NCHRP 3-65: Applying Roundabouts in the United States Preliminary Findings

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1 NCHRP 3-65: Applying Roundabouts in the United States Preliminary Findings Lee A. Rodegerdts, P.E. AASHTO Subcommittee on Design, Chicago, IL June 27, 2005 Bhagwant Persaud, Canada David Harkey, USA George Mason University, USA McDonald & Partners, UK Rensselaer Polytechnic University, USA Rod Troutbeck, Australia Ruhr-University Bochum, Germany University of Idaho, USA

AASHTO Subcommittee on Design, Chicago, IL June 27, 2005 Bhagwant Persaud, Canada David

2 Topics of Discussion Project panel and team Project need and objective Preliminary findings Safety Operations Design Anticipated products

3 Project panel Beatriz Caicedo-Maddison, Florida DOT (chair) Maria Burke, Texas DOT Jerry Champa, California DOT Leonard Evans, Science Serving Society Steve King, Kansas DOT Robert Limoges, New York State DOT Richard Long, Western Michigan University Ron Pfefer, HSM liaison Brian Walsh, Washington State DOT Mohsin Zaidi, City of Kansas City, MO Joe Bared, FHWA Hari Kalla, FHWA Rich Cunard, TRB Ray Derr, NCHRP

State DOT Richard Long, Western Michigan University Ron Pfefer, HSM liaison Brian Walsh, Washington

4 Project team P.I.: Lee Rodegerdts (KAI) (Bruce Robinson, Co-P.I. Emeritus) USA Kittelson & Associates, Inc. University of Idaho Rensselaer Polytechnic Institute George Mason University David Harkey John Mason Australia Rod Troutbeck Canada Bhagwant Persaud Germany Werner Brilon United Kingdom Richard Hall

David Harkey John Mason Australia Rod Troutbeck Canada Bhagwant Persaud

5 U.S. practice relies heavily on the experience from other countries. Current U.S. procedures depend on international methods without having U.S. data for calibration Use of roundabouts in the U.S. may differ from that experienced in other countries

6 Overview of research tasks 1. Summarize Existing Relationships 2. Model Formulation 3. Data Collection Plan 4. Interim Report 5. Execute the approved data-collection plan 6. Inventory U.S. Roundabout Sites 7. Operational Performance Methods 8. Safety Performance Methods 9. Design Criteria 10. Final Report 11. Prepare marketing materials

Execute the approved data-collection plan 6. Inventory U.S. Roundabout Sites 7.

7 Preliminary Safety Findings Roundabout-level accident models Approach-level accident models Before-after study of intersections converted to roundabouts

8 Roundabout-Level Crash Prediction Used for comparing performance to other intersection types Baseline prediction on which approach-level CMFs could be applied Form: Crashes = α(aadt) β Factors affecting coefficients: Number of lanes Number of approaches

which approach-level CMFs could be applied Form: Crashes =

9 Approach-Level Crash Data (139 approaches) Total Number of Approach Crashes Total Number of Crashes Single Lane Multi-Lane Entering Circulating Exiting/Circulating Rear End on Approach Loss of Control Pedestrian Bicycle Crash Type

Single Lane Multi-Lane Entering Circulating

10 % change in crashes from candidate approach level models per unit change in variable Variable Entering/ Circulating Exiting/ Circulating Approach Entry Radius (ft.) 1% reduction Entry Width (ft.) 5% increase Approach Half Width (ft.) 3% increase Inscribed Circle Diameter (ft.) 2.2% increase Central Island Diameter (ft.) 0.5 to 0.8% reduction 1.4% increase Circulating Width (ft.) 12% increase Angle To Next Leg (degree) 3% reduction

) 5% increase Approach Half Width (ft.) 3% increase Inscribed Circle Diameter (ft.) 2.

11 Before-After Results All Sites (55) All Injury Crashes recorded in after period EB estimate of accidents expected after without roundabouts Reduction (Standard error) 35.4 % (3.4) 75.8 % (3.2) CONTROL BEFORE All Injury SIGNALS (9) 48% 78% TWO WAY STOP (34) 44% 82% ALL-WAY STOP (10) Insignificant increase

Reduction (Standard error) 35.4 % (3.4) 75.8 % (3.

12 Preliminary Operations Findings Analysis of existing models Driver behavior and effect of geometry HCM recommendations

13 Analysis of Existing Models All international models (including SIDRA and RODEL) predict capacities higher than observed 2000 WA04-N (Port Orchard) Max Entering Flow (pcus/hr) Raw Data Akcelik Uncalibrated Kimber Uncalibrated Linear (Raw Data) Conflicting Flow (pcus/hr)

Max Entering Flow (pcus/hr) 1500 1000 500 Raw Data Akcelik Uncalibrated

14 Influence of Flow & Geometry on Driver Behavior Entry lane width = entry width / # lanes All tf All tc Linear (All Gap Parameters y = x R 2 = y = x R 2 = Entry Width

