operational analysis overview - kittelson & associates,...

Operational Analysis OverviewRoundabout Traffic Operations AnalysisShort Course

Operational AnalysisOperational Analysis

Basics of roundabout operationsMeasures of effectivenessModeling techniques

Basics of Roundabout Operations: Gap AcceptanceBasics of Roundabout Operations: Gap Acceptance

Drivers make their own decisions about when to enter

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Characteristics that Affect Roundabout PerformanceCharacteristics that Affect Roundabout Performance

The driver making the decision to enterThe stream of conflicting trafficGeometry

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NCHRP Report 572: Roundabouts in the United StatesNCHRP Report 572: Roundabouts in the United States

Most comprehensive study of U.S. roundabout performance to date

NCHRP Project 3-65

Published in 2007

Anchors new HCM 2010 procedure

NCHRP Report 572 (cont.)NCHRP Report 572 (cont.)

U.S. capacities lower than observed in other countries

Capacity clearly sensitive to geometry in the aggregate (number of lanes)

Secondary effects of geometry on capacity (e.g., lane width, diameter) masked by variations in driver behavior

Lane-by-lane analysis needed

NCHRP Report 572 (cont.):Possible Reasons for Lower U.S. CapacitiesNCHRP Report 572 (cont.):Possible Reasons for Lower U.S. Capacities

Driver unfamiliarity with roundabouts

Larger vehicles

Prevalence of stop control

Lack of use of turn signals on exits

Suboptimal geometry affects lane use at multilane roundabouts

Trends may change over time or by region

NCHRP Report 572 (cont.):Capacity CurvesNCHRP Report 572 (cont.):Capacity Curves

0

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500

750

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0 200 400 600 800 1000 1200 1400 1600 1800 2000

Conflicting Flow (pcu/hr)

Max

Ent

ry F

low

(pcu

/hr)

All RdbtsCritical LaneSingle LaneCritical Lane RegressionSingle Lane RegressionExtropolated RegressionExtropolated Regression

y=1130e-0.001x

y=1130e-0.0007x

Need for Continuing ResearchNeed for Continuing Research

More roundabouts in operation today (~2000 in 2010 versus ~300 in 2000)

More multilane roundabouts today

Longer adaptation period by drivers

HCM 2010 procedure has calibration tools to allow procedure to be adapted to empirical, local conditions

Degree of Saturation (Volume-to-Capacity Ratio)Degree of Saturation (Volume-to-Capacity Ratio)

Degree of saturation = volume / capacityTypical desirable maximum = 0.85Consider context of location in determining appropriate thresholds for maximum

Higher degrees of saturation may be ok in less critical areas where impacts of adding capacity exceeds benefitMore care may be appropriate in areas where queuing is more sensitive (e.g., closely spaced intersections, interchange off-ramps)

DelayDelay

Control delayIncludes initial deceleration delay, queue move-up time, stopped delay, and final acceleration delay

Geometric delayDelay experienced by a single vehicle with no conflicting flowsCaused by geometric features

Total delay = Control + GeometricTypical measure used: control delay

Other Performance MeasuresOther Performance Measures

Operational measuresQueuesStops

Environmental measures derived from operational measuresFuel economyEmissions

Other time framesDailyAnnual

Level of ServiceLevel of Service

Based on control delayLOS assigned to each lane, approach, and intersectionDelay thresholds same as for TWSC and AWSC intersections

Similar delay formulationDrivers must select gaps – no guarantee of service

Research to affirm existing thresholds or support new thresholds desired

Level of Service (cont.)Level of Service (cont.)

LOSControl Delay

(s/veh)

A 0 − 10

B > 10 − 15

C > 15 − 25

D > 25 − 35

E > 35 − 50

F > 50

Modeling TechniquesModeling Techniques

Variety of modeling techniques used for all types of transportation facilities

No one method inherently superior to any other – they are only as good as their calibration to reality

How to Model Capacity and Performance?How to Model Capacity and Performance?

