November/December 2002
Does Your Interchange Design Have You Going Around in Circles?
by Joe G. Bared and Evangelos I. Kaisar
America is facing a national crisis as increased traffic
and the ensuing congestion and delays negatively affect commerce,
the environment, and quality of life. Traffic congestion is such
a problem that engineers and researchers across the country are
making it their personal missions to find innovations that will
enhance traffic flow, ultimately leading to alleviation of congestion.
The roundabout might be one alternative to diamond interchanges.
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Roundabouts, like this one in the United Kingdom, are
common in Europe.
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An informal study of four case problems and several
simulation scenarios examining geometric design and control delay
suggests that well-designed roundabouts may be a viable option for
some stop- or signal-controlled diamond interchanges with low-to-moderate
volumes. The study compares the delay caused by a diamond interchange
with the delays at interchanges containing double- or single-roundabouts.
Comparisons were made using data from computer-simulated models
for roundabout traffic operations and for signalized intersections.
Based on the modeling scenarios, roundabout interchanges
provide noticeable reductions in control delays, which directly
affect the amount of time that drivers sit in traffic. The modern
roundabout also uses a narrower bridge, therefore contributing to
savings in construction costs.
Quick Overview on Selecting Appropriate Interchanges
In A Policy on Geometric Design of Highways and
Streets, the American Association of State Highway and Transportation
Officials (AASHTO) presents six warrants (i.e., selection criteria)
for interchanges and grade separations, including reductions in
bottlenecks, crashes, and traffic volumes. Selecting the most appropriate
type of interchange depends on various factors such as the number
of intersection approaches, expected traffic movements, expected
volumes, design controls, rights-of-way, and topographies. Planners
should perform engineering reviews prior to any construction to
determine the appropriate interchange configuration for a given
situation. Moreover, accommodations for bicyclists and pedestrians
must be considered in order to provide access to all users, including
people with disabilities.
For additional guidance on the design selection process,
engineers can reference AASHTO's policies, Guidelines for Preliminary
Selection of Optimum Interchange Type for Specific Location,
by N.J. Garber and M.D. Fontaine; Single Point Urban Interchange
Design Operations Analysis by C.J. Messer, J.A. Bonneson, S.D.
Anderson, and W.F. McFareland; or Grade Separated Intersections:
Intersection and Interchange Design by J.P. Leisch.
In Guidelines for Preliminary Selection of Optimum
Interchange Type for Specific Location, Garber and Fontaine
recommend using a diamond interchange for low-traffic volumes of
less than 1,500 vehicles per hour (vph) and a single-point urban
interchange for volumes between 1,500 and 5,500 vph. A single-point
urban interchange yields higher delays when the crossroad and left-turn
volumes do not balance. Additionally, Garber and Fontaine contend
that a single-point interchange design is too expensive and intricate
to construct where there are rights-of-way restrictions. Garber
and Fontaine's results also indicate that when compared with diamond
interchanges, single-point urban interchanges yield approximately
5-second delay savings per vehicle for up to a total flow of 4,500
vph. These delay savings do not apply to single-point interchanges
with designs requiring a frontage road, where a diamond interchange
(or a tight diamond interchange) often will be a more favorable
design configuration.
This study compares conventional diamond interchanges
with round-abouts at ramp terminals in terms of delay only.
Introducing Double- and Single-Roundabouts
In both rural and suburban areas, the most predominant
interchange is the diamond type, featuring a relatively simple design
and implementation that accommodates low-to-medium traffic volumes,
with partial access control and limited right-of-way. Although a diamond
interchange is the most common interchange type, it creates unnecessary
delay at signals and stop signs and may cause spillback onto a freeway.
An alternative to the conventional or tight diamond interchange is
a double-roundabout interchange.
Note: This image does not reflect facilites for pedestrians
or bicyclists. For details, refer to the sidebar, "Pedestrians
Also Need to Get Around."
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This illustration shows two roundabouts at the
ends of both ramp terminals in place of a signal- or stop-contolled
intersection onto the cross-street.
