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Safety Evaluation of Roundabouts in North Carolina
Carrie L. Simpson, PESafety Evaluation Group, NCDOT Transportation Mobility & Safety Division
NCDOT Transportation Mobility & Safety Division
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Evaluation Objectives:The Purpose of this Evaluation is to Determine the Safety Effectiveness of Roundabouts Installations Statewide
• Crash Frequency • Severity • Crash Types• Night Crashes & Lighting Conditions• Relationships between Intersection Geometry & Crashes • Relationships between Entering Volume & Crashes •Vehicle Speeds•Signing and Marking Practices
Introduction
NCDOT Transportation Mobility & Safety Division
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Roundabout Study Locations
NCDOT Transportation Mobility & Safety Division
54 Total Study Locations(30 Locations for Before & After)
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Roundabout Study Locations
NCDOT Transportation Mobility & Safety Division
Buncombe County
Mecklenburg County
Brunswick County
Union County
Mecklenburg County
Suburban
Urban
Rural
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Roundabout Study Locations
NCDOT Transportation Mobility & Safety Division
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Crash Analysis Results
NCDOT Transportation Mobility & Safety Division
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North Carolina Crash Analysis Results
* As crash classified in DMV 349( ) = standard deviation of an estimated value
NCDOT Transportation Mobility & Safety Division
Percent Crash Reductions at NC Roundabouts (Naïve Before & After with Linear Traffic Factor)
All 30 SitesTotal Crashes 46.2% (5.2)Injury Crashes - All Types 75.3% (4.9)Injury Crashes - KAB 85.0% (6.5)
Frontal Impact Crashes* 75.6% (3.9)Rear End Crashes* 29.9% (13.2)Sideswipe Crashes* 20.1% (28.9)
Day 56.0% (5.0)Night 2.8% (18.2)
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National Roundabout Safety Statistics
* As crash classified in DMV 349( ) = standard deviation of an estimated value
NCDOT Transportation Mobility & Safety Division
SOURCE: NCHRP 672 (2010)
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Crash Analysis Results - Immediately After
NCDOT Transportation Mobility & Safety Division
0
0.5
1
1.5
2
2.5
3
Before: Two-Way Stop [24] Before: Signalized [6] After: First Year [54] After: Remainder of AfterPeriod [50]
Safety Performance First Year After Installation
Tota
l Cra
shes
/ Yr/
Site
2.11.9
0.91.2
RoundaboutFirst Year
RoundaboutRemaining Years
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Crash Analysis Results:Crash Types
NCDOT Transportation Mobility & Safety Division
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Roundabout Crash Types
NCDOT Transportation Mobility & Safety Division
( ) = Crash Frequency0 Pedestrian Crashes ( ) = Crash Frequency
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NCDOT Transportation Mobility & Safety Division
No Pedestrian Crashes (Before or After)
Bicycle Crashes
Before (30 Sites) After (54 Sites)
Sites with Bicycle CrashesStudy Years #
Injury Type
Vehicle Impact Speed Location #
Injury Type
Vehicle Impact Speed Location
WT Weaver @ University Heights 7 1 B 10 mph inside 0 --- --- ---
US 421 NB Ramp @ Williams 6 0 --- --- --- 1 B 5 mph entry
US 421 SB Ramp @ Williams 9 0 --- --- --- 1 C 15 mph inside
Ninth @ Davidson 8 1 B 5 mph xwalk 0 --- --- ---
Voit Gilmore @ Knoll 4 0 --- --- --- 1 B 20 mph xwalk
Pullen @ Stinson 6 1 C 5 mph xwalk 1 C 5 mph entry
SUM 3 4
Bicycle and Pedestrian Involved Crashes
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Heavy Vehicle Involvement
NCDOT Transportation Mobility & Safety Division
( ) = Crash Frequency
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Day Vs. Night Crashes
( ) = standard deviation of an estimated value
