FHWA-SA-10-006

Technical Summary

Roundabouts
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Foreword

This technical summary is designed as a reference for State and local transportation officials, Federal Highway 

Administration (FHWA) Division Safety Engineers, and other professionals involved in the design, selection, and 

implementation of roundabouts. Its purpose is to provide an overview of safety considerations in the design, 

implementation, and operation of roundabout intersections in urban, suburban, and rural environments where 

design considerations can vary as a function of land uses, travel speeds, volumes of traffic by mode (e.g., car, 

pedestrian, or bicycle), and many other variables.

This technical summary explores the characteristics of modern roundabouts while reinforcing the need to 

apply a principles-based approach to design. It provides readers with an overview of the key considerations 

for planning, analysis, and design of single-lane and multilane roundabouts. Section 1 of this document 

summarizes the characteristics of roundabouts. Section 2 presents benefits of roundabout intersections 

compared to traditional signalized and/or stop-controlled intersections. Sections 3-6 provide an overview of 

user, location, operational and design considerations respectively.

The information presented in this summary outlines the principles described in the FHWA document 

Roundabouts: An Informational Guide [1] and the forthcoming 2nd Edition [2] of that document (hereafter 

referred to as the Roundabout Guide), which is in progress at the time of this writing and due to be published in 

2010. Specific considerations for mini-roundabouts are summarized in a separate FHWA document titled MiniRoundabouts Technical Summary [3]. Figures are from the Roundabout Guide unless otherwise noted. 

This publication does not supersede any publication; and is a Final version.
FHWA | Roundabouts iii

Contents

Introduction 1

Section 1: Characteristics of Roundabouts . . . . . . . . . . . . . . . .3

Section 2: Benefits of Roundabouts . . . . . . . . . . . . . . . . . . . .4

Section 3: User Considerations 5

3.1 Motorists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Pedestrians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.3 Bicycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.4 Emergency Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Section 4: Location Considerations 7

4.1 Common Site Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.2 Site Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Section 5: Operational Analysis . . . . . . . . . . . . . . . . . . . . . . .9

Section 6: Design Considerations 10

6.1 Horizontal Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

6.2 Pedestrian Design Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

6.3 Bicycle Design Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

6.4 Sight Distance and Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

6.5 Vertical Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

6.6 Pavement Markings and Signs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
iv FHWA | Roundabouts

6.7 Lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.8 Landscaping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

6.9 Other Design Details and Applications . . . . . . . . . . . . . . . . . . . . . . . . . .24

Section 7: Costs 25

Section 8: References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
FHWA | Roundabouts 1

Introduction

Modern roundabouts are a type of intersection characterized by a generally circular shape, yield 

control on entry, and geometric features that create a low-speed environment. Modern roundabouts 

have been demonstrated to provide a number of safety, operational, and other benefits when 

compared to other types of intersections. On projects that construct new or improved intersections, 

the modern roundabout should be examined as an alternative. This technical summary explores the 

characteristics of modern roundabouts while reinforcing the need to apply a principles-based approach to design. It provides readers with an overview of the key considerations for planning, analysis, and design of single-lane and multilane roundabouts.

The information presented in this summary outlines 

the principles described in the FHWA document 

Roundabouts: An Informational Guide [1] and the 

forthcoming 2nd Edition [2] of that document (hereafter 

referred to as the Roundabout Guide), which is in 

progress at the time of this writing and due to be 

published in 2010. Specific considerations for miniroundabouts are summarized in a separate FHWA 

document titled Mini-Roundabouts Technical Summary

[3]. Figures are from the Roundabout Guide unless 

otherwise noted.

Adapted from Photo by Lee Rodegerdts (used with permission)
2 FHWA | Roundabouts
FHWA | Roundabouts 3

Section 1: Characteristics of Roundabouts

Circular intersection forms have been part of the transportation system in the United States for over 

a century. Their widespread usage decreased after the mid-1950s, as rotary intersections began 

experiencing problems with congestion and safety. However, the advantages of the modern roundabout, including modified and improved design features, have now been recognized and put to the 

test in the United States. There are now estimated to be well over a thousand roundabouts in the 

United States and tens of thousands worldwide, with the number estimated to be increasing in the 

United States each year.

A modern roundabout has the following distinguishing 

characteristics and design features: 

•	 Channelized approaches; 

•	 Yield control on all entries;

•	 Counterclockwise circulation of all vehicles around the 

central island; and

•	 Appropriate geometric curvature to encourage slow travel 

speeds through the intersection. 

Figure 1 and Figure 2 illustrate these characteristics and 

design features, respectively. 

Figure 1: Key Roundabout Characteristics.

Can have

more than 

one lane

Yield signs

at entries

No need to

change lanes

to exit

Geometry that 

forces slow 

speeds

Counterclockwise

circulation

Figure 2: Roundabout Design Features
4 FHWA | Roundabouts

Roundabouts have been classified into three basic 

categories according to size and number of lanes 

to facilitate discussion of specific performance 

or design issues: mini-roundabouts, single-lane 

roundabouts, and multilane roundabouts1. These are 

summarized in Table 1.

Modern roundabouts are different from other types 

of circular intersections in use in some parts of the 

United States. Roundabouts are typically smaller than 

the large, high-speed rotaries still in use in some parts 

of the country, and they are typically larger than most 

neighborhood traffic calming circles. Further discussion 

can be found in the Roundabout Guide.

Table 1: Roundabout Category Comparison

Design Element Mini Roundabout Single-Lane Roundabout Multi-Lane Roundabout

Desirable maximum entry 

design speed

15 to 20 mph

(25 to 30 km/h)

20 to 25 mph

(30 to 40 km/h)

25 to 30 mph

(40 to 50 km/h)

Maximum number of entering 

lanes per approach 1 1 2+

Typical inscribed circle diameter 45 to 90 ft

(13 to 27 m)

90 to 180 ft

(27 to 55 m)

150 to 300 ft

(46 to 91 m)

Central island treatment Fully traversable Raised (may have traversable 

apron)

Raised (may have 

traversable apron)

Typical daily service volumes on 

4-leg roundabout below which 

may be expected to operate 

without requiring a detailed 

capacity analysis (veh/day)*

Up to approximately 

15,000 veh/day

Up to approximately 

25,000 veh/day

Up to approximately 45,000 

veh/day for two-lane 

roundabout

*Operational analysis needed to verify upper limit for specific applications.

Section 2: Benefits of Roundabouts 

Roundabouts are becoming more popular based on the multiple opportunities to improve safety 

and operational efficiency, and provide other benefits. Of course, roundabouts are not always feasible and do not always provide the optimal solution for every problem. The benefits of roundabout 

intersections, and some constraining factors, are described below.

•	 Traffic Safety – Numerous studies have shown 

significant safety improvements at intersections converted 

from conventional forms to roundabouts. The physical 

shape of roundabouts eliminate crossing conflicts that 

are present at conventional intersections, thus reducing 

the total number of potential conflict points and the most 

severe of those conflict points. The most comprehensive 

and recent study showed overall reductions of 35 percent 

in total crashes and 76 percent in injury crashes [4]. Severe, 

incapacitating injuries and fatalities are rare, with one 

study reporting 89-percent reduction in these types of 

crashes [5] and another reporting 100-percent reduction in 

fatalities [6]. 

•	 Operational Performance – When operating 

within their capacity, roundabouts typically have lower 

overall delay than signalized and all-way stop-controlled 

intersections. The delay reduction is often most significant 

during non-peak traffic periods. These performance 

benefits can often result in reduced lane requirements 

between intersections. When used at the terminals of 

freeway interchanges, roundabouts can often reduce 

lane requirements for bridges over or under the freeway, 

thus substantially reducing construction costs. However, 

as yield-controlled intersections, roundabouts do not 

provide priority to specific users such as trains, transit, or 

emergency vehicles.

