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 NCHRP 3 NCHRP 3 NCHRP 3 NCHRP 3- - -65: Applying 65: Applying 65: Applying 65: Applying Roundabouts in the United States Roundabouts in the United States Roundabouts in the United States Roundabouts in the United States Preliminary Findings Lee A. Rodegerdts, P.E. AASHTO Subcommittee on Design, Chicago, IL June 27, 2005
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U.S. practice relies heavily on the experience U.S. practice relies heavily on the experience U.S. practice relies heavily on the experience U.S. practice relies heavily on the experience
from other countries.from other countries.from other countries.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
Overview of research tasksOverview of research tasksOverview of research tasksOverview of research tasks
�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
ApproachApproachApproachApproach----Level Crash Data (139 approaches)Level Crash Data (139 approaches)Level Crash Data (139 approaches)Level Crash Data (139 approaches)
Total Number of Approach Crashes
0
50
100
150
200
Enter
ing
Circ
ulat
ing
Exitin
g/C
ircul
atin
gR
ear E
nd o
n App
roac
h
Loss
of C
ontro
l
Pedes
trian
Bicyc
le
Crash Type
Tota
l Num
ber
of C
rashes
Single Lane
Multi-Lane
% change in crashes from candidate approach
level models per unit change in variable
3% reduction Angle To Next Leg
(degree)
12% increase Circulating Width
(ft.)
1.4% increase 0.5 to 0.8%
reduction
Central Island Diameter
(ft.)
2.2% increase Inscribed Circle Diameter
(ft.)
3% increase Approach Half Width
(ft.)
5% increase Entry Width
(ft.)
1% reduction Entry Radius
(ft.)
ApproachExiting/
Circulating
Entering/
Circulating
Variable
BeforeBeforeBeforeBefore----After Results After Results After Results After Results –––– All Sites (55)All Sites (55)All Sites (55)All Sites (55)
All Injury Crashes recorded in after period
726 72
EB estimate of accidents expected after without roundabouts
Analysis of Existing ModelsAnalysis of Existing ModelsAnalysis of Existing ModelsAnalysis of Existing Models
�All international models (including SIDRA and RODEL) predict capacities higher than observed
WA04-N (Port Orchard)
0
500
1000
1500
2000
0 250 500 750Conflicting Flow (pcus/hr)
Ma
x E
nte
rin
g F
low
(p
cu
s/h
r) Raw Data
Akcelik Uncalibrated
Kimber Uncalibrated
Linear (Raw Data)
Influence of Flow & Geometry on Driver BehaviorInfluence of Flow & Geometry on Driver BehaviorInfluence of Flow & Geometry on Driver BehaviorInfluence of Flow & Geometry on Driver Behavior
Current FHWA speed prediction method is Current FHWA speed prediction method is Current FHWA speed prediction method is Current FHWA speed prediction method is
based on AASHTO speedbased on AASHTO speedbased on AASHTO speedbased on AASHTO speed----radius function.radius function.radius function.radius function.
�Anecdotal evidence suggests that their correction can substantially improve safety
performance
Example: Clearwater Beach, FL, before and Example: Clearwater Beach, FL, before and Example: Clearwater Beach, FL, before and Example: Clearwater Beach, FL, before and
after design modificationsafter design modificationsafter design modificationsafter design modifications
� 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
How do motorists behave when encountering How do motorists behave when encountering How do motorists behave when encountering How do motorists behave when encountering
pedestrians?pedestrians?pedestrians?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
How do pedestrian behaviors at roundabouts How do pedestrian behaviors at roundabouts How do pedestrian behaviors at roundabouts How do pedestrian behaviors at roundabouts
compare to other forms of control?compare to other forms of control?compare to other forms of control?compare to other forms of control?