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Note: This image does not reflect facilites for pedestrians
or bicyclists. For details, refer to the sidebar, "Pedestrians
Also Need to Get Around."
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This illustration shows two roundabouts at the
ends of both ramp terminals in place of a signal- or stop-contolled
intersection onto the cross-street.
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Note: This image does not reflect facilites for pedestrians
or bicyclists. For details, refer to the sidebar, "Pedestrians
Also Need to Get Around."
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In this drawing, one roundabout is located at
the ends of both ramp terminals in lieu of a signalized, single-point
crossing onto the surface street.
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Some of the first modern double-roundabout interchanges
in the United States were built in the mid-1990s in Colorado and Maryland.
According to the National Cooperative Highway Research Program (NCHRP)
Report No. 264, the new design creates smooth flows with less delay
and eliminates spillback onto the freeway. In practice, preliminary
results show that both Colorado and Maryland experienced notable success
in improving traffic operations and safety. In Colorado, double roundabouts
replaced stop-controlled intersections that were assisted by traffic
officers during peak flow conditions.
The single-roundabout interchange is suitable for tight
urban areas with moderate capacity requirements. A single-roundabout
interchange requires two curved bridges as part of the circulatory
roadway, whether the roundabout is above the mainline or under the
mainline. The number of lanes at entries and exits are comparable
to those in a double-roundabout interchange.
Roundabouts with large inscribed diameters greater than
90 meters (295 feet) are not advisable because they encourage speeding
and diminish the expected safety benefits. A disadvantage of the single-roundabout
interchange is the need to widen the bridges to meet intersection
sight-distance requirements at the off-ramp terminals and the necessity
to comply with stopping sight-distance requirements for circulating
vehicles.
In addition to reducing delay, roundabouts can handle
more than four legs of traffic efficiently when a frontage road is
present. Expected advantages of well-designed double-roundabout interchanges
include crash reductions (approximately 20 to 70 percent fewer fatal
injury crashes) and delay reductions when operating below capacity.
Savings in construction costs for the double roundabout are noticeable
because the bridge size is reduced by at least two left-turn lanes.
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Completed and opened to traffic in November
1998, these two single-land roundabouts move traffic between
MD 103 and MD 100 in Howard County, MD.
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Pedestrians Also Need to Get Around
A message from FHWAs Office of Civil Rights
When reviewing a given intersection to improve
safety or mobility, planners and engineers need to consider
pedestrian, bicyclist, and other user access as part of the
traffic solution. The solution should accommodate non-motorized
traffic, unless non-motorized travel is prohibited. Intersection
configuration evaluations should include how to safely accommodate
these users, especially people with disabilities.
Although roundabouts may be appropriate for decreasing
vehicle-related crashes and increasing vehicle flow at intersections,
the absence of a stop sign or signal might present problems
for
other users crossing streets. For example, motorists exiting
the roundabout often may not yield to those on foot, and busy
roundabouts provide few gaps long enough for pedestrians to
cross safely. A constant stream of traffic might be especially
problematic and unsafe for children, the elderly, and those
with mobility-related, visual, or cognitive impairments. Visually-impaired
pedestrians, who rely on auditory cues from traffic to cross
safely, may find it difficult to interpret the direction of
oncoming traffic and gaps for crossing because of the constant
sound of circulating traffic.
In July 2002, representatives from FHWA and The
Access Board, an independent Federal agency charged with developing
design standards for complying with the Americans with Disabilities
Act (ADA), met to examine options that improve roundabout accessibility.
Roundabouts, as with other public rights-of-way, should accommodate
pedestrians with disabilities. As a minimum,
the group established a goal to educate design engineers more
fully about accommodation issues and share best practices on
accommodating all pedestrians and other non-motorized
users in the design of intersections.