0 All tf All tc Linear (All Gap Parameters 5.0 4.0 3.0 y = -0.

15 Multilane Modeling Issues Several sites exhibit strong queuing in only one lane Possible causes: Turning movement effects Lane use assignment (or lack thereof) Geometric effects (vehicle path overlap) Driver unfamiliarity Model intended to allow designer to capture these first-order effects apparent in U.S. data

thereof) Geometric effects (vehicle path overlap) Driver unfamiliarity Model

16 Proposed HCM Capacity Models Single-lane: Current HCM model with new t c, t f Multi-Lane: Exp. regression model for critical lane Entry capacity (veh/h) Roundabout Entry Capacity Conflicting flow (veh/h) Single-Lane Capacity Multilane Critical Lane Capacity

regression model for critical lane Entry capacity (veh/h) 1400 1200 1000 800

17 Preliminary Design Findings Design speed modeling Other design findings for motor vehicles Pedestrian and bicycle observations

18 Current FHWA speed prediction method is based on AASHTO speed-radius function Speed (mph) Radius (ft) e=+0.02 e=-0.02

40 35 30 Speed (mph) 25 20 15 10 5 0 0 50

19 Design speed modeling: V4, Left-turn turn circulating speed (all sites) 35 Actual Speed, V4a (mph), All Sites y = x R 2 = Predicted Speed, V4p (mph), All Sites Data Match Line 85th %ile (15+ obs.) Linear (85th %ile (15+ obs.))

20 Design speed modeling: Exit speed (all sites), unadjusted Actual Exit Speed, V3a or V6a (mph), All Sites y = x R 2 = Unadjusted Predicted Exit Speed, V3pbase or V6pbase (mph), All Sites V3 Data Match Line 85th %ile (15+ obs.) V6 Data Linear (85th %ile (15+ obs.))

2933 0 5 10 15 20 25 30 35 40 45 50 Unadjusted Predicted Exit Speed, V3pbase or

21 Proposed exit speed equation V 3 where: = min V 3 = V 3 speed, in mph V (1.47V 3 pbase 2 ) 2 + V 3pbase = V 3 speed predicted based on path radius, in mph V 2 = V 2 speed predicted based on path radius, in mph 2a 23 d a 23 = acceleration along the length between the midpoint of V 2 path and the point of interest along V 3 path = 6.9 ft/s 2 d 23 = distance between midpoint of V 2 path and point of interest along V 3 path, in ft 23 Speed where exit radius is limiting factor Speed where circulating speed and acceleration distance is limiting factor

22 Design speed modeling: Exit speed (all sites), adjusted Actual Speed, V3a or V6a (mph), All Sites y = x R 2 = Adjusted Predicted Speed, V3p2 or V6p2 (mph), All Sites Match Line 85th %ile (15+ obs.) Linear (85th %ile (15+ obs.))

23 Implications for design Tangential or nearly tangential exits do not appear to cause excessive vehicle exit speeds if the following conditions are met: The speed of circulation (V2 and V4) is kept low The distance between the start of the exit path and the point of interest (e.g., crosswalk) is kept short Similar prediction adjustment for entry speeds Entry speed appears to be limited by drivers anticipation of the speed needed for circulation However, recommend continued reliance on entry path curvature as a primary method to control entry speed

24 Entry width and lane width Narrow lane widths (entry and circulating) at multilane roundabouts appear to have a detrimental effect on safety Entry width: Aggregated entry width (number of lanes) has a clear safety and operational effect Variations of lane width appear to be second-order effects

25 Multilane roundabout issues Higher crash frequencies and crash rates than single-lane roundabouts Vehicle path overlap, poor striping apparent contributors Anecdotal evidence suggests that their correction can substantially improve safety performance

26 Example: Clearwater Beach, FL, before and after design modifications Before (2001) Photo: Bruce Robinson After (2005) Photo: Lee Rodegerdts

27 Non-motorized users Examination of observed field behaviors for two groups: Pedestrians Bicyclists Pedestrian data: 10 approaches at 7 sites; 769 events Bicyclist data: 14 approaches at 7 sites; 690 events Geographic diversity: California, Florida, Maryland, Nevada, Oregon, Utah, Vermont, Washington

28 How do motorists behave when encountering pedestrians? Motorists failing to yield to pedestrians All sites: 30 percent Entry leg: 23 percent Exit leg: 38 percent 1-lane approaches: 17 percent 2-lane approaches: 43 percent

29 How do pedestrian behaviors at roundabouts compare to other forms of control? Crossing control Uncontrolled Percent of normal crossings 70% Percent of nonyielding vehicles 48% Roundabout 85% 32% Stopcontrolled Signalcontrolled 90% 100% 15% 4%

30 Anticipated products Final report Draft Highway Capacity Manual procedure Components compatible with a possible Highway Safety Manual procedure Updated design research for use in future updates to FHWA Roundabout Guide, AASHTO Green Book Data that is accessible for future research Problem statement(s) for continued research Anticipated completion: December 2005

31 Questions? (503) or (800) Photo: Lee Rodegerdts

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP REPORT 572. Roundabouts in the United States

NCHRP REPORT 572 NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Roundabouts in the United States TRANSPORTATION RESEARCH BOARD 27 EXECUTIVE COMMITTEE* OFFICERS CHAIR: Linda S. Watson, CEO, LYNX Central