1 Please see the Mini-Roundabouts Technical Summary for information on mini-roundabouts. 
FHWA | Roundabouts 5

•	 Environmental Factors – Roundabouts often provide 

environmental benefits by reducing vehicle delay and the 

number and duration of stops compared with signalized 

or all-way stop-controlled alternatives. Even when there 

are heavy volumes, vehicles continue to advance slowly 

in moving queues rather than coming to a complete stop. 

This can reduce noise and air quality impacts and fuel 

consumption significantly by reducing the number of 

acceleration/deceleration cycles and the time spent idling.

•	 Access Management – Because roundabouts 

can facilitate U-turns, they can be a key element of a 

comprehensive access management strategy to reduce 

or eliminate left-turn movements at driveways between 

major intersections. 

•	 Traffic Calming – Roundabouts can have traffic calming 

effects on streets by reducing vehicle speeds using 

geometric design rather than relying solely on traffic 

control devices.

•	 Pedestrian Safety – Due to the reduction of vehicle 

speeds in and around the intersection, roundabouts can 

improve pedestrian crossing opportunities. Additionally, 

the splitter island refuge area provides the ability for 

pedestrians to focus on one traffic stream at a time while 

crossing. However, pedestrians with visual impairments 

may not receive the same level of information at a 

roundabout as at a typical signalized intersection, and 

they may require additional treatments, such as pedestrian 

signalization. Specific design treatments for enhancing 

accessibility for visually impaired pedestrians are receiving 

continued study [7].

•	 Aesthetics – The central island and splitter islands 

offer the opportunity to provide attractive entries 

or centerpieces to communities through use of 

landscaping, monuments, and art, provided that they 

are appropriate for the speed environment in which the 

roundabout is located. 

•	 Land Use – Roundabouts can provide a transition 

area between high-speed rural and low-speed urban 

environments. They can also be used to demarcate 

commercial areas from residential areas. 

•	 Ongoing Operations and Maintenance – A 

roundabout typically has lower operating and maintenance 

costs than a traffic signal due to the lack of technical 

hardware, signal timing equipment, and electricity needs. 

Roundabouts also provide substantial cost savings to 

society due to the reduction in crashes, particularly fatal 

and injury crashes, over their service life. As a result, the 

overall life cycle costs of a roundabout can be significantly 

less than that of a signalized intersection.

•	 Approach Roadway Width – A roundabout may 

reduce the amount of widening needed on the approach 

roadways in comparison to alternative intersection 

forms. While signalized or stop-controlled intersections 

can require adding lengthy left-turn and/or right-turn 

lanes, a roundabout may enable maintaining a narrower 

cross section in advance of the intersection. However, 

roundabouts usually require more space for the circulatory 

roadway, central island, and sidewalks than the typically 

rectangular space inside traditional intersections. 

Therefore, roundabouts often have greater right-of-way 

needs at the intersection quadrants compared with other 

intersection forms. 

Section 3: User Considerations

The various user types of a roundabout have unique characteristics that should be considered in the 

planning and design processes. Some of the characteristics of four primary user groups—motorists, 

pedestrians, bicyclists, and emergency vehicles—are discussed here; a more complete discussion 

can be found in the Roundabout Guide.

3.1 Motorists 

Research indicates roundabouts address some of 

the problems drivers experience in dealing with 

intersections. One of the key design features of a 

roundabout is the geometric shape of the roundabout 

that causes all traffic to slow down as it enters the 

intersection. Roundabouts can enhance the safety for 

drivers, including older drivers, by:

•	 Allowing more time to make decisions, act, and react; 

•	 Reducing the number of directions in which a driver needs 

to watch for conflicting traffic; and

•	 Reducing the need to judge gaps in fast traffic accurately. 

Attention should be paid to the layout of signs and 

pavement markings to make them clear, visible, and 
6 FHWA | Roundabouts

unambiguous to all users, including older drivers. Trucks 

and other large vehicles can be accommodated at a 

roundabout with proper attention to design. Further 

details on design vehicles are provided later in this 

technical summary.

3.2 Pedestrians

Pedestrians are accommodated at pedestrian crosswalks 

around the perimeter of the roundabout. By providing 

space to pause on the splitter island, pedestrians can 

consider one direction of conflicting traffic at a time, 

which simplifies the task of crossing the street. The low 

vehicular speeds through a roundabout also allow more 

time for drivers and pedestrians to react to one another 

and to reduce the consequences of error. As a result, few 

crashes involving pedestrians have been reported at 

roundabouts [4].

Pedestrians with vision impairments may have more 

difficulty crossing roundabouts due to the following 

key factors:

•	 Pedestrians with vision impairments may have trouble 

finding crosswalks because crosswalks are located 

outside the projection of approaching sidewalks and the 

curvilinear nature of roundabouts alters the normal audible 

and tactile cues they use to find crosswalks.

•	 Roundabouts do not typically include the normal 

audible and tactile cues used by pedestrians with vision 

impairments to align themselves with the crosswalk 

throughout the crossing maneuver.

•	 The sound of circulating traffic masks the audible cues that 

blind pedestrians use to identify the appropriate time to 

enter the crosswalk (both detecting a gap and detecting 

that a vehicle has yielded).

The Americans with Disabilities Act requires that all new 

and modified intersections, including roundabouts, 

be accessible to and usable by people with disabilities. 

Further discussion on treatments can be found later in 

this technical summary and in the Roundabout Guide.

3.3 Bicycles

Bicyclists have a broad range of skills and experiences, 

and roundabouts are typically designed to 

accommodate that wide range. Bicyclists should be 

provided similar options to negotiate roundabouts 

as they have at conventional intersections, where 

they navigate either as motor vehicles or pedestrians 

depending on the size of the intersection, traffic 

volumes, their experience level, and other factors. 

Bicyclists are often comfortable riding through singlelane roundabouts in low-volume environments in 

the travel lane with motor vehicles, as speeds are 

comparable and potential conflicts are low. At larger 

or busier roundabouts, many cyclists may be more 

comfortable and safer using ramps connecting to a 

sidewalk or multiuse path around the perimeter of the 

roundabout as a pedestrian. 

3.4 Emergency Vehicles

Roundabouts provide emergency vehicles the benefit 

of lower vehicle speeds, which may make roundabouts 

safer for them to negotiate than signalized crossings. 

Unlike signalized intersections, emergency vehicle 

drivers will not encounter through vehicles unexpectedly 

running the intersection and hitting them at high 

speed. Emergency services personnel may have some 

concern about their ability to navigate a roundabout 

in an emergency vehicle, although this can be readily 

addressed in design (see the “Design Vehicle” section of 

this technical summary).

On emergency response routes, the delay for the 

relevant movements at a planned roundabout should 

be compared with alternative intersection types and 

control. As with conventional intersections, motorists 

should be educated not to enter a roundabout when an 

emergency vehicle is approaching on another leg. Once 

entered, they should clear out of the circulatory roadway 

if possible, facilitating queue clearance in front of the 

emergency vehicle. 
FHWA | Roundabouts 7

Section 4: Location Considerations

In the planning process for a new or improved intersection where a traffic signal or stop control is 

under consideration, a modern roundabout should likewise receive serious consideration as an alternative. This begins with understanding the site characteristics and determining a preliminary configuration. There are a number of locations where roundabouts are commonly found to be advantageous and a number of situations that may adversely affect their feasibility. As with any decision 

regarding intersection treatments, care should be taken to understand the particular benefits and 

trade-offs for each project site. This section outlines some location considerations to help determine 

whether a roundabout is a feasible intersection alternative.

