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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
Roundabout History
- Rotaries and Traffic Circles Emerge
- Columbus Circle in NYC credited as the first
- Circular intersections out of favor
- Great Britain tries variants of circular intersections
- Adopted mandatory "yield at entry" rule
- Modern roundabouts widely used in Europe and Australia
- Modern roundabouts start to be built in the US
speaker notes:
Major points to make:
Note: Click to build the slide as you talk about the history.
- The concept of a circular intersection dates back to 1903, when a "gyratory" operation for traffic control was adopted by the French at Place de l'Etoile (now Place Charles de Gaulle.) During the early half of the 20th century, numerous gyratory systems (traffic circles or rotaries) were built throughout the United States.
- Unfortunately, both the design and traffic conditions at that time led to severe problems with gyratory systems. Gyratory systems typically required traffic inside the circle to yield to traffic entering the circle. The lack of support for modern roundabouts in the United States — as contrasted with their overwhelming popularity in Europe and Australia — can usually be traced back to the problems with gyratory systems (traffic circles or rotaries) initially encountered by motorists. During the time when these systems were first installed, local ordinances were not enforceable and uniform rules of the road were still being developed. Consistent, right-of-way rules did not exist throughout the country; practices differed throughout the United States.
- Adopted by the British in 1960, the mandatory yield at entry rule was a very dramatic and important change in the success and use of modern roundabouts.
- Modern roundabouts were developed in the United Kingdom in the 1960s, and have been widely used throughout Europe and Australia as a form of intersection control during the past four decades.
- The first modern roundabouts in the United States were constructed in Nevada in 1990. More than 23 states, including California and New York, have active programs to construct roundabouts. More than a thousand modern roundabouts have been built in the USA since 1990.
slide 7
Roundabouts in the U.S.
speaker notes:
Major points to make:
- The figures shown here are reported in NCHRP Report 572: Roundabouts in the United States (completed in 2003). It attempted to document every known modern roundabout constructed in the U.S. While many more have been built since 2003, a follow up study of similar magnitude has not been published.
- The report is downloadable from the TRB web site. Or just Google: NCHRP 572
slide 8
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 9
Yield Control
- Vehicles yield upon entry in a modern roundabout.
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.
slide 10
Circulatory Roadway
- No traffic control in circular roadway.
- Movement is counterclockwise.
speaker notes:
Major points to make:
- 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 11
Central Island
- Central island deflects vehicles from a straight line path.
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.
slide 12
Splitter Island
- Splitter islands separate, deflect, and slow traffic.
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)
slide 13
Landscaping
- Landscaping is needed as a visual element to drivers.
speaker notes:
Major points to make:
- While landscaping in the central island is very appealing for aesthetic reasons (which can be a big selling point to communities), its 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 14
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 15
Truck Apron
- Where trucks are common, a properly designed apron may be necessary.
speaker notes:
Major points to make:
- 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.
- Note about drivers and emergency vehicles: If a driver is in a roundabout when an emergency vehicle also enters, motorists are to clear the roundabout and not simply pull over within the roundabout.
slide 16
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 photos/diagrams on this slide represent signing for a simple urban roundabout. The next slide illustrates advance signing for more complex roundabouts.
- The proposed changes in the upcoming MUTCD include much more on roundabout signing and marking than previous versions.
slide 17
Signing and Marking
- Proper signing helps drivers navigate the roundabout.
speaker notes:
Major points to make:
- The photos/diagrams on this slide represent signing for a more complex roundabouts.
- The proposed changes in the upcoming MUTCD include much more on roundabout signing and marking than previous versions.
slide 18
Signing and Marking
- Proper pavement markings help drivers navigate the roundabout.
speaker notes:
Major points to make:
- The photos/diagrams on this slide show the various pavement markings that are used at modern roundabouts.
- Left: A lane-use "fishhook" diagram that can be used for multi-lane roundabouts.
- Upper right: Dotted circulatory roadway delineation line
- Lower right: Yield line marking
- The proposed changes in the upcoming MUTCD include much more on roundabout signing and marking than previous versions.