Some design treatments that engineers and planners
might use to facilitate foot traffic in roundabouts include:
- Providing pedestrian yield signs for motorists in both directions
of a crossing requiring drivers to stop for pedestrians waiting
on the crosswalk
- Placing speed tables at pedestrian crossings to further
slow the entering and exiting traffic
- Adding auditory and tactile indicators to identify crossing
locations for pedestrians with visual disabilities (Since
July 26, 2001, detectable warnings/truncated domes are required
on all curb ramp crossings.)
- Incorporating barriers, such as low-growing bushes or guardrails,
along the street side of sidewalks to guide pedestrians to
the crossing location and prevent people with visual disabilities
from inadvertently crossing a roundabout roadway at unsafe
locations
- Installing pedestrian-activated crossing signals, including
devices that halt traffic only when a pedestrian is present
in the crosswalk (This treatment enables pedestrians who are
unable to determine gaps to cross the street.)
- Designing single-lane roundabouts with single entry lanes,
rather than multilane roundabouts, to shorten the crossing
distance and enhance pedestrian visibility at vehicle
entry and exiting points (This eliminates the possibility
of multiple-threat pedestrian crashes.)
- Adding pedestrian-accessible medians and splitter islands
to reduce crossing distances and allow those on foot to negotiate
one direction of traffic at a time
- Installing rumble strips to reduce the speed of vehicles
entering or exiting the roundabout and make the sound of cars
more detectable to visually-impaired pedestrians
- Moving pedestrian crossings slightly away from the inscribed
circle by a convenient distance, such as one or two car lengths,
may eliminate some pedestrian/vehicle conflicts
- Adding landscaping buffers, truncated-domes, etc., as physical
boundaries to help guide pedestrians with visual disabilities
to crossings
With half of all injury-related highway crashes
and one-fifth of all fatality crashes occurring at intersections,
finding solutions to reduce the number and severity of intersection-related
crashes is clearly a vital ingredient in meeting the goal of
improving highway safety. Roundabouts have a role in improving
motorist safety and increasing mobility at intersections.
The risk to pedestrians, especially people who have visual or
mobility impairments, the elderly, or children, must be evaluated
carefully when deciding on intersection configurations. Slower
motorist approaches and exit speeds, and shorter pedestrian
crossing distances, are needed at roundabouts to increase pedestrian
safety. Additional research is warranted for designing
roundabouts that better accommodate pedestrians and people with
disabilities.
The Access Boards proposed guidelines currently
indicate that a traffic signal is needed at roundabouts to accommodate
sight-impaired pedestrians, who may not be able to differentiate
the direction of traffic or traffic gaps when vehicles are moving
in a circular direction. This is one option.
The challenge for design engineers and planners
is to find ways to build in pedestrian accessibility where appropriate,
says Ed Morris, FHWAs director of civil rights. FHWA
and our partners are looking for viable alternatives that will
help pedestrians reach their destinations in a safe and timely
manner.
For more information about this topic, please
contact the FHWA Office of Civil Rights at 202 366
4634 or the Office of Safety at 202 493 3314.
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Analysis Methodology
The typical double-roundabout interchange analyzed
in this study includes a four-lane crossroad intersecting two-lane
off- and on-ramps from the freeway.
The study used the measurement units of effectiveness
recommended in the Transportation Research Board's Highway Capacity
Manual to compare the three types of interchanges for control
delay. Control delay encompasses deceleration, acceleration, move-up,
and stop delays.
The United Kingdom TRL Software Bureau's Assessment of Roundabout
Capacity and Delay (ARCADY) computer program, which aids in roundabout
design, crash predictions, and traffic flow, was used to determine
delay and queuing for roundabouts. ARCADY 4 can model peak periods
and applies to single-island roundabouts with three to seven legs.
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These two single-lane roundabouts are located
at the ramp ends of the interchange between U.S. 301 and MD
291 in Kent County, MD.
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The study model used the Texas Transportation Institute's
PASSER III software to help determine cycle length (ranging from
60 to 120 seconds), optimum phase timing, and time offset between
the two signals for diamond interchanges. PASSER III minimizes intersection
delay only for undersaturated conditions; however, phase timing and
offset are reliable in oversaturated conditions. The diamond interchange
is controlled by two three-phase signals that are coordinated according
to five given sequences.