NCDOT Transportation Mobility & Safety Division
236
111
9 7 29 32 25 260
50
100
150
200
250
Cra
shes
Daylight Dusk/Dawn Dark-LightedRoadway
Dark-RoadwayNot Lighted
Lighting Condition
BeforeAfter
% Crash Reduction With Linear Traffic Factor
Day 56.0% (5.0)
Night 2.8% (18.2)( ) = standard deviation of an estimated value
BeforeAfter
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Crash Analysis Results:Intersection Features & Crashes
NCDOT Transportation Mobility & Safety Division
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Crash Analysis Results – Size Categories
NCDOT Transportation Mobility & Safety Division
SINGLE LANE Sites Total Crashes KAB Injury Crashes
Mini & Compact 9 60.7% (7.5) 100%Mini 4 57.7% (17.1) 100%Compact 5 61.9% (8.1) 100%
Standard & Large 14 56.3% (6.3) 84.8% (9.0)Standard 10 58.9% (7.3) 90.5% (9.0)Large 4 51.4% (12.1) 80.7% (13.6)
Percent Crash Reductions at NC Roundabouts (Naïve Before & After with Linear Traffic Factor)
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Crash Analysis Results – Size Categories
NCDOT Transportation Mobility & Safety Division
Sites Total Crashes KAB Injury Crashes
Single Lane (All, Non-Ramp) 23 57.8% (4.9) 89.6% (6.1)
Double Lane 1 -11.2% (42.1) N/A
Ramp 6 2.5% (19.8) 75.8% (14.5)
Percent Crash Reductions at NC Roundabouts (Naïve Before & After with Linear Traffic Factor)
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Crash Analysis Results – Prior Control
NCDOT Transportation Mobility & Safety Division
Percent Crash Reductions at NC Roundabouts (Naïve Before & After with Linear Traffic Factor)
Before Period Control Type Sites Total Crashes
KABInjury Crashes
Two-Way Stop 24 47.7% (5.7) 78.1% (9.6)
Signalized 6 41.2% (11.8) 100%
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Crash Analysis Results – Number of Legs
NCDOT Transportation Mobility & Safety Division
Percent Crash Reductions at NC Roundabouts (Naïve Before & After with Linear Traffic Factor)
Number of Legs Sites* Total CrashesKAB
Injury Crashes
3 Leg 8 57.1% (9.7) 89.3% (10.1)
4 Leg 15 58.2% (5.6) 90.2% (6.9)* All Sizes of Single Lane, Non-Ramp Roundabouts
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Crash Analysis Results – Bypass Lanes
NCDOT Transportation Mobility & Safety Division
Avg. Sideswipe Crashes/Yr/Site Sites Before After
Avg. AADT (Excluding Ramps)
Bypass Lane 5 0.2 0.1 10,800
No Bypass Lane 25 0.1 0.1 8,000
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Crash Analysis Results:Entering Volume & Crashes
NCDOT Transportation Mobility & Safety Division
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NCDOT Transportation Mobility & Safety Division
Comparison of Single Lane Roundabouts, All-Way Stops, Two-Way Stops & SignalsEntering Volume vs. Crashes/Yr
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NCDOT Transportation Mobility & Safety Division
Comparison of Multi Lane Roundabouts, 2-Way Stops & SignalsEntering Volume vs. Crashes/Yr
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Crash Analysis Results:Speed Related Data
NCDOT Transportation Mobility & Safety Division
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Crash Analysis Results - Speed Limits
NCDOT Transportation Mobility & Safety Division
Percent Crash Reductions at NC Roundabouts (Naïve Before & After with Linear Traffic Factor)
Approach Speed Limits Sites Total CrashesKAB
Injury Crashes
Low Speed (< 45 mph) 19 39.9% (7.9) 90.9% (6.4)
High Speed (≥ 45 mph) 11 52.6% (6.7) 79.0% (11.1)
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Speeds at Impact (mph)
NCDOT Transportation Mobility & Safety Division
Estimated Speeds at Crash Impact (30 Sites) Before After % Difference
Average Speed 18.2 15.1 -17%
Average Speed: Vehicle 1 16.8 16.4 -2%
Average Speed: Vehicle 2 19.7 13.3 -32%
Max Speed 70 55
Interquartile Range (IQR)* 5-30 [25] 5-20 [15]*IQR is the range of the middle 50% of data.
