Roundabout Capacity Analysis Framework Presented By: Dave Stanek & Joel Rabinovitz Fehr & Peers Presented By: Dave Stanek & Joel Rabinovitz Fehr & Peers Source: FHWA, Dokken Engineering

Presentation Overview Roundabout Analysis Framework Roundabout Analysis Methods & Software Case Study #1 – SR 32 in Chico, CA Case Study #2 – Kietzke Lane in Reno, NV 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

FHWA Guidelines –ADT volume thresholds –Peak hour entering vs. circulatory flow thresholds Analysis Software –Conduct sensitivity testing –Understand methodology used 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.) Review LOS results Review queuing results Visually inspect simulation Conduct sensitivity testing –NCRHP 572 gap acceptance When to Use –V/C ratios are > 0.85 –Corridor or system –Unique geometry (5-legs, adjacent signal, etc.) 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 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 Only one-lane roundabouts LOS thresholds are undefined Results are a range of v/c ratios Use Synchro or HCS+ to calculate 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 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 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 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

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 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 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 Trafficware, USA Stochastic simulation model Uses Synchro for data input Driver behavior and vehicle characteristics Can change headway factors to calibrate to local conditions 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 Quadstone, UK Stochastic simulation model Driver behavior and vehicle characteristics Link/node network structure Automatically creates roundabout Lane change animation is unrealistic

Paramics

VISSIM PTV, Germany Stochastic simulation model 3D animation features Link/connector network structure Specify gap acceptance parameters by lane for each approach 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 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 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 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 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 right- turn delay even if vehicle queues block access to bypass lanes RODEL does not consider the capacity of the circulatory roadway 20% increase in volume yields LOS F RODEL manual does not identify the upper limit of the empirical data RODEL does not account for right- turn 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 th 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 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 Neil Rd Between Kietzke Ln and US 395 SB Ramps 3-lane section allows both southbound lanes on Kietzke Ln to enter freeway

Modified Design Results Maximum Queue Length ApproachLength (feet) WB Neil Road535 SB Kietzke Ln (left-turn)270 EB Del Monte Ln160 NB Kietzke Ln (through)240 NB Kietzke Ln (right)225 Level of Service IntersectionDelayLOS US 395 NB Ramps / Neil Rd42D US 395 SB Ramps / Neil Rd25C Kietzke Ln / Neil Rd10A SB Neil Rd at Kietzke Ln 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 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. 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?