4.1 Common Site Applications

The following applications represent some of the 

situations at which roundabouts are commonly found to 

be feasible and advantageous (further applications can 

be found in the Roundabout Guide): 

•	 New residential subdivisions – Roundabouts offer a 

low-speed, low-noise intersection form that requires little 

ongoing maintenance.

•	 Schools – A primary benefit is the reduction of vehicle 

speeds in and around the roundabout. Roundabouts 

improve pedestrian crossing opportunities, providing midblock refuge and the ability for pedestrians to focus on 

one traffic stream at a time while crossing with or without 

crossing guards. Single-lane roundabouts are generally 

preferable to multilane roundabouts near schools because 

they offer simpler crossings for children. However, if the 

traffic volume is sufficiently high, a multilane roundabout 

may still be preferable to a large signalized intersection.

•	 Corridors – Roundabouts present opportunities to shape 

the cross section of a corridor in ways that are perhaps 

different from those afforded by signalized intersections. 

Signalized intersections operate most efficiently when 

they manage the advancement of platoons of traffic. 

This requires sufficient through lanes between signals to 

maintain the integrity of these platoons. Roundabouts, 

on the other hand, produce efficiency through a gap 

acceptance process and thus do not carry the same need 

for platoon progression. As a result, roundabouts can 

be made as large as needed for node capacity, keeping 

the links between nodes more narrow. This concept is 

sometimes referred to as a “wide nodes, narrow roads” 

concept. The reduced number of travel lanes between 

intersections may make it feasible to reduce right-of-way 

impacts and to accommodate parking, wider sidewalks, 

planter strips, and bicycle lanes. 

•	 Interchanges – Roundabouts often can make more 

efficient use of the bridge structure between ramp 

terminals, extending design life or substantially reducing 

construction costs if improvements are needed.

•	 Gateway treatments – Roundabouts 

present opportunities to create community 

focal points, landscaping, and other gateway 

features within an intersection form that is also 

safe and efficient.

•	 Intersections with high delay – A 

roundabout can be an ideal application to 

reduce delay at stop-controlled or signalized 

intersections.

•	 Rural intersections – Roundabouts 

have been demonstrated to significantly 

reduce fatal and injury crash experience at 

Figure 3: Roundabout near a School (Clearwater, Florida) 

Photo: Lee Rodegerdts (used with permission)
8 FHWA | Roundabouts

rural, high-crash locations, even those with 

high-speed approaches (greater than 55 mph).

•	 Commercial developments – 

Roundabouts are an aesthetically pleasing 

design alternative to traffic signals and have 

the ability to meet similar capacity needs.

4.2 Site Constraints

Certain site-related factors may significantly 

influence the design requiring that a more 

detailed investigation of some aspects of the 

design or operation be carried out. A number 

of these factors (many of which are valid for 

any intersection type) are listed below:

•	 Physical complications such as right-of-way 

limitations, utility conflicts, environmental constraints, 

drainage problems, intersection skew, grades or 

unfavorable topography, etc, that make it politically or 

economically infeasible to construct a roundabout.

•	 Proximity of generators of significant traffic that might 

have difficulty negotiating the roundabout, such as 

high volumes of trucks or oversized vehicles (sometimes 

called “superloads”).

•	 Proximity of other conditions that would require preemption, such as at-grade rail crossings, drawbridges, etc.

•	 Proximity of bottlenecks that would routinely back 

up traffic into the roundabout, such as over-capacity 

signals, etc. The successful operation of a roundabout 

depends on generally unimpeded flow on the circulatory 

roadway. If traffic on the circulatory roadway comes to a 

halt, roundabout operation is impeded. In comparison, 

other control types may be able to serve some 

movements under these circumstances.

•	 Intersections where an unacceptable delay to the major 

road could be created. Roundabouts introduce some 

delay to all traffic entering the intersection, including the 

major street.

•	 Heavy pedestrian or bicycle movements in conflict with 

high traffic volumes that might require supplemental traffic 

control (e.g., signals).

•	 Intersections located on arterial streets within a 

coordinated signal network. In these situations, the level 

of service on the arterial might be better with a signalized 

intersection incorporated into the system. 

The existence of one or more of these conditions may or 

may not preclude installing a roundabout. Roundabouts 

have, in fact, been built at locations that exhibit one 

or more of the conditions listed above. To address 

these conditions, additional analysis, design work, and/

or coordination with affected parties may be needed 

to resolve conflicts and help in the decision-making 

process. In some cases, the conditions identified above 

cannot be overcome, and another intersection type may 

be more suitable.

Figure 4: Roundabouts at an Interchange (Vail, Colorado) 

Photo: Lee Rodegerdts (used with permission)
FHWA | Roundabouts 9

If the volumes fall within the ranges 

identified in Figure 5 where “additional 

analysis is needed,” a single-lane or doublelane roundabout may still function quite 

well, but it requires using the procedures 

described in the following section to obtain 

a closer look at the actual turning movement 

volumes during the design hour. Variablesized roundabouts (e.g., one lane for part of 

the circulatory roadway, and two lanes at 

other parts within the same roundabout), 

roundabouts with peak-period metering, and 

three-lane roundabouts have been successful 

in some locations. 

The 2010 Highway Capacity Manual (HCM) [8] 

employs a number of models to reflect the 

capacity of roundabout entries with up 

to two lanes. The capacity of each entry 

lane is calculated based on the conflicting 

traffic flow in the circulatory roadway, which 

comprises the various turning movements 

from other approaches that pass in front of 

(and thus conflict with) the subject entry. 

Figure 6 shows the capacity curves for 

various one- and two-lane roundabout 

scenarios. The lower curve can be used to 

calculate the capacity of a one-lane entry to 

a one-lane roundabout, or either lane of a 

two-lane entry conflicted by one circulating 

lane. For a roundabout with two circulatory 

lanes, the two curves representing the left 

and right entry lanes should be used. As an 

example, for a given circulatory flow rate of 

600 passenger cars per hour (pc/h) across 

two lanes, the left lane of a two-lane entry 

would have a capacity of approximately 720 

pc/h, and the right lane of a two-lane entry 

Section 5: Operational Analysis

A basic question that needs to be answered at the planning level is how many entering and circulating lanes a roundabout would require to serve the traffic demand. The number of lanes affects not 

only the capacity of the roundabout, but also the size of the roundabout footprint. Figure 5 presents 

ranges of average annual daily traffic (AADT) volumes to identify scenarios under which one-lane 

and two-lane roundabouts may perform adequately. These ranges represent total entering volume 

thresholds where a one-lane or two-lane roundabout should operate acceptably and ranges of 

volumes over which more detailed analysis is required. This procedure is offered as a simple, conservative method for estimating roundabout lane requirements. 

Figure 5: Planning Level AADT Intersection Volumes

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

0% 10% 20% 30% 40%

Left-Turn Percentage

AADT

Double-lane roundabout 

likely to operate 

acceptably

Single-lane roundabout may be 

sufficient (additional analysis 

needed)

Single-lane roundabout 

likely to operate 

acceptably

Double-lane roundabout may be 

sufficient (additional analysis needed)

Figure 6: Capacity of Single-Lane and Multilane Entries

Source: 2010 HCM [8]

0

200

400

600

800

1,000

1,200

1,400

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

Conflicting Flow Rate (pc/h)

Capacity (pc/h)

Dashed regression extrapolated beyond the data 

Capacity of one-lane entry or right lane of 

two-lane entry against two conflicting lanes

Capacity of left lane of two-lane entry 

against two conflicting lanes

Capacity of one-lane or either lane of twolane entry against one conflicting lane
10 FHWA | Roundabouts

would have a capacity of approximately 740 pc/h. More 

detail, including sample calculations of roundabout 

volumes, conversion of vehicles per hour (veh/h) to 

passenger cars per hour (pc/h), lane use, capacity, and 

performance measures, can be found in the 2010 HCM.