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Signing & Marking Practices
NCDOT Transportation Mobility & Safety Division
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Advanced Warning Signs
NCDOT Transportation Mobility & Safety Division
MUTCD 2009
MUTCD 2009
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NC Advanced Warning Sign Practices
NCDOT Transportation Mobility & Safety Division
10 10 10 14 7 9 10 5 12
13
12 14 9
9
9
9* 10*
12*
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NCDOT Transportation Mobility & Safety Division
NC Advanced Warning Sign Practices
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Advisory Speed Limits
NCDOT Transportation Mobility & Safety Division
2009 MUTCD
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Advisory Speed Limits
NCDOT Transportation Mobility & Safety Division
NCHRP 672 Roundabout Guide (2010):
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Advisory Speed Limits & Crashes/Yr
NCDOT Transportation Mobility & Safety Division
Average Crashes Per Year at Sites With and Without Advisory Speed Limits
2.4
1.61.3
0.9 0.9 0.8
0.0
0.5
1.0
1.5
2.0
2.5
Ramp with AdvisorySpeed
Ramp withoutAdvisory Speed
High Speed withAdvisory
High Speed withoutAdvisory
Low Speed withAdvisory Speed
Low Speed withoutAdvisory Speed
Ave
rage
Cra
shes
Per
Yea
r
20 mph 20 mph15 mph
20 mph15 mph10 mph
None None None
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NCDOT Transportation Mobility & Safety Division
Number of Signs Per Approach
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NCDOT Transportation Mobility & Safety Division
NCDOT Signing Layout - RURAL
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NCDOT Transportation Mobility & Safety Division
Rural Signing & Marking Examples
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NCDOT Transportation Mobility & Safety Division
NCDOT Signing Layouts - URBAN
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NCDOT Transportation Mobility & Safety Division
Pedestrian Accommodations
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NCDOT Transportation Mobility & Safety Division
Pedestrian Accommodations
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NCDOT Transportation Mobility & Safety Division
Pedestrian Accommodations
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NCDOT Transportation Mobility & Safety Division
Bicycle Accommodations
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NCDOT Transportation Mobility & Safety Division
Pavement Marking PracticesMarking Type % of Roundabouts [#]
Yield Entry Lines72% [39]
Dashed Entry Lines
72% [39]
“Yield” Markings6% [3]
Advance Arrow Markings
13% [7]
In Circle Arrow Markings
19% [10]
Marked Crosswalks
59% [32]
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Collision Diagrams
NCDOT Transportation Mobility & Safety Division
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Crash Diagrams: Westview @ Buckingham, Forsyth Co.MINI SINGLE LN
BEFORE (8 Years of Data)
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Crash Diagrams: Westview @ Buckingham, Forsyth Co.
AFTER (8 Years of Data)
MINI SINGLE LN
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Crash Diagrams: WT Weaver @ University Heights, Buncombe Co.
COMPACT SINGLE LN
BEFORE (7 Years of Data)
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Crash Diagrams: WT Weaver @ University Heights, Buncombe Co.
COMPACT SINGLE LN
AFTER (7 Years of Data)
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Crash Diagrams: NC 751 @ Erwin Rd, Durham Co.COMPACT SINGLE LN WITH BYPASS LN
BEFORE (6 Years of Data)
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Crash Diagrams: NC 751 @ Erwin Rd, Durham Co.COMPACT SINGLE LN WITH BYPASS LN
AFTER (6 Years of Data)
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Crash Diagrams: Lake Jeanette @ Elm, Guilford Co.STANDARD SINGLE LANE
BEFORE (3 Years of Data)
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Crash Diagrams: Lake Jeanette @ Elm, Guilford Co.STANDARD SINGLE LANE
AFTER (3 Years of Data)
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Crash Diagrams: Gaston Day School @ Kendrick, Gaston Co.STANDARD SINGLE LANE
BEFORE (4 Years of Data)
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Crash Diagrams: Gaston Day School @ Kendrick, Gaston Co.STANDARD SINGLE LANE
AFTER (4 Years of Data)
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Crash Diagrams: Ninth @ Davidson, Mecklenburg Co.LARGE SINGLE LANE
BEFORE (8 Years of Data)
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Crash Diagrams: Ninth @ Davidson, Mecklenburg Co.LARGE SINGLE LANE
AFTER (8 Years of Data)
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Crash Diagrams: US 421 @ Williams Rd, Forsyth Co.INTERCHANGE
BEFORE (9 Years of Data) BEFORE (6 Years of Data)
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Crash Diagrams: US 421 @ Williams Rd, Forsyth Co.INTERCHANGE
AFTER (9 Years of Data) AFTER (6 Years of Data)
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Crash Diagrams: Main @ Salem, Forsyth Co.MULTI LANE
BEFORE (4 Years of Data)
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AFTER (4 Years of Data)
Crash Diagrams: Main @ Salem, Forsyth Co.MULTI LANE
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CONCLUSIONRecommended Crash Reduction Factors:Total Crashes -46%Injury Crashes -75%Frontal Impact Crashes -76%
NCDOT Transportation Mobility & Safety Division