FHWA-SA-09-018

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 the case 

study to answer the following questions:

•	 What isan intersection alternativetoimprovesafetyat rural two-way

stop-controlledintersections?

•	 How manycrashesdidthetreatment reduce?

•	 Arethereimplementation issuesassociated with roundabouts,andif so,

how can theybeovercome?

Roundabouts—

The Maryland Experience

A Maryland Success Story

Intersection Safety Case Study

Photo courtesy of KLS (used with permission).
2 FHWA | Roundabout—Maryland Experience

Introduction

Intersections represent one of the most dangerous places for drivers on the road today. 

According to the National Highway Traffic Safety Administration (NHTSA) 733,000 injury 

crashes and 7,196 fatal intersection-related crashes occurred in the United States in 2008[1]. 

Intersections are inherently dangerous because of the conflicts that occur when vehicles 

cross or turn in traffic. Modern roundabout intersections are a dramatically safer intersection 

choice because they eliminate numerous vehicular conflicts.

“Roundabouts can offer a good 

solution to safety and capacity 

problems at intersections. 

Roundabouts can also offer 

high capacity at intersections 

without requiring the expense 

of constructing and maintaining 

a traffic signal.”

Maryland State Highway 

Administration 

Objective

The following case study presents selected findings from a study conducted by the Maryland State Highway 

Administration (MDSHA) that evaluated the safety benefits of modern roundabout intersections compared to 

two-way stop-controlled intersections. Maryland has more than 60 roundabouts in operation.

Photo provided by Maryland State Highway Administration (used with permission). 
FHWA | Roundabout—Maryland Experience 3

Treatment Summary

All intersection examples used in this report are 

from Maryland. This case study examines conversion 

of five two-way stop-controlled intersections to 

roundabout intersections.

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. 

Roundabout intersections eliminate a number of 

vehicle conflict points typically associated with 

traditional intersections. A four-legged, single-lane 

roundabout has 75 percent fewer vehicle conflict 

points than a traditional stop-controlled intersection 

[2]. Roundabouts also enhance safety by reducing 

vehicle speeds both in and through the intersection.

Two-lane roundabout

Eight vehicle to vehicle conflicts

Eight vehicle to pedestrian conflicts

Two-lane road standard intersection

32 vehicle to vehicle conflicts

24 vehicle to pedestrian conflicts

Figure 1: Eight Vehicle Conflict Points

Source: FHWA Roundabouts: An Informational Guide

Figure 2: 32 Vehicle Conflict Points 

Source: FHWA Roundabouts: An Informational Guide

Evaluation Methodology

This case study examines five intersections, with a 

high incidences of crashes (many with injuries), that 

were converted to roundabouts. Crash reduction 

results were based on a review of “before and after” 

data from these intersections during a minimum of 

15 years, between 1990—20071. (The “before” and 

“after” observation periods ranged from five years to 

more than thirteen years at the treated intersections). 

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. 
4 FHWA | Roundabout—Maryland Experience

Results

Problem: Five rural, two-way, stop-controlled intersections in Maryland were experiencing a high incidence of 

injury crashes. 

Solution: MDSHA wanted to reduce the number of crashes while maintaining capacity and reducing costs. 

They decided to convert five two-way stop-controlled intersections to single-lane roundabout intersections.

Table 1 summarizes the “before and after” crash analysis at the five roundabouts. Following the table is a brief 

discussion of the results at each roundabout.

Table 1: Summary of crash reductions after conversion to roundabout intersections.

Locations

Months

Implem-entation 

Date

Total CrashesInjury CrashesFatal Crashes

Months

Total CrashesInjury CrashesFatal Crashes

Cearfoss

(MD-58 and MD-63/

MD-494)

Lisbon

(MD-94 and MD-144)

Lothian

(MD-2 and MD-408/

MD-422)

Taneytown

(MD-140 and MD-832/

Antrim Boulevard)

Leeds

(MD-213 and Leeds 

Road/Elk Mills Road)

TOTAL 300 150 82 3 640 99 21 0 69.1% 88.0% 100.0% Total Crashes Injury Crashes Fatal Crashes

Percent Reduction 

In Crashes/Year Before After

60 19 8 1 121 9 1 0

60 42 19 0 161 18 4 0

60 39 26 1 122 40 11 0

60 30 15 0 112 10 3 0

60 20 14 1 124 22 2 0

76.5% 93.8% 100.0%

84.0% 92.2% 0.0%

49.6% 79.2% 100.0%

82.1% 89.3% 0.0%

46.8% 93.1% 100.0%

Dec-95

Apr-93

Oct-95

Aug-96

Aug-95
FHWA | Roundabout—Maryland Experience 5

Cearfoss Roundabout, Washington 

County

This single-lane rural roundabout is located at the 

intersection of MD-58 and MD-63/MD-494. This was 

a two-way stop-controlled intersection prior to the 

construction of the roundabout. MD-63 from the north 

and south has the right-of-way. The posted speed limit 

on MD-63 is 50 mph in both directions and 30 mph 

along both approaches of MD-58 and MD-494. 