As noted on Slide #9:
- 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.
slide 19
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 20
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 21
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 22
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 23
Study Results
Convert signalized intersection to roundabout.
- 48% reduction in all crashes
- 78% reduction in fatal/injury crashes
- 60% reduction in fatal/injury crashes in urban areas
speaker notes:
Major points to make:
- This slide shows typical crash reduction data for converting a signalized intersection to a modern roundabout. Data 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 24
Study Results
Convert two-way stop intersection to roundabout.
- 44% reduction in all crashes
- 87% reduction in fatal/injury crashes in rural area
- 78% reduction in fatal/injury crashes in urban area on one-lane road.
- 72% reduction in fatal/injury crashes in urban area on two-lane road.
speaker notes:
Major points to make:
- This slide shows typical crash reduction data for converting a two-way stop intersection to a modern roundabout. Data comes from Intersection Safety Issue Brief #8 (September 2007) and originally reported in NCHRP Report 572: Roundabouts in the United States.
- The top two 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.
- There have been some studies done on converting 4-way stop intersections to roundabouts. While operations were improved (less delay), the affect on crash reduction was very little (or negative).
slide 25
Older Drivers and Safety
- Narrowing of visual field
- Restricting of the area of visual attention
- Decreased motion sensitivity
- Decline in selective attention
- Decline in divided attention
- Decline in perception-reaction time (PRT)
- Loss of flexibility
speaker notes:
Major points to make:
As human beings age, a number of our abilities begin to diminish. And many of them are critical to the driving task.
- Narrowing of the visual field diminishes the ability to see objects in the periphery, such as signs, signals, vehicles, pedestrians and cyclists.
- Restricting of the area of visual attention diminishes the ability to see potential conflicts in the periphery and to discriminate relevant from irrelevant information.
- Decreased motion sensitivity diminishes the ability to accurately estimate closing speeds and distances and is need for judging gaps to safely perform left turns at conventional intersections with oncoming traffic or to cross an intersecting traffic stream.
- Decline in selective attention or the ability to filter out less critical information and continuously re-focus on the most critical information, such as detecting a lane-use restricted message on an approach to a busy intersection or detecting a pedestrian crossing while watching oncoming traffic to locate a safe gap.
- Decline in divided attention or the ability to perform multiple tasks simultaneously and process information from multiple sources.
- Decline in PRT, the time required to perceive a situation, evaluate it, decide an appropriate response, and execute that response. PRT increases disproportionately for older motorists with increase in complexity of the driving situation.
- Loss of flexibility needed to rapidly glance in each direction from which a vehicle conflict might be expected.
When comparing roundabouts to the conventional intersection design, most (if not all) of these factors are reduced, thereby making the roundabout a safer choice for older drivers.
slide 26
Older Drivers and Safety
| Conventional Intersection |
Roundabout |
| High speeds |
Low speeds |
Little response time
|
Situation changes slowly/More PRT |
High energy crashes
|
Low energy crashes |
Unforgiving environment
|
Forgiving environment |
High severity crashes
|
Low severity crashes |
| Complexity |
Easier to judge gaps |
Wide visual scans
|
Narrow visual scans |
speaker notes:
Major points to make:
This table illustrates the differences between conventional intersections and roundabouts. While the focus here is on older drivers in particular, it should always be noted that what is good for older drivers is almost always good for all categories of drivers.
