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Home > Intersection Safety > Roundabouts > Presentation: Safety Aspects of Roundabouts
Safety Aspects of Roundabouts
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speaker notes:
This presentation is on the safety aspects of the modern roundabout. It covers the following topics:
- Terminology, What is and what isn't a modern roundabout
- History
- Key Features
- Why install roundabouts
- Safety benefits
- Other benefits
- Special considerations
- Implementation
slide 2
Terminology
speaker notes:
Major points to make:
- Not all circular intersections are roundabouts; Roundabouts are a subset of circular intersections.
- Not much time needs to be spent on this slide. The point is that there are numerous terms being used to describe various configurations of similar roadway configurations. Not all are the equivalent of the modern roundabout. This presentation will help explain the differences.
slide 3
What isn't a Modern Roundabout?
speaker notes:
Major points to make:
- Before explaining what a modern roundabout is, let's describe a few examples of what isn't a modern roundabout.
Clockwise from upper left:
- Rotary (like in England, east coast): high speeds on approaches and usually within the circle, not pedestrian friendly
- Traffic circle (like in D.C., Rome): often signalized and/or bi-directional, sometimes nothing more than a circular roadway with numerous "T" intersections
- Neighborhood circle: traffic calming countermeasure, usually no traffic control, location would not likely warrant a traffic signal, might be in lieu of a 2-way or 4-way stop
slide 4
What is a Modern Roundabout?
speaker notes:
Major points to make:
These four points are what differentiate a modern roundabout from other similar or related traffic control features. Use mouse clicks to bring up each point along with an illustration on the photograph:
- A compact one-way, circular intersection in which traffic flows counterclockwise around a center island
- Other styles (i.e. rotaries, traffic circles) may have 2-directional flow and are typically much larger than the modern roundabout. This compactness helps keep speeds low and makes it easier for drivers to stay oriented.
- Entering traffic yields
- Other styles may have circulating traffic yield to entering traffic
- Approaches are channelized to deflect traffic into the flow
- Other styles may not use channelization, or deflection may not exist
- Designed to slow the speed of vehicles through deflection of the vehicle path
- Other styles may not be designed to slow traffic
slide 5
What is a Modern Roundabout?
speaker notes:
Major points to make:
- This slide illustrates the size difference between a high-speed rotary and a modern roundabout. Click the mouse to reveal the modern roundabout that is being constructed to replace the rotary. This photo is taken from a project in Kingston, NY.
- If your computer has a live Internet connection, you should be able to click on the Google Maps icon to look at a satellite view of the completed roundabout.
slide 6
Key Features
- Yield control
- Circulatory roadway
- Central island
- Splitter island
- Pedestrian access
- Landscaping
- Truck apron
- Signing and marking
speaker notes:
Major points to make:
- Some have called a modern roundabout a "geometric feature," others consider it more of a "traffic control device." Both are true in some respects.
- This is a pass-through slide. Note that there are numerous features of a modern roundabout that must be properly designed and maintained in order to function properly and to achieve the safety benefits. Each of these will be discussed in more detail on the following slides.
slide 7
Key Features
- Vehicles yield upon entry in a modern roundabout.
- No traffic control in circular roadway. Movement is counterclockwise.
speaker notes:
Major points to make:
- A modern roundabout is different from the older-style rotary or traffic
circle in that the approaching vehicles ALWAYS yield to vehicles in the roundabout.
- This is true no matter how many lanes are on the approach or within the
circle.
- A YIELD sign is often accompanied by a YIELD line and pavement legend.
- Circulatory roadway edge line: Solid white line in front of the splitter
island, and dotted across the entry lanes
- Yield line: a row of solid white isosceles triangles pointing toward approaching
vehicles
- Yield lines are optional and may be used to indicate where approaching vehicles
should yield, supplementing the dotted circulatory roadway edge line extension
across the entry lanes of a roundabout. A YIELD word pavement marking may
also be used at a roundabout entrance to supplement the YIELD sign.
- Once inside the roundabout, there are no traffic control devices to impede
the movement of vehicles.
- Traffic circulates in a counter-clockwise direction. Thus every point of
entry to the traffic circle is a right-in/right-out only access. Hence there
are no LEFT TURNS being made in the roundabout, which, at a traditional intersection,
often lead to severe crashes.
