SAFETY BENEFITS OF
ROUNDABOUTS

Prepared for:
Dr. Massoud Moradi, PE, AICP
University of Central Florida
TTE 5805, Geometric Design, Summer 2014

Prepared by:
Kimberly Braunsdorf
July 2014

TTE 5805: Geometric Design, Safety Benefits of Roundabouts

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Table of Contents
Section

Page

I.

Introduction. .............................................................................................................................3
I.1 Roundabout Safety Description ........................................................................................3

II.

Vehicular Safety.......................................................................................................................4
II.1 Single Lane Roundabouts................................................................................................4
II.2 Multilane Roundabouts ...................................................................................................7

III.

Bike/Pedestrian Safety ...........................................................................................................10
III.1 Pedestrian Conflicts .....................................................................................................10
III.2 Bicycle Conflicts ..........................................................................................................12

IV.

Conclusion .............................................................................................................................14

V.

References ..............................................................................................................................15

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I. Introduction
I.1

Roundabout Safety Description

Countries around the world have embraced the use of roundabouts. Although they are less common in the
United States, the safety benefits of this type of intersection have not gone unnoticed. Various studies and reports have described the operational, environmental, cost, and safety benefits. According to the FHWA
“There are an estimated 300,000 signalized intersections in the United States. About one‐third of all intersection fatalities occur at these locations, resulting in roughly 2,300 people killed each year.
Furthermore, about 700 people are killed annually in red‐light running collisions.”
The implementation of roundabouts have significant safety benefits and could reduce the fatalities occurring at the aforementioned signalized intersections.
Currently most roundabouts are one to two lanes and follow the general guidelines set by the Federal Highway
Administration. Each design will vary according to the parameters and needs of the specific project, but the safety benefits have been shown to occur in all roundabouts. (Benefits for small and medium capacity roundabouts show greater benefits than large capacity roundabouts). Intersections that involve a high number of crashes (such as right angle, head-on, left/through, and U turns) can benefit from the implementation of a roundabout. Vehicular, pedestrian, and bike safety as a result of this geometric design will be explained in this report. TTE 5805: Geometric Design, Safety Benefits of Roundabouts

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II. Vehicular Safety
II.1

Single Lane Roundabouts

The foremost benefit of roundabout safety is the reduction of conflict points. Traditional intersections have a larger number of conflict points, while roundabouts have eliminated all but a few. Shown below are exhibits from the NCHRP Report 672: Roundabouts: An Informational Guide, Second Edition. The types of conflicts have been described as follows:


Queuing Conflicts – Are caused by a vehicle running into the back of another vehicle while approaching the intersection that has a queue for through-movements or left turn movements. These types of conflicts are a concern in both traditional intersections and roundabouts. However they are generally the least severe due to the low speeds at which they occur.



Diverging Conflicts – Occur when two traffic streams separate. When the speed of one traffic stream is significantly less than another (a right turn and a through movement) the risk of a rear end collision increases. 

Merging Conflicts – Occur when two traffic streams join. For example, a right turn from a side street onto the mainline merges with the through traffic of the mainline. These types of conflicts increase the chances of side-swipes and rear end crashes. A side swipe is considered to be more severe than a rearend crash because the side of a car is generally less protected than the front or back.



Crossing Conflicts– Occur when the path of two opposing directions cross. These conflicts result in the most severe crashes, such as right-angle and head-on crashes, and are likely to involve fatalities.

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It is important to note that the following exhibits do not account for the use of turn-lanes, signalization, or signage. Exhibit 2.1 shows that by replacing a T-Intersection with a single-lane roundabout, all crossing conflict points are eliminated. Since the crossing conflicts are the most severe and include the highest number of injuries and fatalities, this is a significant improvement for vehicular safety. According to the Highway Safety Manual
“By converting from a two-way stop control mechanism to a roundabout, a location can experience an 82 percent reduction in severe (injury/fatal) crashes and a 44 percent reduction in overall crashes.”

Exhibit 2.1 – Vehicle Conflict Points for T-Intersections with Single-Lane Approaches

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Exhibit 2.2 compares the number and types of conflict points between a 4-leg intersection and a 4-leg roundabout. As you can see the traditional 4-leg intersection has thirty two conflict points. Whereas the roundabout has only eight. Once again the crossing conflicts have been eliminated with the implementation of the roundabout. As noted earlier, these exhibits do not take signalization and signage (traffic control) into account. The number of conflicts would be reduced by using these measures, as well as designated turn lanes.
However, it would not completely eliminate them as the roundabout does. It is also important to recognize that the most severe crashes occur when traffic control is not followed and a vehicle runs a light or a stop sign. It has been found that the geometric restrictions of a roundabout is a more effective method of impacting the number of crashes than relying on the drivers’ obedience of traffic lights and signs. According to the Highway Safety
Manual “By converting from a signalized intersection to a roundabout, a location can experience a 78 percent reduction in severe (injury/fatal) crashes and a 48 percent reduction in overall crashes.”