Different methods of analysis are available and are in 

common use for a variety of applications, including 

software programs with specific roundabout analysis 

procedures and simulation models. These models 

may be capable of analyzing situations beyond 

the methodologies presented in the 2010 HCM or 

Roundabout Guide; refer to these documents for 

further discussion. Regardless of the analytical tools 

used, it is critical to understand that each model 

and analysis method makes certain operational 

and performance assumptions. Along with an 

understanding of the inherent imprecision of traffic 

forecasting, this makes the application of engineering 

judgment crucial in the analytical process.

Section 6: Design Considerations

The geometric design of a roundabout requires the balancing of competing design objectives. 

Roundabouts operate most safely when their geometry forces traffic to enter and circulate at slow 

speeds. Poor roundabout geometry has been found to negatively impact roundabout operations by 

affecting driver lane choice and behavior through the roundabout. Many of the geometric parameters are governed by the maneuvering requirements of the design vehicle and the accommodation 

of nonmotorized users. Thus, designing a roundabout is a process of determining the optimal balance among safety provisions, operational performance, and accommodation of design users.

This design balance is further influenced by physical, 

environmental, economic, and political constraints and 

opportunities, which further increases the variability 

from site to site. For example, a roundabout that is 

built to its ultimate configuration on opening day may 

have different design characteristics from one that is 

initially built in an interim configuration (e.g., a singlelane roundabout converted later to a double-lane 

roundabout), and the techniques for those conversions 

can vary (e.g., adding lanes to the outside versus 

the inside). For these reasons, roundabout design 

techniques are difficult to standardize, and there is 

rarely only one correct or even best way to design a 

roundabout.

Fundamentally, roundabout design involves achieving 

the following key objectives:

•	 Slow entry speeds and consistent speeds through the 

roundabout by using deflection;

•	 The appropriate number of lanes and lane 

assignment to achieve adequate capacity, lane volume 

balance, and continuity of lanes through the roundabout;

•	 Smooth channelization that is intuitive to drivers and 

results in vehicles naturally using the intended lanes;

•	 Adequate accommodation for the design vehicles; 

•	 A design that meets the needs of pedestrians and 

bicyclists; and

•	 Appropriate sight distance and visibility.

Since roundabouts are applied in many different 

situations and under differing site specific conditions, 

each roundabout design requires distinctive design 

choices. The general nature of the roundabout design 

process is an iterative one. Minor adjustments in 

geometric design attributes can result in significant 

effects on the operational and safety performance of 

the roundabout. Also, many of the individual design 

components interact with each other, and therefore 

considering the roundabout design in whole (the 

outcome of the design) is more important than focusing 

on the isolated components. Because of this iterative 

process, it may be advantageous to prepare initial 

layout drawings to a “hand-sketch” level of detail and 

investigate the compatibility of the design principles 

presented below before further design effort is invested. 

The optimal position of the roundabout may not be 

established until geometrics are roughly investigated for 

various location options.

The key design parameters and methods for checking 

designs are summarized in the remainder of this section. 
FHWA | Roundabouts 11

6.1 Horizontal Design

Three of the key considerations that affect horizontal 

design of roundabouts include design speed, path 

alignment, and design vehicle. These in turn influence 

the size of the roundabout and the design of the central 

island and splitter islands. This section highlights these 

considerations and design details.

6.1.1 Design Speed

Achieving appropriate vehicular speeds 

entering and traveling through the 

roundabout is a critical design objective 

as it has profound impacts on safety. 

A well-designed roundabout reduces 

vehicle speeds upon entry and achieves 

consistency in the relative speeds between 

conflicting traffic streams by requiring 

vehicles to negotiate the roundabout along 

a curved path.

Generally speaking, although the frequency 

of crashes is most directly tied to volume, 

the severity of crashes is most directly 

tied to speed. Therefore, careful attention 

to the design speed of a roundabout is 

fundamental to attaining good safety 

performance [4]. 

The recommended design speed of a 

roundabout is primarily a function of the 

number of lanes rather than the design 

speed of the intersecting roadways. The 

design speed of a roundabout is defined 

by the theoretical speed that drivers 

could achieve through the roundabout 

if taking the fastest path through the 

roundabout without regard to lane line 

striping, if present. In practice, actual 

speeds through the roundabout will be 

less than these theoretical values, as drivers 

will be decelerating into the roundabout, 

yielding to other users, and staying within 

their lanes (for multilane roundabouts). 

For single-lane roundabouts, typical 

maximum theoretical entering speeds 

of 20 to 25 mph are recommended; for 

multilane roundabouts, typical maximum 

theoretical entering speeds of 25 to 30 mph 

are recommended. This design technique 

ensures that speeds observed in practice 

will fall within a reasonable range. 

To determine the theoretical speed of each movement, 

the fastest path allowed by the geometry is drawn. 

This is the smoothest, flattest path possible for a single 

vehicle given the absence of any other traffic and given 

that the driver ignores all lane markings, traverses 

the entry, and travels around the central island and 

through the exit. Usually the fastest possible path is 

the through movement, but in some cases it may be a 

right turn movement. Figure 7 and Figure 8 illustrate the 

construction of the fastest vehicle paths at a single-lane 

roundabout and at a multilane roundabout, respectively. 

Figure 7: Fastest Vehicle Path Through a Single-Lane Roundabout

Figure 8: Fastest Vehicle Path Through a Multilane Roundabout
12 FHWA | Roundabouts

Figure 9 provides an example of a right-turn 

path. Dimensions are provided to show how the 

centerline of the vehicular path is drawn relative 

to control points (e.g., curbs). The theoretical 

speeds estimated from these paths are checked 

against the target design speed for the type of 

roundabout (single-lane versus multilane) to 

determine if the geometry produces reasonable 

speeds.

The fastest path should be drawn and checked 

for all approaches of the roundabout. Figure 10 

illustrates the five critical path radii that are 

commonly checked for each approach.

Once the fastest paths are drawn, the above radii 

are measured and corresponding design speeds 

are calculated using standard horizontal curve 

guidelines from the American Association of State 

Highway and Transportation Officials (AASHTO) 

[9]. Typically, roundabouts are designed with a 

cross slope of 2 percent toward the outside (i.e., 

a superelevation of -0.02). Figure 11 displays a 

graphical representation of the speed-radius 

relationships (in U.S. Customary units). 

In addition to achieving an appropriate design 

speed for the entry movements, another 

important objective is to achieve consistent 

speeds for all movements, which are influenced 

by choices on geometric elements. The key 

benefits of achieving speed consistency among 

the movements are safety related. Typically, the 

relationships between the speeds associated 

with radii R1

, R2, and R3 and radii R1 and R4 are 

of primary interest. In practice, by keeping the 

recommended maximum entry design speed 

below the recommended values, the goal of 

consistent speeds for all movements can be 

readily achieved. 

There are differences of opinion on the 

importance of tangential versus curved exit 

geometry for the purpose of controlling exit 

speeds, particularly at the pedestrian crosswalk. 

Some designers advocate for a relatively tight 

exit radius to minimize exit speeds; however, 

others advocate for a more relaxed exit radius for 

improved drivability. Theoretical exit speeds can 

be checked using the above method. However, 

research has found that observed exit speeds are 

more commonly limited by circulating speeds 

Figure 9: Example of Critical Right-Turn Movement

Figure 10: Vehicle Path Radii

R4

R5

R1 R3

R2

R1 = entry path radius

R2 = circulating path radius

R3 = exit path radius

R4 = left-turn path radius

R5 = right-turn path radius

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300 350 400

Radius (ft)

Speed (mph)

e=+0.02 e=-0.02

Figure 11: Speed-Radius Relationship (U.S. Customary Units)
FHWA | Roundabouts 13

and acceleration out of the roundabout 

than by the radius of the exit path. More 

information on calculating exit speeds can 

be found in the Roundabout Guide [2]. It is 

important to understand the relative tradeoffs of design choices, and choices may 

vary based upon the location context.