To estimate stop delay, signal-timing data were fed
into the Federal Highway Administration's (FHWA) traffic microsimulation
model, Corridor Simulation (CORSIM), which provides comprehensive
capabilities such as traffic operational analysis, geometric design/traffic
operational evaluation, and assessment of mitigation strategies under
congested conditions. Control delay was assumed to be 1.3 times stop
delay for the sub-network (at the crossroad) of the signalized diamond
interchange.
Three case problems for two-lane roundabouts and one
case problem for a single-lane roundabout were included in this
study. The scenarios for the two-lane roundabouts were:
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Weekday peak—30 percent left-turning traffic
volume from the cross-street and 60 percent left-turning volume
from the off-ramps
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Weekday off-peak—20 percent left-turning
volume from the cross-street and 40 percent left-turning volume
from the off-ramps
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Weekend—20 percent left-turning volume from
the cross-street and 60 percent left-turning volume from the off-ramps
Proportions of turns were assumed to be constant on
all approaches, and 10 percent of the traffic was assumed to be
trucks.
Comparable geometries were selected for all three
interchange configurations—the diamond, the double roundabout,
and the single roundabout. For the diamond interchange, the crossroad
had four through-lanes: two in each direction with exclusive 76-meter
(249-foot) right-turn and 106-meter (348-foot) left-turn lanes.
The two intersections were offset by 90 meters (295 feet) from stop
bar to stop bar.
For the roundabout, the two approach lanes were flared
from 3.7 meters (12 feet) to 4.5 meters (14.8 feet) per lane. The
off-ramps for the diamond interchange were flared from one lane
to two lanes at the entry to provide a 60-meter (197-foot) right-turn
lane. Similarly, the double-roundabout interchange and single-roundabout
interchange off-ramps were flared to two lanes at the entry from
5 meters (16 feet) to 9 meters (30 feet) total width.
The inscribed circle diameter (ICD) of the double-roundabout
interchange was 55 meters (180 feet), while the ICD for the single-roundabout
interchange was 85 meters (279 feet). This relatively small ICD
for the single roundabout can be achieved only by providing tight
retaining walls along the freeway. Except for the ICD, the approach
and entry geometries are similar; however, the diamond interchange
has extra right- and left-turn lanes on the crossroad.
Comparing the Scenarios
For the weekday off-peak and weekend scenarios, the
savings by the roundabouts in control delay range from a few seconds
to about 30 seconds per vehicle. Savings are slightly higher in
the weekday peak case, when the left-turn percentage of the crossroad
is higher.
The capacity of the single roundabout is slightly
higher than that for the double roundabout because of the larger
ICD. Although the savings in delay between the diamond interchange
and the double-roundabout interchange/single-roundabout interchange
are noticeable, the capacities of the roundabouts are relatively
moderate with total entering flow less than 4,500 vph. Capacities
for these roundabouts are limited because the entries have a maximum
of two lanes without storage lanes for left- and right-turning vehicles.
At this point, the constraint of two-lane roundabouts is recommended
for the safety of American users who are slowly adapting to a new
intersection environment.
In the weekday off-peak scenario, the capacity is
approximately 4,300 vph for the double-roundabout interchange and
4,700 for the single-roundabout interchange. A slightly higher capacity
can be attained by allowing a longer distance between the double-roundabout
circles. The weekend scenario shows the capacity of the double-roundabout
interchange as smaller at 4,000 vph, with the single-roundabout
interchange capacity at 4,600 vph. The capacity of the weekday peak
scenario is also 4,000 vph for the double-roundabout interchange
and 4,300 vph for the single-roundabout interchange.
Some traffic flow imbalances between opposing entrances
were selected within and outside the scenarios. Their impacts were
minimal at lower flows and noticeable at higher volumes. A last
scenario was modeled for single-lane roundabouts only.