Geometry, traffic

volumes

CapacityEmpirical Regression Method

Field

Aggregate gap acceptance behavior

Analytical MethodField

Traffic flow theory

Performance Measures

SimulationTraffic flow theory

Gap Acceptance ParametersGap Acceptance Parameters

tc (critical headway)Accepts gap →

Accepts new gap → tc (critical headway)

OR within the same gap → tf (follow-up headway)

1

2

Priority Stream

Non Priority Stream

User behavior characteristicsUser behavior characteristics

Roundabout performance depends on the availability of gaps and the willingness of drivers to accept those gaps

Considerable variation possible from driver to driver, so performance is typically based on the mean observed performance

Geometric CharacteristicsGeometric Characteristics

Roundabout operations are affected by the geometry of the intersection

Number of lanesLane use assignment (e.g., left, left-through, etc.)Usable length of lanesLane width, entry angle, and other relatively minor geometric parameters

Models capture these characteristics to varying degrees

Characteristics of the Surrounding SystemCharacteristics of the Surrounding System

Effects from driveways, pedestrian crossings, upstream signals, railroad crossingsAdvanced modeling may be needed

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Flow Types at a Roundabout: Aggregations of Turning MovementsFlow Types at a Roundabout: Aggregations of Turning Movements

Entering flow (ve)Circulating flow (vc)Exiting flow (vex)

HCM 2010 Exhibit 21-2

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Example: Simple Volume ConversionExample: Simple Volume Conversion

Turning movements

Roundabout volumes

6699

106

16510673

Example Step 1: Calculate Entry VolumeExample Step 1: Calculate Entry Volume

Entry volume = sum of entering turning movements

271

?

? 344

?

602

847

6699

106

16510673

Example Step 2: Calculate Exit VolumeExample Step 2: Calculate Exit Volume

Exit volume = sum of turning movements as shown

271

?

?

?

847

602

3446699

106

16510673

Example Step 3: Calculate Circulatory VolumeExample Step 3: Calculate Circulatory Volume

Circulatory volume = sum of turning movements as shown

271

246

693

222

847

344

602

6699

106

16510673

Example: SolutionExample: Solution

Turning movements

Roundabout volumes

6699

106

16510673 271

246

693

222

847

344

602

HCM 2010 Flow Rate AdjustmentsHCM 2010 Flow Rate Adjustments

Use of 15-minute flows or Peak Hour Factor (PHF) adjustmentsAdjustments for heavy vehiclesAdjustments for lane use

Turning movement patternsDesignated lane useGeometry (e.g., lane alignment)Driver behavior

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Conflicting Flow Rate (pc/h)

Cap

acit

y (p

c/h

)

Dashed regression extrapolated beyond the data

Capacity of one-lane entry or right lane of two-lane entry against two conflicting lanes

Capacity of left lane of two-lane entry against two conflicting lanes

Capacity of one-lane or either lane of two-lane entry against one conflicting lane

HCM 2010 Capacity AnalysisHCM 2010 Capacity Analysis

HCM 2010, Exhibit 21-7

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Capacity: 1 laneCapacity: 1 lane

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Capacity: 2x1 laneCapacity: 2x1 lane

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Calibration of CapacityCalibration of Capacity

Need for calibration to local data (preferred if available) or national averagesAll models—HCM 2010 and all types of software—need to be calibrated

HCM 2010 is anchored to NCHRP Report 572 data (average of US conditions observed in 2003)Form of empirical regression model allows for direct calibration to local field conditions

Example of HCM 2010 Model CalibrationExample of HCM 2010 Model Calibration

Single-Lane Roundabouts

NCHRP 572 (2003 data)

California(Tian et al,

2006)

Bend, Oregon (KAI, 2009)

Critical Headway 5.1 s 4.8 s 4.1 sFollow-up Headway

3.2 s 2.5 s 2.7 s

Higher single-lane capacity in these examplesDrivers more aggressive than national average?Driver history and familiarity?Use of turn signals? (police enforcement/signing)

Alternative ToolsAlternative Tools

HCM 2010 explicitly recognizes that HCM procedures are not the only way to analyze problemsApplicability to roundabouts

Geometric configurations not included in modelOversaturated conditions requiring multiple-period analysisInteraction effects with other intersections

operational analysis overview - kittelson & associates,...

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