- Lower speeds – conditions change slowly allowing for more time to make the right response
- Low energy crashes
- Less complicated situations and decision-making
- Judging gaps is easier and mistakes are rarely fatal
- No demand to accurately judge speeds of traffic
- No wide visual scans needed
slide 27
Reduce Congestion and Pollution
- Efficient during peak and off peak
- Reduced stops
- Reduced idling
- Reduced delay
speaker notes:
Major points to make:
- In addition to the safety benefits described in the previous slides, modern roundabouts can also:
- Reduce congestion because they are efficient during both peak hours and other times
- Impose less delay on drivers
- Reduce pollution and fuel use
- Lead to fewer stops and fewer hard accelerations
- Reduce the amount of time drivers spend idling
slide 28
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 there is 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 is 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 29
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 30
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 31
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 32
Mini-Roundabouts
- Smaller design for urban areas
- Speed zones < 35 mph
- Central island is often painted
- Relatively inexpensive
speaker notes:
Major points to make:
- Small roundabouts used in low-speed urban environments often characterized by having mountable (or painted only) center islands and splitter islands
- The smallest possible design in tight urban areas and low speed zones less than 35 mph but preferably 25 mph.
- Relatively inexpensive in existing sites because it is mostly marked.
- The inscribed perimeter does not need to be circular.
- It can be defined by the swept path of the design vehicle (bus or emergency vehicle) traveling on the most critical path (left-turn, right-turn or through)
- The central island is painted and slightly domed with the center raised by about 5 – 6 inches to make it traversable by larger vehicles.
- A typical inscribed circular diameter (ICD) is about 45-80 feet.
- The splitter islands are usually marked.
- Pedestrians feel comfortable in this low speed environment.
slide 33
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 34
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 35
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 36
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 37
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 38
Roundabouts and Rail Crossings
speaker notes:
Major points to make:
- Light rail lines have successfully been integrated into roundabouts. This
aerial photograph is of a roundabout in Salt Lake City, Utah near the University
of Utah campus and football stadium. The lower right photo is of the LRT going
through the same roundabout. The photo in the lower left is of a heavy rail
line going through a roundabout in Tavares, Florida (intersection of Main
St & Lake Dora Dr).
slide 39
Roundabouts and Schools
speaker notes:
Major points to make:
- Roundabouts have been installed successfully near schools (typically on lower volume arterials).
- This is an aerial photograph taken of a school in Brown County, Wisconsin (north of Green Bay).
slide 40
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 41
Issues to Review
- Context
- Space feasibility
- Physical or geometric complications
- Proximity of railroad grade crossings, drawbridges
- Traffic congestion
- Presence of oversize vehicles
- Presence of pedestrians and bicyclists
speaker notes:
Major points to make:
- Context. What are the regional policy constraints that
must be addressed? Are there site specific and community impact reasons why
a roundabout of any particular size would not be a good choice?
- Space feasibility. Is there enough right-of-way to build
the roundabout? Is right-of-way acquisition required? If "yes,: this introduces
administrative complications that some agencies might want to avoid.
- Physical or geometric complications such as right-of-way
limitations, utility conflicts, drainage problems and unfavorable topography
that may limit visibility or complicate construction.
- Proximity of railroad crossings, drawbridges (and other
features) that may have traffic control devices that would require preemption.
- Traffic congestion that would cause routine back-ups into
the roundabout, such as over-capacity signals or freeway entrance ramps. The
successful operation of a roundabout depends on unimpeded flow on the circulatory
roadway.
- Traffic congestion that would cause routine back-ups into
the roundabout, such as over-capacity signals or freeway entrance ramps. The
successful operation of a roundabout depends on unimpeded flow on the circulatory
roadway.
- Presence of oversize vehicles or significant traffic that
might have difficulty negotiating the roundabout.
- Presence of a large number of pedestrians and/or bicyclists in
conflict with high traffic volumes. (These conflicts pose a problem for all
types of traffic control.)
slide 42
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 43
Roundabout Resistance
speaker notes:
Major points to make:
- Results of a National survey contained in the NCHRP Synthesis 264.
- Uncertainty about functionality/effectiveness (safety and operation), and drivers lack of familiarity prevailed.
- Several jurisdictions who were pioneers in recommending and building roundabouts confronted these doubts by developing guidelines and building them (Maryland, and several cities in Florida).
- Doubts were proven to be unfounded and were dispelled by experience.
slide 44
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 45
Roundabout Resources
speaker notes:
Major points to make:
From left to right:
slide 46
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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