HomeMy WebLinkAbout18. Receive & File Report on Implementation of Roundabouts within the CityI I CT"1111111 0
TO: HONORABLE MAYOR AND MEMBERS OF THE CITY COUNCIL APPROVED BY
CITY %NIAGER
FROM: OFFICE OF THE CITY MANAGER A BY:DELFINO R. CONSUNJI, P.E., DIRECTOR OF PUBLIC WORKS/CITY ENGIN
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That the City Council receive and file this report.
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As requested by the City Council, this report has been prepared to summarize staff's
investigation on the potential use of roundabouts within the City. Various aspects of the use of
roundabouts are presented, including background information, appropriate usage, design
features, advantages and disadvantages, estimated costs and recommended location within the
City. There are generally two types of roundabouts namely, conventional and
roundabouts. Mini -roundabouts are smaller than conventional roundabouts and are intended to
fit within the intersections of two small two-lane streets, such as the intersection of two
residential streets with dimensions similar to those within Downey. Therefore, this report
focuses on mini -roundabouts. Figure 1 includes photos of mini -roundabouts.
A roundabout is a type of intersection which features a small diameter center island and
.. splitter" islands at the entrances requiring all traffic entering the roundabout to yield the right-of-
way to traffic circulating within the roundabout, which circulates in a counter -clockwise direction.
By forcing motorists to traverse the roundabout in a counter -clockwise pattern, traffic safety at
the intersection is enhanced and delays are reduced. Mini -roundabouts were invented in the
United Kingdom and presently, are widely used in Europe and are an emerging form of
intersection control in the United States. Mini -roundabouts are designed to fit within smaller
intersections within existing right-of-way and feature smaller footprints than larger conventional
roundabouts at significantly reduced costs. Roundabouts can be used at existing intersections
to replace two-way STOP controls, all -way STOP controls or traffic signals and can improve the
operation of intersections by reducing the dominance of traffic flow from one direction over
others, facilitating access and reducing delay to minor street movements and improving overall
intersection capacity and safety.
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It is recommended that roundabouts be used at intersections along streets with a posted speed
limit of 30 miles per hour (MPH) or less and with a total entering daily traffic volume of 15,000
vehicles per day or less. Intersections with light traffic volumes on minor streets may not be
suitable locations for roundabouts. Motorists on major streets may become conditioned over
time to ignore the intersection controls due to lack of presence of vehicles on minor streets,
which require motorists traveling on major streets to proceed more cautiously through the
intersection. In fact, at least 10 percent of the total intersection volume, or a minimum of 500
vehicles per day, should be traveling on the minor street in order for a roundabout to be deemed
an effective form of control for a given intersection. Intersections which have existing STOP
controls on some or all approaches and meet traffic volume warrants for signalization as well as
intersections with heavy left -turns are good candidates for roundabouts.
Design Features
Roundabouts incorporate the following design principles:
• Provide slow entry and consistent speeds through the roundabout (15 MPH to 25 MPH)
• Provide smooth channelization that is intuitive to motorists
• Provide adequate accommodation for design vehicles
• Design to meet the needs of pedestrians and bicyclists
• Provide appropriate sight distance and visibility
Traffic within a roundabout is forced to circulate in a counter -clockwise pattern by the placement
of splitter islands at the entrances to the roundabout which are shaped in a manner such that
they create a deflection, compelling motorists to veer to the right as they enter the roundabout
and discourage left -turning movements at entrances. Splitter islands can be either
raised/mountable or painted. The raised/mountable type is preferred since it is more effective in
forcing motorists to divert to the right as they enter the roundabout and still will be able to
accommodate large vehicles such as trucks which have too large of a wheel path to completely
navigate the roundabout. The painted type is used if there is not sufficient area to install
raised/mountable splitter islands at entrances.
