Purdue Road School
March 9, 2011
Wes Butch
DLZ Corporation
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Background Information
Scope of Study
Study Intersections
Design Characteristics
Field Observations
Crash Data
Trucking Industry Input
New Guidelines
Areas for Additional Study
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Accommodating trucks at multi-lane roundabouts
(MLR) - debated since late 1990’s
Wisconsin DOT (WisDOT)
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Aggressive and proactive roundabout program
Very active in freight issues/planning
Have an excellent Roundabout Design Guide
Interested in how trucks operate/best design practices
for MLR’s
Partnered with Minnesota DOT (Mn/DOT) to
perform study of trucks at MLR’s
DLZ is prime consultant for study team
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Definition of “Truck”
 WB-65 in Wisconsin
 WB-62 in Minnesota
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Phase 1: Inventory/synthesis of current design
practice
Phase 2: Targeted field data collection
Phase 3: Develop new design guidelines
Phase 4: Summary materials and information
for commercial driver’s manuals
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Includes evaluation of:
 Geometry
 Operations
 Crash data
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Input from trucking industry throughout
Does not include oversize/overweight permitted
loads
Technical Advisory Committee
NOT a statistically rigorous analysis of data
(funding limitations)
Will recommend areas for further study
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Case 1: MLR where trucks overlap into
adjacent lanes (~95%+ of MLRs built to date)
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Case 2: MLR where trucks stay in lane at
entries, but overlap in circulating road/exits
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Case 3: MLR where trucks stay in lane
throughout entire intersection
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Goal was to find 15-20 representative
intersections for Phase 1 of study
Inventoried hundreds of intersections
nationwide
Findings:
 Case 1 roundabouts are everywhere
 Case 2 roundabouts are concentrated in WI, AZ,
NY, CA with smaller numbers in other states
 Case 3 roundabouts are very rare (only two built
that we could find)
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Case 1 intersections:
 4 in WI, 2 in MN, 1 in MI
 Two are 3-lane, five are 2-lane
 All are built
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Case 2 intersections:
 5 in WI, 2 in AZ
 Two are 3-lane, five are 2-lane
 All are built
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Case 3 intersections:
 1 in AZ, 3 in WI
 All are 2-lane
 Only two are built
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18 total intersections
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Case 1:
 Typically smaller diameter (140’ to 170’), entry
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widths, exit widths, entry radii
Sometimes smaller exit radii
Always included truck aprons in central island
Rarely included truck aprons on outside of entry
radii
Common to have successive curves with relatively
tight radii (approach, circulate, exit)
Circulating lane widths typically equal
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Case 2:
 Typically larger diameter (160’-200’), entry widths, exit
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widths, entry radii
Usually larger exit radii
Always included truck aprons in central island
Never included truck aprons on outside of entry radii
Approach curves typically larger and more sweeping
Rarely had successive curves with relatively tight radii
(approach, circulate, exit)
Almost always have hatched common use area
between lanes on approaches
Lane widths often unequal in circulating road
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Case 3:
 Larger diameter (180’-210’), entry widths, exit widths,
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entry radii
Larger exit radii and flat exits
Always included truck aprons in central island
Never included truck aprons on outside of entry radii
Approach curves larger and more sweeping
No successive curves with tight radii (approach,
circulate, exit)
Have hatched common use area between lanes on
approaches
Lane widths often unequal in circulating road
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Not always correlation between truck
volumes and design type
For Case 2 and Case 3 locations, trucks usually
stay in lane where design allows
Trucks prefer not to use central island apron if
they can avoid it
At Case 1 locations, truck trailers often mount
curb on outside of entry radius
Video Observations at Case 2 Approaches
Location
Approach
I-43 at Mooreland North
I-43 at Mooreland South
I-43 at Mooreland South
STH 42 at Vanguard
STH 42 at Vanguard
Totals
SB
SB
NB
EB
WB
Conflicting Traffic Present
Number of
% of time trucks
Observations
stayed in lane
49
96%
27
89%
13
100%
11
82%
19
84%
119
92%
No Conflicting Traffic Present
Number of
% of time trucks
Observations
stayed in lane
28
86%
25
36%
25
92%
10
70%
7
86%
95
73%
Video Observations at Case 2 Circulatory Roadways
Location
Approach
I-43 at Mooreland North
I-43 at Mooreland South
I-43 at Mooreland South
STH 42 at Vanguard
STH 42 at Vanguard
Totals
SB
SB
NB
EB
WB
Conflicting Traffic Present
Number of
% of time trucks
Observations
stayed in lane
49
98%
27
85%
13
54%
11
82%
19
63%
119
83%
No Conflicting Traffic Present
Number of
% of time trucks
Observations
stayed in lane
28
68%
25
24%
25
12%
10
70%
7
43%
95
40%
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Not statistically rigorous analysis – general trends
only
Compared to Case 1, Case 2 roundabouts appear
to have lower percent of crashes that involve
trucks
Truck crashes often side swipe
Not assessing increases in other crashes due to
larger geometry at Case 2 and Case 3 locations
Representative split of truck/non-truck crashes
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Case 1 locations
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Case 2 locations
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Survey questionnaire (great response) asking
about preferences for signing, design types
Not surprisingly, many prefer Case 2 and Case
3 designs
Want better signing on approaches – stay in
lane or encroach
Prefer to avoid aprons if possible
Surprisingly, liability issues not raised as major
concern
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Phase 3 of Study
Done by Summer 2011
Advice on when to use the three cases
Thresholds - Truck % or volumes?
Cost, safety, and ROW impacts will be
consideration
Specific design techniques that are preferred
for Case 2 and Case 3 designs
INDOT Guide currently allows all cases
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Rigorous statistical analysis of data:
 Do Case 1 locations have higher rate of truck
crashes?
 Do Case 1 locations have overall lower crash rates?
 Any significant differences in severity between
cases?
 How do truck percentages relate to crash rates?
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Do trucks drive Case 2 and Case 3 designs as
intended under different traffic conditions?
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WisDOT and Mn/DOT welcome input from
other state DOT’s and stakeholders
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Arizona DOT
Minnesota DOT
Wisconsin DOT
University of Wisconsin TOPS Lab
Roundabouts and Traffic Engineering, Inc.