6.1.2 Path Alignment

With multilane roundabouts, the designer 

should also consider the alignment of 

vehicles, or the natural path, to ensure 

the proposed geometry directs vehicles 

to stay within the proper lanes through 

the circulatory roadway and exits. Path 

overlap occurs when the natural paths of 

vehicles in adjacent lanes overlap or cross 

one another. The entry design should 

align vehicles into the appropriate lane 

within the circulatory roadway, using the 

technique shown in Figure 12 or others 

that promote good path alignment.

Designing multilane roundabouts with 

good path alignment, while also controlling 

entry speeds through adequate deflection, 

can be difficult. Strategies that improve 

path alignment may result in increased 

fastest path speeds. A good design 

attempts to balance the entry speed, path 

alignment needs, and other factors (e.g., 

design vehicle needs) through design 

iterations and checks of the various factors.

Figure 13 illustrates one possible multilane 

design technique in greater detail. The 

primary objective of this particular design 

technique is to locate the entry curve at the 

optimal placement so that the projection 

of the inside entry lane at the entrance line 

connects tangentially or nearly tangentially 

to the central island. The design of the 

exits should also provide sufficiently large 

exit radii and alignment to allow drivers 

to intuitively maintain the appropriate lane. Other 

techniques involve changes to approach alignment, 

entry curvature, and/or inscribed circle diameter; these 

are discussed in the Roundabout Guide. Each of these 

adjustments could create trade-offs; for example, 

increasing the inscribed circle diameter could result in 

faster circulatory speeds, greater land impacts, and so 

on. A good design attempts to balance these factors 

through design iteration. 

Likewise, problems can also occur when the design 

allows for too much separation between entries and 

subsequent exits. Large separations between legs cause 

entering vehicles to join next to circulating traffic that 

may be intending to exit at the next leg, rather than 

Figure 12: Design to Promote Good Path Alignment

Source: Kansas Roundabout Guide [10] Source: Kansas Roundabout Guide [10]

Figure 13: Possible Design Technique to Promote Good Path Alignment
14 FHWA | Roundabouts

crossing the path of the exiting vehicles. This 

can create conflicts at the exit point between 

exiting and circulating vehicles, as shown in 

Figure 14.

A variety of solutions are possible to 

address this problem, including changes to 

lane configurations, changes to inscribed 

circle diameter, and realignment of the 

approaches. Figure 15 illustrates one of 

these possible solutions, which involves 

realignment of the approach legs to have 

the paths of entering vehicles cross the 

paths of the circulating traffic (rather 

than merging) to minimize the conflict. 

This significantly increases the likelihood 

that entering drivers making a through 

movement will yield to both conflicting 

lanes. 

6.1.3 Design Vehicle

Large trucks, buses, and emergency 

vehicles often dictate many of the 

roundabout’s dimensions, particularly for 

single-lane roundabouts. Therefore, the 

design vehicle is best identified at the 

start of the project and evaluated early 

in the design process. A truck apron will 

often be needed within the central island 

to accommodate larger design vehicles 

(including the common WB-62, WB-65, 

or WB-67 design vehicles) but maintain a 

relatively narrow circulatory roadway to 

adequately constrain passenger car speeds. 

Design details regarding truck aprons are 

provided in the “Grades” section of this 

document. 

Appropriate vehicle-turning templates or a 

CAD-based computer program should be 

used to determine the swept path of the 

design vehicle through each of the turning 

movements. Usually, the left-turn movement 

is the critical path for determining circulatory 

roadway width while the right-turn movement is 

the critical path for entry and exit widths. Figure 16 

illustrates an example vehicle path check.

Buses should generally be accommodated within the 

circulatory roadway without tracking over the truck 

apron, which could cause discomfort to bus occupants. 

Figure 14: Exit-Circulating Conflict Caused by Large Separation between Legs

Figure 15: Realignment to Resolve Exit-Circulating Conflict

(from Figure 14)
FHWA | Roundabouts 15

Table 2: Common Inscribed Circle Diameter Ranges

Roundabout Configuration Typical Design Vehicle Inscribed Circle Diameter Range*

Mini-Roundabout SU-30 (SU-9) 45 to 90 ft (14 to 27 m)

Single-Lane Roundabout B-40 (B-12) 90 to 150 ft (27 to 46 m)

WB-50 (WB-15) 105 to 150 ft (32 to 46 m)

WB-67 (WB-20) 130 to 180 ft (40 to 55 m)

Multilane Roundabout (2 lanes) WB-50 (WB-15) 150 to 220 ft (46 to 67 m)

WB-67 (WB-20) 165 to 220 ft (50 to 67 m)

Multilane Roundabout (3 lanes) WB-50 (WB-15) 200 to 250 ft (61 to 76 m)

WB-67 (WB-20) 220 to 300 ft (67 to 91 m)

* Assumes 90-degree angles between entries and no more than four legs.

For multilane roundabouts, there are 

different philosophies regarding the extent 

to which trucks need to stay in their lane 

throughout their movement; these are 

discussed further in the Roundabout Guide.

6.1.4 Size

The size of a roundabout, measured by 

its inscribed circle diameter (see Figure 

2), is determined by a number of design 

objectives, including design speed, path 

alignment, and design vehicles as discussed 

above. Selection of an initial inscribed circle 

diameter is the first step towards preparing 

a design. The selected diameter may be 

somewhat subjective, but its ultimate size is 

an output of meeting other objectives (e.g., 

speed control, design vehicle, etc.). Smaller 

inscribed circle diameters can be used for 

some local street or collector street intersections where 

the design vehicle may be a fire truck or single-unit 

truck. Larger inscribed circle diameters generally provide 

increased flexibility for the entry design to meet design 

criteria (e.g., speed, adequate visibility to the left, etc.) 

while accommodating large design vehicles. Table 2 

provides common ranges of inscribed circle diameters 

for various roundabout categories and typical design 

vehicles; values outside these ranges are possible but 

less common.

Figure 16: Example Design Vehicle Path Check
16 FHWA | Roundabouts

6.1.5 Central Island

The central island of a roundabout is 

the raised, mainly non-traversable area 

surrounded by the circulatory roadway. It 

may also include a traversable truck apron. 

The island is typically landscaped for 

aesthetic reasons and to enhance driver 

recognition of the roundabout upon 

approach. Raised central islands for singlelane roundabouts are preferred over 

depressed central islands, as depressed 

central islands are difficult for approaching 

drivers to recognize. 

A circular central island is preferred 

because the constant-radius circulatory 

roadway helps promote constant speeds 

around the central island. Oval or irregular 

shapes may be necessary at irregularly 

shaped intersections or intersections with more than 

four legs. Raindrop-shaped islands are sometimes used 

in areas where certain movements do not exist, such 

as interchanges, or at locations where certain turning 

movements cannot be safely accommodated, such as 

roundabouts with one approach on a relatively steep 

grade.

The size of the central island plays a key role in 

determining the amount of deflection imposed on 

the through vehicle’s path. However, its diameter is 

dependent upon the inscribed circle diameter and 

the required circulatory roadway width. Roundabouts 

in rural environments typically need larger central 

islands than urban roundabouts to enhance their 

visibility, accommodate larger design vehicles, enable 

better approach geometry to be designed in the 

transition from higher speeds, and be more forgiving 

to errant vehicles.

The central island may include enhancements (e.g., 

landscaping, sculptures, fountains) serving both 

an aesthetic purpose and providing conspicuity of 

the intersection for approaching motorists. These 

treatments should not attract pedestrians to 

the central island, as they should never cross the 

circulating roadway. Furthermore, care is needed when 

including any fixed objects within the central island in 

environments where the speeds on the approaching 

roadways are higher.

