Roundabout Capacity Analysis Framework

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

2. 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

3. Analysis Framework

4. Analysis Framework • FHWA Guidelines • ADT volume thresholds • Peak hour entering vs. circulatory flow thresholds • Analysis Software • Conduct sensitivity testing • Understand methodology used

5. 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

6. 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

7. HCM 2000 • Only one-lane roundabouts • LOS thresholds are undefined • Results are a range of v/c ratios • Use Synchro or HCS+ to calculate

8. 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

9. 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

10. 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

11. RODEL

12. 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

13. 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

14. SIDRA

15. 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

16. SimTraffic

17. Paramics • Quadstone, UK • Stochastic simulation model • Driver behavior and vehicle characteristics • Link/node network structure • Automatically creates roundabout • Lane change animation is unrealistic

18. Paramics

19. VISSIM • PTV, Germany • Stochastic simulation model • 3D animation features • Link/connector network structure • Specify gap acceptance parameters by lane for each approach

20. VISSIM

21. 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

22. 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

23. 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

24. Proposed Design Source: Mark Thomas & Co.

25. RODEL Results: LOS A

26. SIDRA Results: LOS C

27. VISSIM Results: LOS F

28. 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

29. 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

30. Review of SIDRA Results 95th percentile queue is 883 feet V/C Ratios on all approaches exceed 0.85

31. Review of VISSIM Results Insufficient Circulatory Capacity Blocked Right-turn Bypass Lanes

32. 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

33. Study Area Proposed Roundabout Location

34. Proposed Design Plan 2-lane section matches existing roadway configuration

35. Proposed Design Results Northbound vehicles will experience long queues and delay Southbound queue as long as existing queue with all-way-stop

36. 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

37. Modified Design Results SB Neil Rd at Kietzke Ln NB Neil Rd at Kietzke Ln

38. Modified Design Results

39. 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

40. 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.

41. Questions?