The roundabout was constructed with a 116 ft inscribed 

circle diameter, including a circulatory roadway 16 

ft wide. The landscaped central island has a radius 

of 42 ft with an apron width of 12 ft. The approach 

and departure lane widths of MD-63 from the south 

are 10 ft, with entry and exit widths of 13 ft and 15 ft, 

respectively. The approach and departure lane widths of 

MD-63 from the north are 10 ft and 11 ft, with entry and 

exit widths of 11 ft and 13 ft, respectively. 

The roundabout cost approximately $386,145 to design 

and construct, and opened to traffic in December 1995 

to serve a total peak hour approach volume of 800 

vehicles. The Cearfoss Roundabout resulted in a 

crash reduction of 76.5 percent and reduced injury 

crashes by 93.8 percent per year. Fatal crashes were 

eliminated.

Lisbon Roundabout, Howard County

This roundabout is located at the rural intersection of MD94 and MD-144 in Lisbon, Maryland. Prior to construction 

of the roundabout, this was a two-way stop-controlled 

intersection. The posted speed limit is 45 mph on MD-94 

and 35 mph on MD-144. 

The roundabout was constructed with an inscribed circle 

diameter of 100 ft, including a circulatory roadway 18 ft 

wide. The landscaped central island has a radius of 32 ft 

with an apron width of 12 ft. The entry and exit widths on 

MD-94 and MD-144 are 18 ft.

The roundabout cost approximately $194,000 to design 

and construct, and opened to traffic in April 1993 to serve 

a peak hour total approach volume of 696 vehicles. The 

Lisbon Roundabout resulted in a crash reduction of 

84 percent and reduced injury crashes by 92.2 percent 

per year. No fatal crashes have occurred.

Photo provided by Maryland State Highway Administration (used with permission). 

Photo provided by Maryland State Highway Administration (used with permission). 

Figure 3: Cearfoss Roundabout, MD

MD-63

MD-58

MD-63

MD-58

MD-494

Figure 4: Lisbon Roundabout, MD

MD-144

MD-144

MD-94

MD-94
6 FHWA | Roundabout—Maryland Experience

Lothian Roundabout, Anne Arundel 

County 

This single-lane roundabout is located at the 

intersection of MD-2 and MD-408/MD-422. Prior to 

construction of the roundabout, this was a two-way 

stop-controlled intersection. MD-2 from the east and 

MD-408 from the west have the right-of-way. The 

posted speed limit along MD-2 and MD-408/MD-422 

for both approaches is 40 mph. 

The roundabout was constructed with an inscribed 

circle diameter of 120 ft, including a circulatory 

roadway 18 ft wide. The landscaped central island has 

a radius of 42 ft, with an apron width of 12 ft. Approach 

and departure lane widths on all the approaches are 

10 ft. 

The roundabout cost approximately $493,880 to design 

and construct, and opened to traffic in October 1995 

to serve a total peak hour approach volume of 1,400 

vehicles. The Lothian Roundabout resulted in a 

crash reduction of 49.6percent and reduced injury 

crashes by 79.2 percent per year. Fatal crashes were 

eliminated.

Taneytown Roundabout, Carroll County 

This single-lane roundabout is located at the intersection of 

MD-140 and MD-832/Antrim Boulevard. Prior to construction 

of the roundabout, this was a two-way stop-controlled 

intersection. MD-140 from the north and MD-832 have the 

right-of-way. The posted speed limit on MD-140 from the 

north is 30 mph and MD-832 from the south is 40 mph. MD140 from the east has a posted speed limit of 50 mph and 

Antrim Boulevard has a posted speed limit of 30 mph. 

The roundabout was constructed with an inscribed circle 

diameter of 150 ft, including a circulatory roadway width of 

30 ft. The landscaped central island has a radius of 42 ft, with 

an apron width of 10 ft. The intersection geometrics are such 

that MD-140 from the north and MD-832 from the south are 

provided with channelized free right-turn lanes and a single 

through/left-turn lane. MD-140 from the east and Antrim 

Boulevard from the west share a single through/left-turn 

lane in each direction. 