6.1.6 Splitter Island

Splitter islands should be provided on all roundabouts, 

and these islands should be raised on all but those with 

small diameters. Their purpose is to provide refuge for 

pedestrians, assist in controlling speeds, guide traffic 

into the roundabout, physically separate entering and 

exiting traffic streams, and deter wrong-way movements. 

Additionally, splitter islands can be used as a place for 

mounting signs.

When performing the initial layout of a roundabout 

design, a sufficiently sized splitter island “envelope” 

should be identified prior to designing the entry and 

exits of an approach. This will ensure that the design 

will eventually allow for a raised island that meets the 

minimum dimensions (e.g., offsets, tapers, length, 

widths). It is recommended that control points for the 

splitter island envelope be identified prior to proceeding 

to the design of the entry and exit geometry to ensure 

that a properly sized splitter island will be provided.

The total length of the raised island should generally 

be at least 50 ft (15 m), although longer is desirable, 

to provide sufficient protection for pedestrians and to 

alert approaching drivers to the roundabout geometry. 

Additionally, the splitter island should extend beyond 

the end of the exit curve to prevent exiting traffic from 

accidentally crossing into the path of approaching 

traffic. For approaches in high-speed locations (typically 

rural), the splitter island should be at least 200 feet long 

and preferably of a length needed for the comfortable 

deceleration length as measured from approach speed 

to entry speed. The splitter island width should be a 

minimum of 6 ft (1.8 m) at the crosswalk to adequately 

Figure 17: Use of Longer Splitter Islands in a Rural Environment (Skagit County, Washington)

Photo: Skagit County Public Works Department (used with permission)
FHWA | Roundabouts 17

provide refuge for pedestrians, including 

those using wheelchairs, pushing a stroller, 

or walking a bicycle.

There are benefits to providing larger 

splitter islands. An increase in the splitter 

island width results in greater separation 

between the entering and exiting traffic 

streams of the same leg and increases the 

time for approaching drivers to distinguish 

between exiting and circulating vehicles. 

In this way, larger splitter islands can help 

reduce confusion for entering motorists. 

However, increasing the width of the 

splitter islands generally requires increasing 

the inscribed circle diameter to maintain 

speed control on the approach. Thus, these 

safety benefits may be offset by higher 

construction cost and greater land impacts.

Standard AASHTO guidelines for island design should 

be followed for the splitter island. This includes using 

larger nose radii at approach corners to maximize island 

visibility and offsetting curb lines at the approach ends 

to create a funneling effect. The funneling treatment 

also aids in reducing speeds as vehicles approach the 

roundabout. Additional details can be found in the 

Roundabout Guide. 

6.2 Pedestrian Design Treatments

Wherever possible, sidewalks at roundabouts should be 

set back from the edge of the circulatory roadway by 

a landscape buffer. The buffer discourages pedestrians 

from crossing to the central island or cutting across the 

circulatory roadway of the roundabout, and it helps 

guide pedestrians with vision impairments to the 

designated crosswalks. A buffer width of 5 ft (1.5 m) 

(minimum 2 ft [0.6 m]) or greater is recommended, and it 

is best to plant low shrubs or grass in the area between 

the sidewalk and curb to maintain sight distance needs. 

Figure 18 shows this technique. 

Crosswalks should be located in vehicle-length 

increments away from edge of the circulatory roadway. 

A typical (and minimum) crosswalk setback of 20 ft (6 

m) is recommended. The raised splitter island width 

should be a minimum of 6 ft (1.8 m) at the crosswalk 

to adequately provide shelter for persons pushing a 

stroller or walking a bicycle. At some roundabouts, it 

may be desirable to place the crosswalk two or three 

car lengths (45 ft [13.5 m] or 70 ft [21.5 m]) back from the 

edge of the circulatory roadway. This longer setback is 

typically used in situations with relatively high volumes 

of pedestrian crossings that may cause queues on the 

exit roadway to frequently extend into the circulatory 

roadway. Other treatments for the accommodation of 

pedestrians, including signalization, are discussed in the 

Roundabout Guide. 

6.3 Bicycle Design Treatments

Bicycle lanes are not recommended within the 

circulatory roadway of roundabouts, as it has been 

demonstrated internationally to have adverse safety 

effects (see the Roundabout Guide). Where bicycle 

lanes or shoulders are used on approach roadways, 

they should be terminated in advance of roundabouts. 

Bicyclists may choose to merge with traffic and travel 

like other vehicles, or they may choose to exit the 

roadway onto the sidewalk (or shared use path) and 

travel as pedestrians. 

The full width bicycle lane should normally end at least 

100 feet before the edge of the circulatory roadway. 

An appropriate taper (a rate of 7:1 is recommended) 

should be provided to narrow the combined travel lane 

and bike lane width down to the appropriate width 

necessary to achieve desired motor vehicle speeds on 

the roundabout approach. Because some bicyclists may 

not feel comfortable traversing some roundabouts in 

the same manner as other vehicles, bicycle ramps can 

be provided to allow access to the sidewalk or a shared 

use path at the roundabout. Figure 19 displays a possible 

layout of bicycle treatments. To minimize confusion 

between bicycle ramps and pedestrian ramps, the 

detectable warning surfaces are placed at the top of the 

Figure 18: Sidewalk Treatments
18 FHWA | Roundabouts

bicycle ramps rather than at the bottom as 

is the practice with pedestrian ramps.

In general, bicycle ramps should only be 

used where the roundabout complexity or 

design speed may result in less comfort for 

some bicyclists. Ramps may not be needed 

at urban one-lane roundabouts, as the 

low-speed and lower-volume environment 

will typically allow cyclists to navigate as 

comfortably as vehicles. 

6.4 Sight Distance and Visibility

Adequate sight distance and visibility 

is needed for a roundabout to operate 

safely. These factors can be contradictory: 

sight distance at the roundabout can be 

increased in some cases at the expense 

of the visibility of the roundabout from a 

distance. Evaluation of sight distance at 

roundabouts includes both intersection 

sight distance and stopping sight distance. 

The fundamental principles of both 

forms of sight distance are the same 

at roundabouts as for other types of 

intersections and roadways. 

Intersection sight distance is evaluated 

at each entry to ensure a driver can see 

and safely react to potentially conflicting 

vehicles. Providing intersection sight 

distance ensures drivers can safely 

enter the circulatory roadway without 

impeding the flow of traffic within the 

circulatory roadway. Figure 20 illustrates the 

measurement of intersection sight distance. 

As can be seen in the exhibit, the distance 

between the entering vehicle and the 

circulatory roadway is fixed. The other legs 

of the sight distance “triangle” are based 

on two conflicting approaches that are 

typically checked independently:

1. Entering stream, comprised of vehicles 

from the immediate upstream entry. 

The speed for this movement can be 

approximated using the average of the entering speed 

and circulating speed. 

2. Circulating stream, comprised of vehicles that entered 

the roundabout prior to the immediate upstream 

entry. This speed can be approximated using the speed 

of left turning vehicles.

In both cases the distance is a function of the speed of 

those vehicles and a design value of the critical headway 

that drivers can reasonably be expected to accept. 

Figure 19: Possible Bicycle Design Treatments

Figure 20: Intersection Sight Distance
FHWA | Roundabouts 19

Detailed design guidelines for evaluating intersection 

sight distance are provided in the Roundabout Guide.

Stopping sight distance should be provided at every 

point within a roundabout and on each entering and 

exiting approach, as illustrated in Figure 21. The required 

distance is based on speed, as determined from the 

fastest path speed checks, and can be calculated using 

AASHTO guidelines. 

