FHWA-SA-09-013

This case study is one in a series documenting successful 

intersection safety treatments and the crash reductions that 

were experienced. Traffic engineers and other transportation 

professionals can use the information contained in this case 

study to answer the following questions:

•	 What isan intersection alternativethat can dramaticallyreducecrashes

(particularlydangerousanglecrashes)at high-speed, rural stopcontrolledintersections?

•	 How manycrashesdidthetreatment reduce?

•	 Arethereanyimplementation issuesassociated with roundabouts in

generalandif so, how can theybeovercome?

Minnesota Roundabout

A Scott County Success Story

Photo by Jody K. Hassel (used with permission).

Intersection Safety Case Study
2 FHWA | Minnesota Roundabout

“It’s worked great. 

We could not have 

hoped for better 

results.... All of the 

serious crashes have 

just gone away.” 

Paul Kachelmyer 

Minnesota Department of 

Transportation

Introduction

The first roundabout in Scott County, Minnesota was constructed at the intersection of 

State Highway 13 and County Road 2 (see Figure 1). The intersection, previously with twoway stop-control on County Road 2, was the site of two fatal crashes and 50 injury crashes 

in a five-year period between June 2000 and June 2005. 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.

Prior to converting the intersection to a roundabout, the 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 concerns that neither of these alternatives would resolve the crash problem. 

As Paul Kachelmyer, Project Manager with Mn/DOT stated, “Only in a fraction of the cases 

where signals and four-way stop signs are put in do they actually reduce crashes[1].”

Then, Kachelmyer learned that Maryland reduced crashes by 90 percent by converting 

a similar intersection to a modern roundabout[2].

Figure 1: Intersection of State Highway 13 and County Road 2

Photo by Jody K. Hassel (used with permission).
FHWA | Minnesota Roundabout 3

Objective

The following case study showcases the measurable safety benefits experienced by Scott County, Minnesota, 

following conversion of a high-speed rural, two-way stop-controlled intersection to a roundabout.

Treatment Summary

The existing intersection of Highway 13 and 

County Road 2 met minimum Manual on Uniform 

Traffic Control Devices (MUTCD) standards. 

This case study examines conversion of a highspeed, two-way stop-controlled intersection to a 

roundabout.

Roundabouts are circular intersections with 

specific design and traffic control features. These 

features include yield control of all entering 

traffic, channelized approaches, and appropriate 

geometric curvature to ensure that travel speeds 

on the circulatory roadway are typically less than 

30 miles per hour (mph). Also, traffic movement 

is possible only in a counter-clockwise direction 

within the roundabout. Federal Highway 

Administration’s (FHWA) Roundabouts: An 

Informational Guide provides more information 

on the defining characteristics and definitions 

for each of the key features of a roundabout (see 

Figures 2 and 3)[5]. 

Roundabout intersections eliminate a number of 

vehicle conflict points typically associated with 

traditional intersections. A four-legged, singlelane roundabout has 75 percent fewer vehicle 

conflict points than a traditional stop-controlled 

intersection[5]. Roundabouts also enhance safety 

by reducing vehicle speeds both in and through 

the intersection.

Two-lane roundabout

Eight vehicle to vehicle conicts

Eight vehicle to pedestrian conicts

Two-lane road standard intersection

32 vehicle to vehicle conicts

24 vehicle to pedestrian conicts

Figure 2: Eight vehicle conflict points

Source: FHWA Roundabouts: An Informational Guide

Figure 3: 32 vehicle conflict points 

Source: FHWA Roundabouts: An Informational Guide
4 FHWA | Minnesota Roundabout

1 Note that crash reduction averages in this report reflect the percent reduction per year based on the difference between the total number of “before” and “after” crashes. 

Injury reductions per year are noted where supporting data was available.

Evaluation Methodology

This case study examines crash data before and after 

conversion of a high-speed, rural, two-way stopcontrolled intersection in Scott County, MN, with a 

high incidence of crashes (many with injuries) due 

to driver violation of the stop-control condition. 

Crash reduction results were based on a review of 

“before and after” data from this intersection during 

a minimum of five years, between 2002-20071. 

(The “before” and “after” observation periods were 

approximately 36 and 24 months respectively). 

Photo by Jody K. Hassel (used with permission).
Months

Total 

Crashes

Injury 

Crashes

Months

Total 

Crashes

Total 

Crashes

Injury 

Crashes

Injury 

Crashes

Angle 

Crashes

Angle 

Crashes

Angle 

Crashes 

(2)

Location Implementation

Date

Before After Reduction In Crashes/Year

State 

Highway 13 

and County 

Road 2

Aug-05 36 19 14 18 24 3 2 0

(1-fatality)

76.2% 78.7% 100% 

FHWA | Minnesota Roundabout 5

Results

Problem: A rural, high-speed intersection, controlled 

by a two-way stop was the site of two fatal crashes 

and 50 injury crashes (most of which were angle 

crashes) in a five-year period. Attempts to reduce 

crash frequency with a variety of safety treatments at 

the intersection (including larger stop signs, striping, 

and flashing lights) were relatively unsuccessful.