- Note that the green arrows shown on the photo are NOT any type of pavement
marking. They are simply an illustration of the direction that traffic moves
in the roundabout.
slide 8
Key Features
- Central island deflects vehicles from a straight line path.
- Landscaping is needed as a visual element to drivers.
speaker notes:
Major points to make:
- The purpose of the central island is not only to define the circulatory
roadway, but it also must deflect vehicles from a straight path, causing them
to slow their speed, thereby reducing the severity of potential crashes.
- Central islands are not always circular (as shown in the photo). They can
be oblong shaped to fit unique conditions.
- The geometric element "Inscribed Circle" is different from the "Central
Island Diameter." The inscribed circle is the sum of the central island diameter
(which includes the apron, if present) and twice the circulatory roadway.
- While landscaping in the central island is very appealing for aesthetic
reasons (which can be a big selling point to communities), it’s primary
purpose is to visually cue drivers that the roundabout is there and to help
prevent drivers from continuing on a straight path.
- Landscaping can also be used on the approaches to encourage pedestrians
to cross only at designated locations.
- Landscaping on the approaches can also be used as a buffer between pedestrians
and vehicles
slide 9
Key Features
- Splitter islands separate, deflect, and slow traffic.
- Where trucks are common, a properly designed apron may be necessary.
speaker notes:
Major points to make:
- Splitter islands separate entering and exiting traffic.
- But perhaps more importantly, a properly designed splitter island deflects
traffic and positions their alignment to enter the circulatory roadway.
- This deflection is critical to slowing vehicles before they enter the circulatory
roadway.
- The photo on the left is of a splitter island approaching a rural (higher
speed) modern roundabout.
- Not shown in these particular photographs, splitter islands also provide
storage space for pedestrians crossing the road in two stages. A photograph
of this is shown in the slide on Pedestrian access (slide #14)
- Truck aprons should be used when there are significant numbers of large
trucks present.
- The curb on the apron is traversable.
- Aprons may be necessary in rural roundabouts for use by farm equipment.
- In urban areas, they are often used by emergency vehicles.
slide 10
Pedestrian Access
- Pedestrian crossings must conform to ADA standards.
speaker notes:
Major points to make:
- Pedestrian crossings must be provided at all roundabouts and those crossings
should conform to current ADA standards.
- The crossing location is set back from the yield line.
- The splitter island is cut to allow peds, wheelchairs, strollers, and bicycles
to pass through (or take refuge during a two-stage crossing).
- Tactile surfaces should be used to warn peds with visual disabilities.
- Refer to NCHRP Project 3-78A: Crossing Solutions at Roundabouts and
Channelized Turn Lanes for Pedestrians with Vision Disabilities (http://www.trb.org/).
This project will be recommending a range of geometric designs, traffic control
devices, and other treatments to make pedestrian crossings at multilane roundabouts
useable by pedestrians with vision impairment.
slide 11
Signing and Marking
- Proper signing helps drivers navigate the roundabout.
speaker notes:
Major points to make:
- 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 pedestrians as well as motorcyclists and bicyclists,
who are the most vulnerable of all roundabout users. Signing needs are different
for urban and rural applications and for different categories of roundabouts.
- The proposed changes in the upcoming MUTCD include much more on roundabout
signing and marking than previous versions.
slide 12
Why a Roundabout?
- Improve safety
- Reduce congestion and pollution
- Save money
- Complement other common community values
speaker notes:
Major points to make:
- This is primarily a pass-through slide. Each of the bullet points will be covered in the subsequent slides.
slide 13
Vehicle Conflict Points
speaker notes:
Major points to make:
- These diagrams point out the difference in vehicle conflict points at a basic roundabout (8) and 4-leg intersection (32).
- In some respects, the graphic on the right is over simplified (it doesn't
illustrate left or right-turn lanes or multiple lanes). And the graphic on
the left only is illustrative of a single-lane roundabout.
- BUT the point is, that the roundabout has ZERO vehicle crossing conflict points as opposed to the 16 vehicle crossing conflict points at the 4-leg intersection. This is where most of the safety benefits arise from.
