0. Minnesota Roundabout A Scott County Success Story FHWA-SA-09-013
This case study is one in a series documenting successful intersection safety treatments and the crash reduction that might be expected if implemented. Traffic engineers and other transportation professionals can use the information contained in this case study to answer the following questions:
What is an intersection alternative that can dramatically reduce crashes, particularly dangerous angle crashes, at high-speed, rural, stop-controlled intersections?
How many crashes did the treatment reduce?
Are there any implementation issues associated with roundabouts in general and if so, how can they be overcome?
A Scott County Success Story
FHWA-SA

1. The Challenge
The intersection of State Highway 13 and County Road 2 in Scott County, Minnesota, was the site of 2 fatal crashes and 50 injury crashes between June 2000 and June 2005.
Minnesota Department of Transportation (Mn/DOT) tried several treatments, including larger stop signs, striping, and flashing lights, with little success.
Mn/DOT sought to reduce the number and severity of crashes at this two-way stop-controlled intersection.
The intersection of State Highway 13 and County Road 2 in Scott County, Minnesota, was the site of 2 fatal crashes and 50 injury crashes between June 2000 and June The intersection was two-way stop-controlled and met minimum Manual on Uniform Traffic Control Devices (MUTCD) standards. Minnesota Department of Transportation (Mn/DOT) tried several safety improvement treatments to reduce the number of crashes. However these treatments, including larger stop signs, striping, and flashing lights, met with little success. Mn/DOT also considered converting the intersection to a four-way stop-controlled intersection or to a signalized intersection, but there were fears that neither of these alternatives would resolve the crash problem. 1

2. Solution Single Lane Roundabout: 4 approach lanes 1 circulatory lane
Central Island diameter of 110 feet
Approach lane widths of 16 feet
Departure lane widths of 14 feet
200-foot long median on County Road 2
300-foot long median on State Highway 13
25 feet wide splitter islands
10 feet wide truck apron
Transverse rumble strips placed 20 feet center-to-center on apron
This case study examines the safety benefits experienced by Scott County, Minnesota, following conversion of the intersection of two high-speed rural roads to a roundabout.
The roundabout was constructed with a central island 110 feet in diameter and for a future peak hour traffic volume of 1,000 vehicles per hour. Approach lanes to the roundabout are 16 feet wide, and departure lanes from the roundabout are 14 feet wide. A 200-foot long median island is provided on County Road 2, and a 300-foot long median island is provided on State Highway 13. Splitter islands are 25 feet wide, and the truck apron width is 10 feet with transverse rumble strips placed 20 feet center to center.
The TOP PHOTO illustrates the construction of the roundabout.
The LOWER PHOTO illustrates the signage on the approach to the roundabout.
NOTES:
This is of course a summary. Please see the Case Study for intersection details. 2

3. Technical Data Location: Speed Limits: Average Daily Traffic (ADT):
County Road 2 and State Highway 13, Scott County, MN
Speed Limits:
Both County Road 2 and Highway 13 have a posted speed limit of 55 mph
Average Daily Traffic (ADT):
County Road 2 - 4,300
Highway 13 – 4,650
Construction Cost:
$725,000
Construction Time:
Constructed in 2005 and lasted for 3 months
Implementation Issues:
MnDOT experienced no implementation issues with the roundabout
Both rural roads have speed limits of 55 miles per hour (mph). State Highway 13 has an average daily traffic (ADT) of 4,650, and County Road 2 has an ADT of 4,300. The construction of the roundabout took place in 2005 and lasted 90 days. During this time, both State Highway 13 and County Road 2 were closed at the intersection, and motorists were advised to follow the posted detour routes during the closure. The cost of constructing the roundabout was $725,000 (no additional right-of-way acquisition was required). 3

4. Results Average Crash Reductions:
100% reduction in angle crashes per year
78.7% reduction in injury crashes per year
76.2% reduction in total crashes per year
These results are based on a review of data from “before and after” study, conducted over a minimum duration of five years, between (The “before” and “after” observation periods were months respectively).
The “before” conditions at this intersection met minimum MUTCD standards. In the three-year period “before” construction of the roundabout ( ), there were 19 total crashes, 14 injury crashes and 18 right-angle crashes (one of which involved a fatality). In the two-year period following the construction of the roundabout ( ), there were only three total crashes and two injury crashes reported. No angle crashes have been reported since the intersection was converted to a roundabout. The roundabout was successful in reducing total crashes at this intersection by 76.2 percent, injury crashes by 78.7 percent, and angle crashes by 100 percent per year.
The average reductions in crashes achieved by the treatments exceed the overall crash reductions of 72 percent for all crash types for a rural two-way stop-controlled single lane roundabout mentioned in the Desktop Reference for Crash Reduction Factors (September 2007), published by the United States Department of Transportation (USDOT) Federal Highway Administration (FHWA).
Note that crash reduction averages in this report reflect the percent reduction based on the difference between the total number of “before” and “after” crashes, calculated on a per year basis. 4

5. For More Information
Visit FHWA’s intersection safety web site to download this and other case studies highlighting proven intersection safety treatments from across the country:
Or Contact:
Again, this is one of a series of case studies documenting proven intersection safety techniques implemented across the country. Check out all of the summaries on the Office of Safety’s intersection safety web site at the address above. You can also contact Ed Rice from the Office of Safety, or Anthony Winiecki of Scott County Public Works Department for more information.
Anthony J. Winiecki, P.E., PTOE
Scott County Public Works Department,
Jordan, MN
Phone: (952)
Ed Rice
Office of Safety (HSSD)
Federal Highway Administration
Phone: (202)