Exhibit 2.2 – Vehicle Conflict Points for Intersections with Single-Lane Approaches

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II.1

Multilane Roundabouts

Multilane roundabouts contain the same conflicts points as single lane roundabouts plus the following three types of conflicts listed below. The corresponding exhibits and descriptions are also from the NCHRP Report
672: Roundabouts: An Informational Guide, Second Edition. These additional conflict points are the result of the increased number of entry lanes and the need to provide wider lanes. Therefore it is important to use the minimum number of lanes for entry, circulatory, and exit for capacity requirements.

1) Drivers do not maintain lane position

Exhibit 2.3

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2) Driver enters next to a vehicle that is perceived to be circulating

Exhibit 2.4
3) Driver Improper Turn Conflicts at Multilane Roundabouts

Exhibit 2.5
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Although the number of conflict points has increased, the crossing conflicts are still eliminated with the use of a multilane roundabout. This option, when appropriate, is still the preferred option when it comes to safety. As with any intersection, the number of lanes directly impacts the number of conflict points. Roundabouts are no exception. As seen below, from the NCHRP Report 672: Roundabouts: An Informational Guide, Second
Edition, the number of crashes increases with the number of lanes.

Exhibit 2.6

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III. Bike/Pedestrian Safety
III.1

Pedestrian Conflicts

Pedestrian safety is a large concern when designing an intersection. Even at low speeds, a collision involving a pedestrian can result in injuries and fatalities. At a traditional intersection, a pedestrian faces various conflict points. These conflicts include:
 All movements coming from the leg of the intersection the pedestrian is crossing (right, left, and through movements)
 Through movements from the opposing side of the intersection
 Right and left turns from the cross street
The frequency and severity of each conflict corresponds to the type of traffic control used at the intersection.
The most common types used are:
 Two-way stop control intersections: conflicts at these intersections include the through traffic on the major leg, vehicles turning right and left from the through onto the minor legs, and vehicles turning left and right from the minor leg onto the major street. In most cases when a vehicle is turning, they are watching oncoming traffic and may not see the pedestrian to their side.
 All-way stop control intersections: most pedestrian/vehicle conflicts at these types of intersections are low. Since all four legs are required to stop, the hazards are usually a result of failure to obey the traffic signs placed at the intersection. When a driver fails to stop at the stop sign and proceeds through the intersection without pause, they might not notice a pedestrian crossing the street.
 Signalized intersections: These types of intersections offer designated timing and protection of pedestrians while crossing by limiting the legal movements of vehicles at any particular time. The following conflicts can be observed at signalized intersections: o Red light running: red light running can be extremely hazardous to pedestrians due to the high speed of the offending vehicle and likely surprise of the person crossing the street. Conflicts can be from left, right, and through movements. o Right turns on green: drivers have the greatest visibility of pedestrians while completing this movement, however they sometimes fail to see the pedestrian. o Left turns on green: Protected turns do not pose as great of a risk as permissive turns to pedestrians. During a permissive turn, a driver is looking for gaps in traffic and my not notice a pedestrian crossing the street. o Right turns on red: These turns also present a hazard due to vehicles looking for gaps in oncoming traffic and not checking for pedestrians. There is also the added hazard of drivers entering the cross walk to increase their line of sight and causing pedestrians to cross in front of the vehicle or behind it.
Pedestrians and vehicles at roundabouts only have two conflict points. These are with vehicles entering and exiting the roundabout. With each additional lane addition, a conflict point is added. Below are exhibits from the NCHRP Report 672: Roundabouts: An Informational Guide, Second Edition, showing a comparison of conflicts points in one crosswalk of a signalized intersections (Exhibit 1.7) and single-lane roundabout (Exhibit
1.8). The safety benefits for pedestrians are obvious as Exhibit 1.7 shows sixteen conflict points for the entire intersection and Exhibit 1.8 shows only eight conflict points for the entire roundabout.
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Exhibit 3.1

TTE 5805: Geometric Design, Safety Benefits of Roundabouts

Exhibit 3.2

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III.2

Bicycle Conflicts

Bicyclists have the same conflict points as vehicles plus the additional conflict points of having to merge with vehicular traffic. Since bicyclists do not have the protection a vehicle offers, any crashes can result in serious injuries or fatalities. Depending on how the bicyclists choose to navigate the intersection, the following conflicts can occur:






Cyclists riding on the right side of the road while going through an intersection have a potential conflict point with vehicles turning right, onto the road. (Known as a right hook)
To avoid the previously described conflict point (the right hook), cyclists may choose to merge into the flow of traffic before entering the intersection. This can result in a merging conflict point.
If a cyclist chooses to make a vehicular style left turn, they must merge with traffic and possibly cross other travel lanes. This results in one or more conflict points.
If a cyclist decides to make a pedestrian style left turn, they face right hook conflicts along the first street they cross and then at the street they cross after turning 90 degrees.
Some cyclists prefer to cross the intersections at pedestrian crosswalks and experience the same potential conflict points as a pedestrian.

When entering a roundabout, bicyclists using the pedestrian crosswalk encounter the same possible vehicle conflicts as a pedestrian. However when sharing the sidewalk with pedestrians, bicyclists and pedestrians may encounter conflicts between each other. Examples are listed below:
 Conflict when bicycle enters the sidewalk or shared-use path with pedestrians
 Possible conflict if the bicyclists rides, instead of walking, across the crosswalk.
 Also, if the bicyclist chooses to ride across the cross walk instead of walking, there is a higher risk of vehicle conflicts due to the speed the bicyclists is traveling. The reaction time for the cyclists and the vehicles is reduced.
 A merging conflict when the bike reenters the roadway is also possible between the vehicle and the bike.
Exhibits 3.3 and 3.4, from the US Department of Transportation, show the conflicts listed above.

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Exhibit 3.3

Exhibit 3.4

Studies comparing the bicycle/vehicular crashes that occur at conventional intersections and roundabouts have mixed results. While crashes appear to be less frequent at roundabouts, the types of crashes at roundabouts result in more serious injuries and a higher amount of fatalities. To improve safety, roundabouts should be designed with the tightest entry curvature, entry width, and radial alignment possible. Also using single-lane roundabouts, instead of multilane, where appropriate will increase the safety. According to the FDOT “Surveys taken from bicyclists indicated that they found roundabout treatment significantly more stressful to negotiate than other forms of treatment, particularly on roads with heavy traffic. Researchers have found that roundabouts affect bicyclists’ choices of routes on regular journeys.” At this time further research and design considerations need to be completed to make roundabouts safer for bicyclists.

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V. Conclusion
At this time, the benefits of roundabouts far outweigh the cons. The FDOT has said that using roundabouts would reduce casualty accident rates by 68%, right-angle accidents by 87%, and overall reported accidents by
47%. Pedestrian related accidents showed a slight reduction. And bicycle related accidents were the majority of accidents occurring in roundabouts. Since single-lane roundabouts have lower speeds, fewer conflict points, and fewer accidents than multilane roundabouts. Recommendations have been made to further increase the safety of pedestrians and bicyclists. These recommendations include the use of special provisions for bicyclists or special bicycle facilities at the outer perimeter of the roundabout in cases of high bicycle volumes. Another solution is to add bicycle and pedestrian crossings to the approach the roundabout so that vehicles can stop before entering the roundabout. It should also be noted that unfamiliarity of roadway users with roundabouts could be a contributing factor to the accidents that occur and may easily be reduced by educating the population.

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VI. References
1.) Florida Intersection Design Guide. Florida Department of Transportation, 2014 http://www.dot.state.fl.us/rddesign/FIDG-Manual/FIDG.pdf 2.) NCHRP Report 672: Roundabouts: An Informational Guide, Second Edition. http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_672.pdf 3.) NCHRP Report 572: Roundabouts in the United States, Transportation Research Board, The National
Academies, 2007 http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_572.pdf 4.) Florida Roundabout Guide. Florida Department of Transportation, 1996 http://www.dot.state.fl.us/trafficoperations/Doc_Library/PDF/roundabout_guide.pdf 5.) Proven Safety Countermeasures, FHWA, 2012 http://safety.fhwa.dot.gov/provencountermeasures/fhwa_sa_12_005.pdf 6.) Bicycle and Pedestrian Considerations at Roundabouts, Florida Department of Transportation, 2000. http://www.dot.state.fl.us/research-center/Completed_Proj/Summary_SF/FDOT_824.pdf 7.) Roundabouts: An Informational Guide, FHA https://www.fhwa.dot.gov/publications/research/safety/00067/000675.pdf 8.) NCHRP Report 674: Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_674.pdf TTE 5805: Geometric Design, Safety Benefits of Roundabouts

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