The approach and departure lane widths along Antrim 

Boulevard from the west are 24 ft, with entry and exit widths 

of 16 ft and 13 ft, respectively. The approach and departure 

lane widths along MD-140 from the east are 24 ft with entry 

and exit widths of 24 ft, as well.

The roundabout cost approximately $464,540 to design and 

construct, and opened to traffic in August 1996 to serve 

a total peak hour approach volume of 1,300 vehicles. The 

Taneytown Roundabout resulted in a crash reduction of 

82.1 percent and reduced injury crashes by 89.3 percent 

per year. No fatal crashes have occurred.

Photo provided by Maryland State Highway Administration (used with permission). 

Photo provided by Maryland State Highway Administration (used with permission). 

Figure 6: Taneytown Roundabout, MD

MD-140

MD-140

MD-832

Antrim Blvd.

Figure 5: Lothian Roundabout, MD

MD-422

MD-408

MD-2

MD-2
FHWA | Roundabout—Maryland Experience 7

Time Frame

Construction time for the roundabouts discussed in this 

report ranged from six to nine months.

Effectiveness 

MDSHA conducted benefit/cost analysis that indicated 

that for every dollar spent, considering the 20-year 

service life of the roundabouts, there was a return of 

approximately $13 to be realized through crash reduction.

Summary of Results

The conversion of these conventional rural, stopcontrolled intersections to modern roundabouts 

cumulatively reduced total crashes by 69.1 percent, 

eliminated fatal crashes, and reduced injury crashes 

by 88.0 percent. The average reductions in crashes 

achieved by the treatments are consistent with the 

overall estimated crash reduction of 72% and fatal/injury 

crash reduction of 87% for two-way, stop-controlled rural 

intersections mentioned in the Desktop Reference for 

Crash Reduction Factors (September 2007), published by 

the United States Department of Transportation (USDOT) 

FHWA[5].

Discussion

Implementation Issues

Maryland experienced no implementation issues 

with installation of the roundabouts.

Cost

The average construction cost of roundabouts 

is estimated at approximately $250,000[2]. 

Roundabouts discussed in this report ranged in cost 

from $194,000 to just under $500,000, depending on 

their size (or “footprint” and right-of-way acquisitions 

that were needed.)

References

1) 2008 Traffic Safety Facts Annual FARS, National Highway 

Traffic Safety Administration, Washington, DC. 

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

Federal Highway Administration, Exhibit 5.2, pp 106, 

Washington, DC, June 2000. 

(www.tfhrc.gov/safety/00068.htm).

3) “Modern Roundabouts and the Environment,” Road 

Commission of Washtenaw County, Michigan. 

(http://www.wcroads.org/news/articles/roundabouts/

enviroment.htm).

4) “Roundabouts – Frequently Asked Questions,” Maryland 

State Highway Administration. 

(http://www.marylandroads.com/safety/oots/

roundabouts/faq.asp#Q3).

5) Federal Highway Administration. Desktop Reference for 

Crash Reduction Factors, FHWA-SA-07-015 (Washington, 

DC: September 2007), p. 30.

Leeds Roundabout, Cecil County

This single-lane roundabout is located at the 

intersection of MD-213 and Leeds Road/Elk Mills Road. 

Prior to the construction of the roundabout, this was a 

two-way stop-controlled intersection. MD-213 has the 

right-of-way. The posted speed limit along MD-213 for 

both approaches is 45 mph and 25 mph along Leeds 

Road/Elk Mills Road. The roundabout was constructed 

with an inscribed circle diameter of 110 ft. 

The roundabout cost approximately $472,014 to 

design and construct, and opened to traffic in August 

1995 to serve a total peak hour approach volume of 

847 vehicles. The Leeds Roundabout resulted in 

a crash reduction of 46.8 percent and reduced 

injury crashes by 93.1 percent per year. No fatal 

crashes have occurred.

Photo provided by Maryland State Highway Administration (used with permission). 

Figure 7: Leeds Roundabout, MD

MD-213

MD-213

Leeds Rd.

Elk Mills Rd.
FHWA-SA-09-018 February 2010

For More Information

Ed Rice

Intersection Safety Team Leader, 

FHWA Office of Safety

202.366.9064

ed.rice@dot.gov

Mike Niederhauser

Maryland State Highway Administration, 

Office of Traffic and Safety

410.787.5879

tmniederhauser@sha.state.md.us