Transcript Roundabout Capacity Analysis Framework

Roundabout Capacity
Analysis Framework
Source: FHWA, Dokken Engineering
Presented By:
Dave Stanek &
Joel Rabinovitz
Fehr & Peers
Presentation Overview
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Roundabout Analysis Framework
Roundabout Analysis Methods & Software
Case Study #1 – SR 32 in Chico, CA
Case Study #2 – Kietzke Lane in Reno, NV
Analysis Framework
Analysis Framework
• FHWA Guidelines
– ADT volume thresholds
– Peak hour entering vs. circulatory flow
thresholds
• Analysis Software
– Conduct sensitivity testing
– Understand methodology used
Simulation Checklist
• When to Use
– V/C ratios are > 0.85
– Corridor or system
– Unique geometry (5-legs, adjacent
signal, etc.)
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Review LOS results
Review queuing results
Visually inspect simulation
Conduct sensitivity testing
– NCRHP 572 gap acceptance
Analysis Methods
• Macroscopic Models
– For isolated locations
– Analyze vehicle flows
– HCM 2000, FHWA, NCHRP 572, RODEL,
& SIDRA
• Microscopic Models
– For system analyses
– Analyze individual vehicles & drivers
– SimTraffic, Paramics, & VISSIM
HCM 2000
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Only one-lane roundabouts
LOS thresholds are undefined
Results are a range of v/c ratios
Use Synchro or HCS+ to calculate
FHWA
• Roundabouts: An Informational
Guide (FHWA, 2000)
• Equations for delay and queue length
• For one & two lane roundabouts with
regular geometry (3 or 4 legs)
• Recommends design to V/C of 0.85
• Use Traffix or formula
NCHRP 572
• NCHRP Report 572: Roundabouts in
the United States (TRB, 2007)
• Equations for delay and queue length
• For one & two lane roundabouts with
regular geometry (3 or 4 legs)
• Based on U.S. data, but a limited
sample size
RODEL
• Barry Crown, UK
• Regression equations based on
observations of UK intersections
• Design elements determine approach
capacity (diameter, entry width, etc.)
• Interactive design / operations
analysis
RODEL
RODEL
• Limitations
– Empirical data includes experimental
observations
– Uses equations calibrated to UK drivers
and UK vehicles
– Capacity is reached with small
increases in volumes
– Ignores circulatory or exit capacity
– Ignores bypass lanes
SIDRA
• Akcelik & Associates, Australia
• Uses gap acceptance and lane
utilization to determine capacity
• Can change headway values to
calibrate to local conditions –
Environmental Factor
• Limitations
– LOS results not always consistent with
predicted V/C or queues
– Ignores bypass lanes
SIDRA
SimTraffic
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Trafficware, USA
Stochastic simulation model
Uses Synchro for data input
Driver behavior and vehicle
characteristics
• Can change headway factors to
calibrate to local conditions
SimTraffic
Paramics
• Quadstone, UK
• Stochastic simulation model
• Driver behavior and vehicle
characteristics
• Link/node network structure
• Automatically creates roundabout
• Lane change animation is unrealistic
Paramics
VISSIM
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PTV, Germany
Stochastic simulation model
3D animation features
Link/connector network structure
Specify gap acceptance parameters
by lane for each approach
VISSIM
Simulation Limitations
• Too many parameters can be
changed that affect results
• Some view simulation as a
methodology to “prove” a
predetermined result
• Many agencies/companies do not
have the expertise to review
simulation models
Review of Methods
• FHWA & RODEL
– Based on British regression equations
– Gap acceptance & lane configuration are not
addressed in the calculation
– Very easy to use
• SIDRA & SimTraffic
– Allow calibration of gap acceptance parameters
to local conditions
– Easy to use
• Paramics & VISSIM
– Most flexible in modeling driver behavior
– More difficult to use as there are more
parameters affecting driver behavior
Case Study #1 – Chico, CA
• Widening of SR 32 in Chico, CA
• Two-lane rural highway with access
control
• Design Year (2030) PM Peak Hour
• Project considered roundabouts at
the 4 study intersections
Proposed Design
Source: Mark Thomas & Co.
RODEL Results: LOS A
SIDRA Results: LOS C
VISSIM Results: LOS F
FHWA Guidelines
• ADT on SR 32 & Bruce Rd at the limit
– Daily volumes on SR 32 are above 40,000
vpd with high left-turn percentages
• Peak-hour volumes exceed entry &
circulatory volume capacity
– Entry + circulatory flow ~ 2,500 vph
• Both indicate that the roundabout
should be at or over capacity
Review of RODEL Results
• 20% increase in volume yields LOS F
• RODEL manual does not identify the
upper limit of the empirical data
• RODEL does not account for rightturn delay even if vehicle queues
block access to bypass lanes
• RODEL does not consider the
capacity of the circulatory roadway
Review of SIDRA Results
V/C Ratios on all
approaches exceed 0.85
95th percentile
queue is 883 feet
Review of VISSIM Results
Insufficient
Circulatory Capacity
Blocked Right-turn
Bypass Lanes
Case Study #2 – Reno, NV
• Initial study performed using RODEL –
Volumes were 40% lower, Result = LOS A
• Asked by local agency to verify results
• Comprehensive traffic analysis was
performed using Vissim
• Volumes analyzed represent 20-year horizon
PM peak hour
• Analysis compared signal and roundabout
Study Area
Proposed Roundabout
Location
Proposed Design Plan
2-lane section matches
existing roadway
configuration
Proposed Design Results
Southbound queue as
long as existing queue
with all-way-stop
Northbound vehicles
will experience long
queues and delay
Modified Design Plan
SB Kietzke Ln
3-lane section allows both
southbound lanes on
Kietzke Ln to enter freeway
Neil Rd Between Kietzke Ln and
US 395 SB Ramps
Modified Design Results
Maximum Queue Length
Approach
Length (feet)
WB Neil Road
535
SB Kietzke Ln (left-turn)
270
EB Del Monte Ln
160
NB Kietzke Ln (through)
240
NB Kietzke Ln (right)
225
SB Neil Rd at Kietzke Ln
Level of Service
Intersection
Delay
LOS
US 395 NB Ramps / Neil Rd
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D
US 395 SB Ramps / Neil Rd
25
C
Kietzke Ln / Neil Rd
10
A
NB Neil Rd at Kietzke Ln
Modified Design Results
Parting Thoughts
• Similar to signalized intersection
analysis, there is no “best”
methodology
• Methodology should be dependant
on the situation
• Concepts in the FHWA Simulation
Guidelines & HCM also hold true in
identifying whether roundabouts
should be simulated
More Information
• Roundabouts: An Informational
Guide (FHWA, 2000)
– New edition due in 2009 or 2010
• TRB National Roundabout
Conference
– 2005 (Vail, CO)
– 2008 (Kansas City, MO)
• ITE Conferences
• DOT Guidance – CA, FL, NY, WI, etc.
Questions?