Smooth channelization and accommodation of design vehicles (i.e., passenger vehicles) for
roundabouts are best achieved through the design of splitter and central islands. Similar to
splitter islands, central islands may be either raised or painted. The raised central island is
more effective since it serves as a better deterrent to errant motorists by forcing a counter-
clockwise circulation pattern within the roundabout. If used, the raised central island should be
designed at a maximum height of 5 inches, constructed with concrete or asphalt concrete and
should be mountable in order to allow large vehicles to traverse the roundabout by constructing
the outside perimeter of the island with a 1:4 slope or with rolled curb. The diameter of the
central island may be constructed to fit within any intersection; however, the diameter should be
maximized to the extent practical such that it will fit within the intersection and still be able to
provide wide enough interior travel lanes (11 feet minimum) so that it can accommodate the
turning path of the largest possible vehicle. The central island should be clearly marked,
vertically located at the highest point of the intersection and as conspicuous as possible such
that it is visible to motorists traversing the roundabout.
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(11 feet minimum) so that it can accommodate the turning path of the largest possible vehicle.
The central island should be clearly marked, vertically located at the highest point of the
intersection and as conspicuous as possible such that it is visible to motorists traversing the
roundabout. The interior travel lane should also be sloped such that water drains away from the
central island.
Intersections with existing cross -gutters may require removal or realignment of cross -gutters as
part of the roundabout conversion process given that they may result in an uncomfortable ride
within the roundabout, particularly for bicyclists as they may be forced to ride within cross -
gutters. The location of YIELD lines placed at the entrances to the roundabout is important to
the geometric design and operation of the roundabout. YIELD lines should not be set too far
back to make it easy for motorists to turn left in front of the central island. Advancing YIELD
lines forward a minimum of two feet behind the outer turning path of vehicles and using skewed
approaches will minimize the likelihood of motorists turning left in front of the central island in
lieu of properly traversing the roundabout.
Pedestrian Accessibility
If the intersection to be converted to a roundabout has existing curb access ramps, the existing
ramps would require removal and new Americans with Disabilities Act (ADA) compliant curb
access ramps with detectable warning and crosswalks would need to be constructed 20 feet in
advance of entrances to the roundabout in order to provide sufficient queuing space for one
vehicle. If splitter islands at the entrances are raised, cut-outs would be provided in -line with the
marked crosswalks. In addition, splitter islands that are at least six feet wide should provide
two -foot wide pedestrian refuge areas with detectable warning provided on both sides.
Bicyclist Accessibility
Bicycles should be ridden through a roundabout similar to the manner in which vehicles are
driven through a roundabout given that bicycles are ridden through a roundabout at speeds
similar to those vehicles. If the roundabout is located along a street with Class II bike lanes, the
bike lanes should be terminated in advance of the roundabout through signage and pavement
markings.
Si nacre and Pavement Markings
Pavement markings for a roundabout typically include YIELD lines, marked crosswalks, and
splitter and central islands (if painted islands are used). If painted splitter islands are used, the
interior of the splitter islands can either be painted with hatch -style markings or rumble strips
can be installed in order to discourage motorists from driving over them. If a painted central
island is used, the interior of the central island may be painted yellow or a color differentiating
the island from the circulating travel lane with a yellow stripe along the perimeter of the island in
order to discourage motorists from traveling across the island.
If the interior of the central island is painted, use of a skid -resistant material is recommended to
help prevent motorcycles and bicycles from slipping on the painted surface, which can become
slippery when the pavement is wet. If there is an occurrence of motorists not complying with the
required counter -clockwise travel pattern through the roundabout, arrow pavement markings
can be added to the interior of the roundabout in advance of the entrances to reinforce the
correct directional pattern. Signage requirements include the use of "YIELD" and roundabout
symbol signs posted at the entrances to the roundabout, pedestrian crossing warning signs at
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IMPLEMENTATION OF ROUNDABOUTS WITHIN THE CITY
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marked crosswalks at each roundabout approach (facing both entering and exiting traffic), and
optional additional roundabout symbol signs posted on each approach in advance of the
roundabout. Street name signs are posted at the roundabout on the same posts as the "YIELD"
and roundabout symbol signs. Since the central island is typically designed to be mountable,
signs are not usually placed on the central island of a mini -roundabout. However, in cases
where the central island is the raised type and designed to be partially mountable, "ONE WAY'
regulatory signs can be installed on the central island facing traffic entering the roundabout at
the approaches if there is a frequent occurrence of motorists violating the required counter-
clockwise circulation pattern through the roundabout.
