6 seconds (sub segment 1US running time) + 2.0 seconds (sub segment 1US geometric delay) + 4.2 seconds (sub segment 1US impeded delay) = 20.8 s. STEP K: DETERMINE SEGMENT AVERAGE TRAVEL SPEED After the travel times are computed, the average segment travel speed is computed by dividing each segment length by the respective average travel time. This performance measure is consistent with the methodology in HCM Chapter 17. The results are shown in Exhibit 4 12: Exhibit 4-11: Average Travel Time for Each Segment Chapter 4–Applications

Evaluating the Performance of Corridors with Roundabouts Chapter 4–Applications Page 4-19 Segment Average Travel Time (s) Segment Length (ft) Average Travel Speed (mph) A 20.8 800 26.2 B 53.2 2,140 27.4 C 46.7 1,740 25.4 D 45.2 1,640 24.7 E 46.7 1,740 25.4 F 53.2 2,140 27.4 G 53.2 2,140 27.4 H 15.5 290 12.8 For example, the average travel speed (ATS) of Segment A is computed using the segment length (800 feet): mph2.26 mi ft280,5 hr s600,3 s8.20 ft800ATS STEP L: DETERMINE SEGMENT LEVEL OF SERVICE Referring to Exhibit 17 2 in the HCM, the level of service can then be computed for each segment using the percentage of the base FFS at which the segment operates. The results are shown in Exhibit 4 13: Segment Average Travel Speed (mph) Base Free-Flow Speed (mph) Travel Speed as a Percentage of Base Free-Flow Speed LOS A 26.2 42.4 61.8 C B 27.4 42.6 64.4 C C 25.4 41.9 60.6 C D 24.7 41.7 59.3 C E 25.4 41.9 60.6 C F 27.4 42.6 64.4 C G 27.4 42.6 64.4 C H 12.8 39.3 32.5 E The results indicate that all but one segment (the short Segment H at the end of the route) operate at LOS C. The final segment, Segment H, operates at LOS E, likely because the entire segment lies within the influence area of Roundabout 7; i.e., vehicles are accelerating or decelerating over most of the segment. Exhibit 4-12: Average Travel Speed for Each Segment Exhibit 4-13: Level of Service for Each Segment

Evaluating the Performance of Corridors with Roundabouts Page 4-20 FACILITY LEVEL OF SERVICE To aggregate the travel times over the entire facility, HCM Chapter 16 is used directly. The facility travel speed is the aggregation of all segment travel speeds. The facility base FFS is the aggregation of all segment FFS. For Beechmont Avenue, the travel speed is 25.8 mph and the facility base FFS is 42.4 mph. Per Exhibit 16 4 of the HCM 2010, the facility operates at LOS C. Roundabout specific models to analyze the performance of a roundabout corridor – that can be included in the Urban Streets procedure of the Highway Capacity Manual – were developed, as well as a framework for comprehensively comparing corridor alternatives and identifying a preferred alternative. Chapter 4–Applications

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 772: Evaluating the Performance of Corridors with Roundabouts provides measurement and evaluation methods for comparing the performance of a corridor with a functionally interdependent series of roundabouts to a corridor with signalized intersections in order to arrive at a design solution.

For the purposes of this research, a “series of roundabouts” is defined as at least three roundabouts that function interdependently on an arterial.

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