Solution: Mn/DOT sought to reduce crashes by 

converting the two-way stop controlled intersection 

to a single-lane roundabout. The roundabout was 

constructed with a central island 110 feet in diameter. 

Approach lanes to the roundabout were 16 feet 

wide, and departure lanes from the roundabout 

were 14 feet wide. A 200-foot long median island 

was provided on County Road 2, and a 300-foot 

long median island was provided on State Highway 

13. Splitter islands were 25 feet wide, and the truck 

apron is 10 feet wide with rumble strips spaced at 

approximately 20 feet, center to center.

Table 1 summarizes the results of the “before and 

after” crash analysis at the intersection. Mn/DOT 

completed the three-month construction of the 

roundabout in August 2005. In the three-year period 

“before” construction of the roundabout, there were 

19 total crashes and 14 injury crashes. In the two-year 

period following the construction of the roundabout, 

there were only three total crashes and two injury 

crashes reported. 

While the old two-way stop-controlled intersection 

experienced 18 right-angle crashes (one of which 

involved a fatality) in the three-year period prior to the 

roundabout conversion, there have been no angle 

crashes since the intersection was converted to a 

roundabout (a 100% reduction). Total crashes per 

year at the intersection have been reduced from 

6.3 to 1.5 (a 76.2 percent reduction) and injury 

crashes per year have been reduced from 4.7 to 

only 1 (a 78.7 percent reduction).

As a result of this project, Mn/DOT has more than a 

dozen roundabouts planned in the state over the next 

few years, especially at rural intersections with high 

crash rates. 

Table 1: Summary of crash reductions after conversion to a roundabout intersection
Construction Cost

The cost of constructing the roundabout was 

approximately $725,000 (Additional right-of-way 

acquisition was not required).

Effectiveness 

The conversion of this intersection from a two-way stopcontrolled intersection to a roundabout was effective at 

reducing total crashes, and angle crashes. “We’re definitely 

going for these in the future,” Kachelmyer said. “This is the 

way to go if we’ve got a crash problem.”

Summary of Results

The “before” conditions at this intersection met minimum 

MUTCD standards. Mn/DOT converted this high-speed 

rural intersection to a roundabout to reduce high crash 

rates that had not been mitigated through other lowercost treatments. Converting this intersection to a 

roundabout resulted in a 76.2 percent reduction 

in total crashes, a 78.7 percent reduction in injury 

crashes, and a 100 percent reduction in angle crashes.

The average reductions in crashes achieved by this 

roundabout conversion 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 by the Department of Transportation 

(USDOT) FHWA[4].

Discussion

Implementation Issues

Scott County experienced no implementation 

issues with the roundabout. Because of the 

rural environment in which the roundabout 

was located, the design of the curb and apron 

were modified to allow wide farm vehicles 

and large snow removal vehicles to effectively 

maneuver through the roundabout. In addition, 

the roundabout design incorporated additional 

signage (as shown in Figure 4), extended medians 

and curbs, and a raised center mound to visually 

convey to drivers the need to slow down and 

navigate the turn. 

Construction Period

The construction of the roundabout took place 

in 2005 and lasted 90 days. During this time, both 

State Highway 13 and Scott County Road 2 were 

closed at the intersection, and motorists were 

advised to follow the posted detour routes during 

the closure. Figure 5 shows the construction 

techniques during construction.

6 FHWA | Minnesota Roundabout

Figure 5: Construction of the roundabout

Figure 4: Sign on the approach to the roundabout

Photo courtesy of MnDOT (used with permission).

Photo courtesy of MnDOT (used with permission).
FHWA | Minnesota Roundabout 7

References

1) “Minnesota successful with Roundabouts”, Independent 

Record, March 20, 2008. 

(http://www.helenair.com/articles/2008/03/20/local/

54lo_032008_minnesota.txt).

2) “Roundabouts: Coming to a road near you”, Chaska Herald, 

Transportation, March 14, 2008. 

(http://www.chaskaherald.com/news/transportation/

roundabouts-coming-road-near-you-3893).

3) Known Modern Roundabouts On Minnesota’s City/

County Systems – July 2007. 

(http://www.richfieldroundabouts.com/RoundaboutsMN.pdf).

4) Desktop Reference for Crash Reduction Factors, 

FHWA-SA-07-015, USDOT FHWA September 2007, p. 30.

5) Roundabouts: An Information Guide, FHWA-RD-00-67, 

Federal Highway Administration, June 2000. 
FHWA-SA-09-013 February 2010

For More Information

Ed Rice

Intersection Safety Team Leader, 

FHWA Office of Safety

202.366.9064

ed.rice@dot.gov

Anthony J. Winiecki, P.E., PTOE

Scott County Public Works Department

Jordan, MN

952.496.8346

twiniecki@co.scott.mn.us

Visit FHWA’s intersection safety web site to download this and 

other case studies highlighting proven intersection safety 

treatments from across the country:

http://safety.fhwa.dot.gov/intersection