- Not shown in the graphics are the typically shorter crossing distances for pedestrians (because they don't cross left and right turn lanes in addition to the through lanes), thereby limiting their exposure time to vehicle paths.
slide 14
Vehicle-Pedestrian Conflict Points
speaker notes:
Major points to make:
- Pedestrians are generally safer at roundabouts although a more active interaction may be necessary with the drivers similar to stop controlled intersections.
- Pedestrians are faced with simpler decisions at a time and they travel shorter distances.
- At a pedestrian crossing one is faced with two less hazardous conflicts then at conventional intersections (whether stopped or signal controlled).
slide 15
Type of Crashes
 
speaker notes:
Major points to make:
- Angle and left turn crashes at a typical 4-leg intersection account for 63% of fatal crashes. Rear-end crashes are another type of crash that is often a problem at signalized intersections. However, these types of crashes are usually (but not always) less severe than the other two types of crashes shown.
- Right angle crashes are typically the most severe because of the speed differential of the two vehicles. One vehicle is typically just starting to accelerate from a stop (> 5 mph), the other vehicle is typically going through the intersection at or above normal operating speeds for that street (perhaps 30-50 mph).
- At a roundabout, vehicles are traveling at a significantly lower speed (<25 mph). Crashes that occur will be less severe because of this reduced speed differential and the more "sideswipe" nature of crashes.
slide 16
Study Results
- Reductions in crashes
- Reduction in fatalities and injuries
- Reduction in stops
- Reduction in idling
- Reduction in delay
speaker notes:
Major points to make:
- Numerous studies have shown the safety and operational benefits of roundabouts.
The data below comes from Intersection Safety Issue Brief #8 (September 2007)
and originally reported in NCHRP Report 572: Roundabouts in the United States.
- Also note that in July 2008, FHWA issued a document called “Guidance
Memorandum on Consideration and Implementation of Proven Safety Countermeasures”
that includes the recommendation to construct roundabouts.
- The numbers shown are for all roundabouts (1- and 2-lane, rural and urban).
However, studies show that crashes are reduced significantly for both 1- and
2-lane configurations.
slide 17
Save Money
- No signal equipment to install, power, and maintain
- May require less right-of-way
- Less pavement may be needed
speaker notes:
Major points to make:
- Frequently, roundabouts can save a jurisdiction money because:
- Often no signal equipment to install, power, and maintain
- Smaller roundabouts may require less right-of-way than traditional intersections (to be discussed in slide 34)
- Often less pavement needed because additional pavement width is not needed for turn lanes.
- A typical 8-phase new traffic signal installation (design and construction) can cost anywhere from $100,000-$200,000. Plus the cost of powering and maintaining the signal must be figured.
- It should be noted that the current Access Board's recommendation is to install a pedestrian-activated traffic signal at multi-lane approaches to roundabouts. It is not yet required, but considered by FHWA to be a "good practice." Therefore it would be prudent to install conduit in preparation for future traffic signal equipment.
- The cost of a modern roundabout depends on several project specific conditions. Some agencies have reported that the cost to design and construct a roundabout is similar to that of a traffic signal. However, when the life cycle cost is analyzed, the roundabout is less expensive in the long run because there are no power requirements (with the exception of possible street lights) and maintenance is limited to routine pavement and sign maintenance.
slide 18
Complement Community Values
- Quieter
- Functional
- Aesthetically pleasing
speaker notes:
Major points to make:
- Roundabouts can operate quieter than conventional signalized intersections because of fewer start-ups and brakings by vehicles.
- Research has shown that roundabouts can be very functional in lieu of other traffic control methods.
- Roundabouts offer the opportunity to provide attractive entries or centerpieces to communities. However, hard objects in the central island directly facing the entries are a safety hazard. The portions of the central island and, to a lesser degree, the splitter islands that are not subject to sight-distance requirements offer opportunities for aesthetic landscaping. Pavement textures can be varied on the aprons as well.
slide 19
Special Considerations
- Pedestrians
- Bicyclists
- Visually-impaired
speaker notes:
Major points to make:
- Pedestrians and bicyclists have far less risk navigating roundabouts than the typical intersections primarily because of the lower speeds. A pedestrian has an 85% chance of being killed by a vehicle traveling at 45MPH. That drops to 15% when the vehicle is traveling at 20MPH. There are also less conflict points (as discussed in slide 21) the crossing distance is usually much shorter, and there is oftentimes a refuge spot in the splitter island.