As shown in Figure 20 and Figure 21, sight distance 

needs may limit the height of landscaping and objects 

around the outer edge of the central island. In general, it 

is recommended to provide no more than the minimum 

required intersection sight distance on each approach. 

Excessive intersection sight distance can lead to higher 

vehicle speeds that may reduce the safety of the 

intersection.

6.5 Vertical Design

As a general practice, a cross slope of 2 percent away 

from the central island should be used for the circulatory 

roadway on single-lane roundabouts. This technique of 

sloping outward is recommended because it:

•	 Promotes safety by raising the height of the central island 

and improving its visibility;

•	 Promotes lower circulating speeds; 

•	 Minimizes breaks in the cross slopes of the entrance and 

exit lanes; and 

•	 Drains surface water to the outside of the roundabout. 

Figure 22 displays a typical section for a single-lane 

roundabout with a truck apron. Where truck aprons are 

used, the slope of the apron should generally be 1 to 

Figure 21: Stopping Sight Distance
20 FHWA | Roundabouts

2 percent; greater slopes may increase the 

likelihood of loss-of-load incidents. Examples 

of traversable curb details can be found in 

the Roundabout Guide.

There are a variety of possible methods for 

the vertical design of a circulatory roadway 

within a multilane roundabout, and many are 

the byproduct of fitting a roundabout to its 

topography. The two most commonly used 

methods are the following:

•	 Outward sloping – This method, used at 

single-lane roundabouts and some multilane 

roundabouts, is the most common type of 

vertical design for roundabouts in the United States. The 

circulatory roadway is graded independently of the rest 

of each approach at a typical outward slope of 2 percent 

(common practice ranges from 1.5 percent to 3 percent). 

This method maximizes visibility of the central island and 

minimizes drainage costs but may in some cases create 

adverse combinations of forces on trucks that may increase 

the possibility of load shifting or overturning.

•	 Crowned circulatory roadway – This method, used 

at some multilane roundabouts, provides a crown on the 

circulatory roadway with approximately two thirds of the 

width sloping toward the central island and one third 

sloping outward. This method may help to physically 

separate turning movements within the roundabout, and 

it may help with truck circulation in reducing the potential 

for load shifting or overturning in some cases. This 

method increases drainage costs by requiring drainage 

on both sides of the circulatory roadway and can reduce 

visibility of the central island unless other measures are 

taken (mounding, signs, etc.). 

It is generally not desirable to locate roundabouts in 

locations where grades through the intersection are 

greater than four percent, although roundabouts have 

been installed on grades of 10 percent or more. Care 

is needed when designing roundabouts on steep 

grades. On approach roadways with downgrades 

steeper than 4 percent, it is more difficult for entering 

drivers to slow or stop on the approach (as with any 

intersection). At roundabouts on crest vertical curves 

with steep approaches, driver sight lines may be 

compromised unless the vertical design is adjusted. In 

addition, significant slope breaks within the roundabout 

can create potential problems for semi-trailer trucks, 

including load shifting and overturning. 

Steep gradients at entries and exits should be avoided 

or flattened to minimize grade breaks at the entry and 

ensure that users are able to safely enter and exit the 

circulatory roadway. This area often requires pavement 

warping or cross-slope transitions to provide an 

appropriate cross-slope transition rate through the entire 

transition area. Grading the vertical profile should also 

ensure that adequate sight distance is provided for the 

intersection and entry. Adjustments to the circulatory 

roadway cross slope may be required to meet these 

criteria, but should be balanced with the effects on the 

circulatory roadway.

6.6 Pavement Markings and Signs

At roundabouts, pavement markings and signs work 

together to create a comprehensive system to guide 

and regulate road users. In order for pavement markings 

at roundabouts to provide appropriate guidance, the 

following general principles should be considered:

•	 Markings and signing are integral to roundabout design, 

especially for multilane roundabouts. Markings, in 

particular, need to be considered at the preliminary design 

stages of the roundabout, rather than fitting them in later 

in the design process.

•	 Markings and signs should facilitate through and turning 

movements in a manner such that drivers choose the 

appropriate lane when approaching a roundabout and 

then do not need to change lanes within the circulatory 

roadway before exiting in their desired direction.

•	 The Federal Highway Administration has published 

the 2009 edition of the Manual on Uniform Traffic 

Control Devices [11], which includes major revisions 

and additions related to signage and markings at 

roundabouts. For more detailed guidelines, designers 

should refer to the 2009 MUTCD and the Roundabout 

Guide. 

Figure 22: Typical Circulatory Roadway Section with Truck Apron
FHWA | Roundabouts 21

6.6.1 Pavement Markings

Typical pavement markings for roundabouts delineate 

the entries, exits, and the circulatory roadway, providing 

guidance for pedestrians and vehicle operators. Example 

markings for single-lane and multilane roundabouts are 

shown in Figure 23. Pedestrian crossing markings (shown 

in the figure) and yield line markings (not shown) may 

be used at any roundabout. Bicycle lanes within the 

circulatory roadway are prohibited.

As shown in Figure 23, solid white lane lines are 

recommended on multilane approaches and departures 

to discourage lane changes in these areas. Multilane 

roundabouts should also have lane line markings 

within the circulatory roadway to channelize traffic to 

the appropriate exit lane. These circulatory roadway 

lane line markings and lane-use arrows (described 

below) should be designed to work together with 

approach lane line markings to ensure that once 

drivers have chosen the appropriate entry lane on the 

approach, they do not have to change lanes within the 

roundabout to exit at their desired exit. One possible 

pattern for marking circulatory roadway lane lines 

is illustrated in Figure 24. However, there are other 

possibilities for the marking pattern of lane lines within 

the circulatory roadway of roundabouts; these are 

discussed in the Roundabout Guide.

In general, lane-use arrows should be used at 

roundabout approaches with exclusive turn lanes 

and at other multilane roundabouts where lane-use 

arrows will improve lane use by drivers. There are four 

different options for the design of lane-use arrows on the 

approach to roundabouts as shown in Figure 25. Normal 

lane-use arrows may be used with or without an oval 

symbolizing the central island. Alternatively, the “fishhook” arrows shown on the right, which also contain 

an optional oval symbolizing the central island, may be 

Figure 23: Example Markings

Figure 24: Example Markings for Multilane Roundabout (Bend, Oregon)

Photo: Casey Bergh (used with permission)
22 FHWA | Roundabouts

used. In choosing a lane-use arrow design, 

designers should consider the general 

practices within a city, region, or State. 

6.6.2Signing

The overall concept for roundabout signing 

is similar to general intersection signing. 

Proper regulatory control, advance warning, 

and directional guidance are required to 

avoid driver expectancy-related problems. 

Signs should be located where they have 

maximum visibility for road users but a 

minimal likelihood of even momentarily 

obscuring more vulnerable users including pedestrians, 

motorcyclists, and bicyclists. Signing needs are different 

for urban and rural applications and for different 

categories of roundabouts.

Figure 26 shows typical layouts of regulatory and 

warning signs for single-lane and multilane roundabouts. 

For multilane roundabouts, lane-use signs can use the 

same range of lane-use arrow options as described for 

pavement markings.

Guide signs at roundabouts generally consist of exit 

guide signs and advance guide signs. Exit guide signs 

are generally recommended at all roundabout exits to 

designate the destinations of each departure leg. These 

signs are similar to conventional intersection direction 

signs or directional route marker assemblies, except that 

a diagonal upward pointing arrow is used, as shown 

in Figure 27. These signs can be placed either on the 

right hand side of the roundabout exit or in the splitter 

island (recommended where feasible to maximize 

sign visibility). Advance destination guide signs should 

Figure 25: Lane-Use Arrow Options for Roundabout Approaches

Optional for

left-most lane

Match arrow(s)

with desired lane

use conguration

A - Normal arrows

Match arrow(s)

with desired lane

use conguration

Optional for

left-most lane

B - Fish-hook arrows

(Optional)

(Optional)

(Optional)

(Optional)

(Optional)

(Optional)

(Optional)

(Optional)

(Optional)

OR

OR OR OR

Figure 26: Example of Regulatory and Warning Sign Layouts
FHWA | Roundabouts 23

be used in all rural locations and in urban/

suburban areas where appropriate. Additional 

examples can be found in the MUTCD.

