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Pages 89-108

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From page 89...
... Chapter 4/Operational Analysis Page 4-1 Roundabouts: An Informational Guide CHAPTER 4 OPERATIONAL ANALYSIS CONTENTS 4.1 INTRODUCTION .
From page 90...
... Roundabouts: An Informational Guide Page 4-2 Chapter 4/Operational Analysis LIST OF EXHIBITS Exhibit 4-1 Calculation of Circulating Flow .
From page 91...
... Chapter 4/Operational Analysis Page 4-3 Roundabouts: An Informational Guide 4.1 INTRODUCTION This chapter presents methods for analyzing the operation of an existing or planned roundabout. The methods allow a transportation analyst to assess the operational performance of a facility, given information about the usage of the facility and its geometric design elements.
From page 92...
... Roundabouts: An Informational Guide Page 4-4 Chapter 4/Operational Analysis • As with other types of intersections, the operational performance of a roundabout is directly influenced by its geometry. The extent to which this influence is affected in the aggregate (e.g., number of lanes)
From page 93...
... Chapter 4/Operational Analysis Page 4-5 Roundabouts: An Informational Guide of the data available at the time of publication of the 2010 HCM dates to 2003, when fewer roundabouts operating at capacity were available for study in the United States. It is therefore probable that capacities will increase over time as drivers become more familiar and as demands on existing roundabouts force drivers to improve efficiency.
From page 94...
... Roundabouts: An Informational Guide Page 4-6 Chapter 4/Operational Analysis 4.3 DATA COLLECTION AND ANALYSIS 4.3.1 Field Data Collection Operational analysis of roundabouts requires the collection or projection of peak period turning-movement volumes. For existing conventional intersections, these can be determined using standard techniques (4)
From page 95...
... Chapter 4/Operational Analysis Page 4-7 Roundabouts: An Informational Guide northbound circulating flow rate are illustrated in Exhibit 4-1. In this exhibit, vc,NB is the circulating flow rate in front of the northbound entry, and the contributing movements are the eastbound through (EBT)
From page 96...
... Roundabouts: An Informational Guide Page 4-8 Chapter 4/Operational Analysis Exhibit 4-3 Conversion of TurningMovement Volumes to Roundabout Volumes Conversion of Turning-Movement Volumes to Roundabout Volumes Prior to conducting a roundabout analysis, turning-movement volumes must first be converted to roundabout volumes. Turning-Movement Data • Percent heavy vehicles for all movements = 2% • Peak Hour Factor (PHF)
From page 97...
... Chapter 4/Operational Analysis Page 4-9 Roundabouts: An Informational Guide Exhibit 4-3 (cont.) Conversion of TurningMovement Volumes to Roundabout Volumes Conversion of Turning-Movement Volumes to Roundabout Volumes Step 3: Determine Entry Flow Rates by Lane The entry flow rate is calculated by summing up the movement flow rates that enter the roundabout.
From page 98...
... Roundabouts: An Informational Guide Page 4-10 Chapter 4/Operational Analysis 4.4 ANALYSIS TECHNIQUES A variety of methodologies are available to analyze the performance of roundabouts. All are approximations, and the responsibility is with the analyst to use the appropriate tool for conducting the analysis.
From page 99...
... Chapter 4/Operational Analysis Page 4-11 Roundabouts: An Informational Guide procedures allow the assessment of the operational performance of an existing or planned one-lane or two-lane roundabout given traffic-demand levels. This section presents an overview of key elements but not a complete representation of the HCM method; details and sample problems can be found in the HCM (1)
From page 100...
... Roundabouts: An Informational Guide Page 4-12 Chapter 4/Operational Analysis The capacity of the left lane of a roundabout approach is lower than the capacity of the right lane. Equation 4-4 gives the capacity of a one-lane roundabout entry opposed by two conflicting lanes as follows: where all variables are as given previously.
From page 101...
... Chapter 4/Operational Analysis Page 4-13 Roundabouts: An Informational Guide 4.5.3 RIGHT-TURN BYPASS LANES Right-turn bypass lanes are right-turn lanes that do not share the same entrance line with the lanes designated for through and left-turning vehicles. Two common types of right-turn bypass lanes are used at single-lane and multilane roundabouts: (1)
From page 102...
... Roundabouts: An Informational Guide Page 4-14 Chapter 4/Operational Analysis 4.5.5 VOLUME-TO-CAPACITY RATIO The volume-to-capacity ratio is a comparison of the demand at the roundabout entry to the capacity of the entry and provides a direct assessment of the sufficiency of a given design. For a given lane, the volume-to-capacity ratio, x, is calculated by dividing the lane's calculated capacity into its demand flow rate, as shown in Equation 4-9.
From page 103...
... Chapter 4/Operational Analysis Page 4-15 Roundabouts: An Informational Guide enters a more unstable range in which conditions could deteriorate rapidly, particularly over short periods of time. Queues that carry over from one 15-minute period to the next may form, and delay begins to increase exponentially.
From page 104...
... Roundabouts: An Informational Guide Page 4-16 Chapter 4/Operational Analysis oversaturated entry and thus cannot conflict with a downstream entry)
From page 105...
... Chapter 4/Operational Analysis Page 4-17 Roundabouts: An Informational Guide roundabouts must make judgments about entering gaps similar to those experienced at two-way stop-controlled intersections; these judgments become more challenging at higher volume-to-capacity ratios. As a result, drivers may not perceive the same amount of control delay the same way at roundabouts as they do at signalized intersections.
From page 106...
... Roundabouts: An Informational Guide Page 4-18 Chapter 4/Operational Analysis The queue length calculated for each lane should be checked against available storage. The queue in each lane may interact with adjacent lanes in one or more ways: • If queues in adjacent lanes exceed available storage, the queue in the subject lane may be longer than anticipated due to additional queuing from the adjacent lane.
From page 107...
... Chapter 4/Operational Analysis Page 4-19 Roundabouts: An Informational Guide by-lane assessments and sensitivity to origin–destination patterns. Even though research in the United States has not necessarily confirmed these findings at American roundabouts, the principles embodied in these tools can be useful to guide a designer in making decisions about potential trade-offs in operational performance due to changes in traffic flows or geometric modifications.
From page 108...
... Roundabouts: An Informational Guide Page 4-20 Chapter 4/Operational Analysis the specific coding of the model can be fine-tuned to reflect localized driver behavior, including look-ahead points for gap acceptance and locations for discretionary and mandatory lane changes. • Volume pattern checking.

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