- Non-motorized traffic must be taken into account at roundabouts. Visually-impaired pedestrians have unique challenges at roundabouts that are very different from the challenges they face at a signalized intersection. Most pedestrians who cross streets at roundabouts use their vision to identify a "crossable" gap between vehicles. While crossing, they visually monitor the movements of approaching traffic and take evasive action when necessary. Blind pedestrians rely primarily on auditory information to make judgments about when it is appropriate to begin crossing a street. To date, little research has been conducted about the usefulness of such non-visual information for crossing streets at roundabouts. Current projects are researching this issue (NCHRP 3-78a).
- Traffic sounds at roundabouts can provide ambiguous cues. Circulating vehicles can mask the sounds of entering and exiting traffic, making it difficult to identify an appropriate time to cross. At exit legs, auditory information may not be adequate to reliably convey whether circulating vehicles will exit or continue around the roadway.
- In the photo on the right, a pedestrian is walking towards the splitter island of a roundabout. Traffic comes from the left in the foreground and from the right in the background (beyond the splitter island).
- For bicyclists, there should be no bike lanes within the circulatory roadway. Mixing bicyclists with vehicles in a single-lane roundabout has shown to be successful because the relative speeds of cars and bikes are much closer. However, for multi-lane roundabouts, a separate or shared bike/ped path is recommended. Bicyclists can also dismount and walk their bikes like a pedestrian.
- NCHRP Project 3-78A Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities (http://www.trb.org/) will be recommending a range of geometric designs, traffic control devices, and other treatments to make pedestrian crossings at multilane roundabouts useable by pedestrians with vision impairment.
- The U.S. Access Board has also been integral in the development of design guidelines for roundabouts as they impact pedestrians with visual and mobility impairments. http://www.access-board.gov/
slide 20
Multi-Lane Roundabouts
speaker notes:
Major points to make:
- Mouse click brings up second photo.
- The most common roundabout is the single-lane roundabout (see chart on slide 7). However, two- and three-lane roundabouts are not unusual. In fact, the second photo shows a 2-lane on the left next to a 3-lane on the right.
- Signing and striping become a little more complex for multi-lane roundabouts. However there is guidance available for how such striping should be done to be consistent.
slide 21
Rural Roundabouts
- Higher approach speeds
- Properly designed splitter island is critical
speaker notes:
Major points to make:
- In order to better accommodate larger vehicles, rural roundabouts are typically designed to have larger diameters. A consequence of a larger diameter is that rural roundabouts tend to have higher design speeds on the vehicle path curvatures. Therefore, rural roundabouts may require supplementary geometric and traffic control device treatments on the approach roadway to encourage drivers to slow to an appropriate speed before entering the roundabout.
- More land is available to build larger inscribed circular diameter (ICD) and allow slightly higher speeds.
- Capacity is higher than urban.
- An apron may be required especially when a WB-50 needs to be accommodated.
- Preferable to design them with curbed boundaries at all approaches and entries to confine motor vehicle paths to the intended design.
- Pedestrians and bicyclists might have to be accommodated in anticipation of future expansions.
- Approach speeds of 50-60 mph are normal.
slide 22
Right-of-Way Requirements
 
speaker notes:
Major points to make:
- Roundabouts can often be constructed within the existing right-of-way of traditional intersections (or requiring very little extra right-of-way).
- The photo in the lower right is a simulation of how the roundabout was illustrated in the planning and design stage. Using the Street View function of Google Maps, you can see the roundabout nearing completion with the buildings still in the same locations. The intersection's location is Main Street and Center Street in Hamburg, New York.