6.7 Lighting

Roundabouts, including their pedestrian 

crossing areas and bicycle design features, 

should be conspicuous and visible to 

approaching drivers. The overall illumination 

of the roundabout should be based on local 

and national guidelines for street lighting. The 

Design Guide for Roundabout Lighting [12], 

published by the Illuminating Engineering 

Society (IES), is the primary resource that 

should be consulted in completing a 

lighting plan for all roundabout types. 

Local illumination standards should also be 

considered when establishing the illumination 

at the roundabout to ensure that the lighting 

is consistent. The Roundabout Guide provides 

a more detailed summary of lighting principles 

and guidelines.

6.8 Landscaping

Landscaping of roundabouts plays an 

important role in improving the aesthetics 

of an area, as shown in Figure 28. However, 

landscaping has a number of functional 

purposes:

•	 It makes the center island more conspicuous; 

•	 It focuses driver attention on key conflict areas 

by blocking the view of other areas; and

•	 It discourages pedestrian traffic through the 

center island.

Any landscaping that is provided should 

be designed to minimize roadside hazards, 

particularly in higher speed environments, 

and to maintain adequate stopping and 

intersection sight distance throughout the 

roundabout.

6.9 Other Design Details and 

Applications

More design details and applications of 

roundabouts exist than can be covered in 

this technical summary; however, some of 

Figure 27: Example Exit and Advance Diagrammatic Guide Signs

Figure 28: Use of Selective Landscaping (Coralville, Iowa)

Photo: Lee Rodegerdts (used with permission)
24 FHWA | Roundabouts

the more notable considerations are described below:

•	 Right-turn bypass lanes – Roundabouts can 

employ right-turn bypass lanes similar to those used at 

conventional intersections. Bypass lanes are designed 

either to yield to exiting traffic or to form an additional 

lane next to exiting traffic (which may then merge into the 

exiting traffic).

•	 Access management – Driveways in the vicinity 

of roundabouts may experience restrictions in access 

similar to those in the vicinity of signalized intersections. 

Roundabouts may offer the opportunity to include 

driveways as a curb cut or a fully developed approach with 

splitter islands depending on the volume characteristics 

and other factors.

•	 At-grade rail crossings – At-grade rail crossings 

through or near a roundabout are possible but introduce 

challenges related to the control of the rail crossing itself, 

queue clearance on the tracks, and the associated effects 

on the roundabout.

•	 Evacuation routes – Roundabouts have been located 

on evacuation routes and have had flow reversed as 

needed to facilitate evacuation.

•	 Bus stops – Bus stops can be provided on either the 

entry or exit side of a roundabout. Bus stops should not 

be provided within the circulatory roadway. Pedestrian 

access to and from the bus stop, including the location 

of the bus stop relative to the nearest crosswalk, should 

be carefully considered.

Refer to the Roundabout Guide for additional 

information on these and other topics. 
FHWA | Roundabouts 25

Section 7: Costs

Construction costs for roundabouts vary widely, from tens of thousands of dollars for minor retrofits 

of small intersections using existing curb lines, existing pavement, and no landscaping to millions 

of dollars for major reconstruction of large intersections with significant earthwork, structures, and 

landscaping. Right-of-way costs also vary widely depending on impact area and land uses. As a 

result, a case-by-case evaluation of construction costs is needed for a reasonable assessment.

A benefit-cost analysis may be useful in alternatives 

analysis, as it recognizes that not all of the benefits 

and costs of an alternative can be quantified by 

pure construction costs. The safety, operational, 

and environmental benefits of roundabouts can be 

quantified and compared to the initial construction 

and ongoing maintenance cost over the life cycle 

of the roundabout. While initial construction costs 

might be higher for a roundabout in a retrofit situation 

(they are often comparable in new installations), the 

roundabout’s ongoing maintenance is often cheaper 

than for signalized intersections, as there is typically no 

signal hardware to power, maintain, and keep current 

in terms of signal timing. Finally, while many factors 

influence the potential service life of a roundabout 

(types of construction materials, weather conditions, 

traffic conditions, growth in the area, etc.), roundabouts 

can often serve for longer periods of time between 

major upgrades (repaving, reconstruction, etc.) than 

comparable signalized intersections. More detail on 

estimating lifecycle benefits and costs can be found in 

the Roundabout Guide.
26 FHWA | Roundabouts

Section 8: References

1. Robinson, B. W., L. Rodegerdts, W. Scarbrough, W. Kittelson, R. Troutbeck, W. Brilon, L. Bondzio, K. Courage, M. 

Kyte, J. Mason, A. Flannery, E. Myers, J. Bunker, and G. Jacquemart. Roundabouts: An Informational Guide. Report 

FHWA-RD-00-067. FHWA, U.S. Department of Transportation, June 2000.

2. Rodegerdts, L. A., et al. Roundabouts: An Informational Guide, 2nd Edition. National Cooperative Highway Research 

Program Project 03-65A. Transportation Research Board, National Academy of Sciences, Washington, D.C. Work in 

progress; estimated publication 2010.

3. Rodegerdts, L. A., W. E. Scarbrough, and J. A. Bansen. Technical Summary on Mini-Roundabouts. FHWA, 

Washington, D.C., 2010.

4. Rodegerdts, L., M Blogg, E. Wemple, E. Myers, M. Kyte, M. Dixon, G. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, 

B. Persaud, C. Lyon, D. Harkey, D. Carter. Roundabouts in the United States. National Cooperative Highway Research 

Program Report 572. Transportation Research Board, National Academies of Science, Washington, D.C., 2007.

5. Persaud, B. N., R. A. Retting, P. E. Garder, and D. Lord. Crash Reductions Following Installation of Roundabouts in the 

United States. Insurance Institute for Highway Safety, Arlington, Virginia, March 2000.

6. Cunningham, R. B. Maryland’s Roundabouts: Accident Experience and Economic Evaluation. Traffic Development 

& Support Division, Office of Traffic and Safety, State Highway Administration, Maryland Department of 

Transportation, March 2007.

7. Hughes, Ronald G., et al. NCHRP 03-78A: Crossing Solutions at Roundabouts and Channelized Turn Lanes for 

Pedestrians with Vision Disabilities. Transportation Research Board, National Academies of Science, Washington, D.C. 

Work in progress, estimated publication 2010.

8. Transportation Research Board. Highway Capacity Manual. Transportation Research Board, National Academies of 

Science, Washington, D.C. Work in progress, estimated publication 2010.

9. American Association of State Highway and Transportation Officials (AASHTO). A Policy on Geometric Design of 

Highways and Streets. AASHTO, Washington, D.C., 2004.

10. Kittelson & Associates, Inc. and TranSystems Corporation. Kansas Roundabout Guide: A Supplement to FHWA’s 

Roundabouts: An Informational Guide. Kansas Department of Transportation, Topeka, Kansas, October 2003.

11. Federal Highway Administration (FHWA). Manual on Uniform Traffic Control Devices. FHWA, Washington, D.C., 2009.

12. Illuminating Engineering Society. Design Guide for Roundabout Lighting. Publication IES DG-19-08. Illuminating 

Engineering Society of North America, New York, February 2008.

For More Information

Ed Rice

Intersection Safety Team Leader, 

FHWA Office of Safety

202.366.9064

ed.rice@dot.gov

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

FHWA-SA-10-006 February 2010