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This chart shows an exponential form of control
delays as a function of total entering flow (vph) for a double
(DRI) and a single roundabout interchange (SRI), compared with
a signalized diamond interchange (DI).
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This chart shows a steady increase in annual
savings in control delay for a double roundabout compared with
a signalized diamond interchange as a function of average daily
traffic.
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Estimated Savings in Delay
Traffic volume distributions for weekdays and weekends
were selected from the Maryland State Highway Administration's
Traffic Trends report. By applying the savings per vehicle to
the daily traffic flow distributions for weekdays and weekends,
the authors of the study derived annual savings in vehicle-hours.
The percentage of daily distribution per hour was
multiplied by a selected average daily traffic (ADT) of 20,000 to
50,000 to determine the hourly traffic flow entering the interchange
at the crossroad. The flow rate then was multiplied by the seconds
per vehicle saved at this flow. All 24 hours of a weekday and weekend
day were added separately to determine respective daily savings.
Annual savings were finally added for 107 days of weekends and holidays
and 258 weekdays. When added up over a year, 30,000 total entering
vehicles a day would yield an annual savings in delay of 35,000
vehicle-hours per year. Although savings in delay generally are
expected, this analysis does not consider a complete daily variation
of directional splits, meaning that the results cannot be generalized.
Conclusions from This Roundabout Study
In addition to the expected safety benefits of well-designed
roundabouts, this study showed that traffic operation is more efficient
in roundabouts than at diamond interchanges for low-to-moderate
traffic flows up to total entering volumes of around 4,500 vph.
Roundabouts noticeably reduce control delay in terms of seconds
per vehicle, with savings in delay being slightly higher when the
proportion of left-turning vehicles is greater and the capacity
is smaller.
The annual savings in delay are considerable at higher
average daily traffic levels, which might better justify the economic
benefits of a double-roundabout interchange. In addition, the bridge
surface required for the double-roundabout interchange is approximately
one-third less than for the diamond interchange.
A roundabout interchange does not necessarily require
more right-of-way than a diamond interchange because the left- and
right-turn lanes are not required. However, in a double-roundabout
interchange, the circles might need to be offset by a greater distance
to accommodate higher flows with long queues.
The single-roundabout interchange may be suitable
for urban environments where the right-of-way is restricted. However,
the bridges might have to be widened to provide required intersection
and stopping sight distances. Similarly, the limitation applies
where the total entering volumes should not exceed approximately
4,500 vph.
In conclusion, the study's simulations show that using
double roundabouts at interchange ramp terminals with low and medium
flows will result in noticeably less delay than stop-controlled
and signalized diamond interchanges. Other side benefits include
increases in safety and the ability to use narrower bridges. Similarly,
for single-roundabout interchanges in tight urban settings, the
delay benefits are significant although the savings in bridge structure
are limited because of sight-distance requirements.
References
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A Policy on Geometric Design of Highways and
Streets, American Association of State Highway and Transportation
Officials, Washington, DC, 1994.
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N.J. Garber and M.D. Fontaine. Guidelines for
Preliminary Selection of Optimum Interchange Type for Specific
Location, Virginia Transportation Research Council, Charlottesville,
VA, 1999.
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Highway Capacity Manual, Transportation Research
Board, National Research Council, Washington, DC, 2000.
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C.J. Messer, J.A. Bonneson, S.D. Anderson, and
W.F. McFareland. Single Point Urban Interchange Design Operations
Analysis, National Cooperative Highway Research Program Report
No. 345, Washington, DC, 1991.
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J.P. Leisch. Grade Separated Intersections: Intersection
and Interchange Design, Transportation Research Record (TRR) No.
1385, Washington, DC, 1993.
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Traffic Trends (data obtained from permanent
automatic traffic recorder stations), Maryland Department of Transportation
and State Highway Administration, 1994.
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Georges Jacquemart. Modern Roundabout Practice
in the United States, National Cooperative Highway Research Program
Report No. 264, Washington, DC, 1998.