- If your computer is hooked to the Internet, you may be able to click on the Google Maps icons to view each of the locations in Google Maps.
slide 23
Where to Consider Roundabouts
- Intersections with high crash rates/high severity rates
- Intersections with complex geometry, skewed approaches, >4 approaches
- Rural intersections with high-speed approaches
- Freeway interchange ramp terminals
- Closely spaced intersections
- Replacement of all-way stops
- Replacement of signalized intersections
- At intersections with high left turn volumes
- Replacement of 2-way stops with high side-street delay
- Intersections with high U-turn movements
- Transitions from higher-speed to lower-speed areas
- Where aesthetics are important
- Where accommodating older drivers is an objective
speaker notes:
Major points to make:
- Roundabouts should be considered under a wide range of conditions, but may be particularly advantageous at those listed in this table. This is just an example of locations that may be good candidates for roundabouts. Nor does it imply that they are the correct choice for each instance. An operational analysis should always be done when considering any traffic control/geometric design options for a given intersection.
slide 24
Roundabouts in Corridors
speaker notes:
Major points to make:
- Roundabouts have been successfully implemented along roadway corridors. Oftentimes this is part of a larger access management program. In the photo, all cross-median access was eliminated between the roundabouts. There is one emergency access cut-through about half-way between the 2 closest roundabouts in the photo. This photo is taken in Malta, NY. If you are connected to a live Internet connection, click on the Google Maps icon to view the corridor in satellite or street view.
- In a study of a roundabout corridor in Golden, Colorado, it was determined that the installation of 4 roundabouts in a half-mile long corridor (replacing two – soon to be three – traffic signals) resulted in slower average travel speed, but lower travel time, less delay, and reduced crash rates (http://ci.golden.co.us/files/roundaboutpaper.pdf). And perhaps equally important, sales tax revenues increased 60% in the corridor since the project was completed and an additional 75,000 sq. ft. of retail/office space has been built.
slide 25
Roundabouts in Interchanges
- Fewer queue backups
- Less bridge width possible
speaker notes:
Major points to make:
- Roundabouts can be used to improve operations at highway interchanges.
- One of the first such installations was on I-70 in Vail, Colorado (left photo).
- More recently, the Arizona DOT installed roundabout interchanges on I-17, just north of Phoenix (right photo).
- By continually moving traffic, fewer queue backups occur on either the ramps or the interior of the arterial.
- Also, by not having to provide left turn lanes on the interior, less bridge width may be required (a significant cost savings).
slide 26
Roundabouts and Driveways
speaker notes:
Major points to make:
- Click to bring up second photograph.
- Some roundabouts have been constructed with driveway access points within the influence area with no adverse safety affects.
- The first photo is in Clearwater, Florida. The other is in Chico, California.
slide 27
Roundabout Resistance
speaker notes:
Major points to make:
- The public usually has an initial fear or negative opinion of roundabouts, but that opinion changes after they've experienced their benefits.
- Most new sites faced uncertain and intense controversies that received media coverage.
- The same media praised the success of the new design after experiencing its benefits and familiarity.
slide 28
Keys to Success
- Proper design
- Public involvement
- Stakeholder support
speaker notes:
Major points to make:
- Proper design is without a doubt the most critical issue for the success of a roundabout. The design elements of a roundabout are discussed only in brief in this presentation, but the importance of proper design cannot be overstated. Even with enthusiastic community support, a poorly designed roundabout will likely be a failure.
slide 29
Roundabout Resources
speaker notes:
Major points to make:
From left to right:
slide 30
For More Information
speaker notes:
Major points to make:
- For more information on roundabouts, you can visit any of these sites.
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Program Contact
Ed Rice
ed.rice@dot.gov
202-366-9064
What’s New
Roundabout Outreach and Education Toolbox
Stop-Controlled Intersection Safety: Through Route Activated Warning Systems
Roundabouts: An Informational Guide, Second Edition (NCHRP Report 672)
Roundabouts Peer-to-Peer Assistance
How to drive a roundabout (WSDOT)
Modern Roundabouts: A Safer Choice
FHWA's Intersection Resources Library CD-ROM
Highlights
Roundabouts Technical Summary
Mini-Roundabouts Technical Summary
Access Management in the Vicinity of Intersections Technical Summary
Intersection Safety Case Studies
Intersection Safety Technologies
Presentation: Intersection Safety
Example Intersection Safety Implementation Plan
Intersection Safety Implementation Plan Workshop
Example Data Analysis Package and Straw Man Outline
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