Lighting- Imrovements
Roundabouts may require additional lighting compared to the intersection prior to roundabout
conversion in order to provide sufficient visibility of splitter and central islands and crosswalks at
night. Landscaping treatments are not typically included at mini -roundabouts due to the
relatively small surface area of mountable splitter and central islands. Landscaping may be
placed along the outer perimeter of the roundabout; however, care needs to be used to
minimize roadside hazards as well as to make sure sufficient stopping and intersection sight
distances are maintained throughout the roundabout.
Other Factors
Driveways located immediately adjacent to intersections, once converted to roundabouts, may
be affected in terms of ingress or egress, particularly if located in the same or nearby location of
the relocated ADA curb access ramps, crosswalks and proposed splitter islands. Existing bus
stops may need to be relocated farther down the street if located immediately adjacent to the
roundabout on the approach or departure side.
Figure 2 illustrates the typical design features of a roundabout.
Advantages and Disadvantages
Roundabouts have certain advantages and disadvantages over other types of intersection traffic
control which are summarized as follows:
Advantages
• Operational efficiency — if used properly, roundabouts generally result in less delay for all
movements at the intersection compared to other intersection alternatives
• Traffic safety — roundabouts have been shown to result in a reduction in the number of
crashes at an intersection up to 30 percent. The potential for the reduction in right-angle
collisions at roundabouts yields the highest traffic safety benefit
• Traffic calming — when deployed properly, roundabouts (particularly mini -roundabouts)
may reduce traffic speeds as part of an overall traffic calming scheme
• Environmental benefits — given the lower delays and number of stops experienced by
motorists at an intersection with a roundabout vs. other types of intersection control,
vehicle emissions, fuel consumption and noise are significantly reduced
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IMPLEMENTATION OF ROUNDABOUTS WITHIN THE CITY
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Figure 2 — Typical layout and design features of a roundabout
Disadvantages
• Low side street traffic volumes - similar to intersections controlled through other means
(i.e., signalized, STOP -controlled), intersections with low traffic volumes on side (minor)
streets compared to traffic volumes on main street approaches are not good candidates
for roundabout conversion due to the tendency for motorists traveling on main streets to
ignore the YIELD controls because of lack of circulating traffic
• Aesthetics — mini -roundabouts may not offer the same opportunities as larger
roundabouts for aesthetic features (i.e., landscaping treatments) and typically require
more signs than intersections controlled through different means (i.e., STOP -controlled).
• Priority — intersections under roundabout control cannot give priority to one direction of
travel or type of vehicle over another (i.e., major street along a corridor, emergency
vehicles, transit vehicles, etc.)
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IMPLEMENTATION OF ROUNDABOUTS WITHIN THE CITY
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Estimated Costs
Construction costs for roundabouts vary between $50,000 for mini -roundabouts and $250,000
for larger or conventional roundabouts. Mini -roundabouts can be installed at intersections with
little or no pavement widening, with only minor changes to the curbs and sidewalks, and
installation of pavement markings and signage.
In terms of long-term maintenance costs, roundabouts (particularly mini -roundabouts) generally
have about the same maintenance costs as the intersections which they replaced depending on
traffic conditions and other factors, and may have lower maintenance costs compared to traffic
signals. The benefits that may be achieved through roundabouts vs. other types of intersection
controls may reduce the overall cost of roundabouts over their life cycles.
Potential Roundabout Location
The Brookshire Avenue at Cherokee Drive intersection is a recommended candidate location for
roundabout conversion. This intersection is currently STOP -controlled and involves the
intersection of a minor arterial (Brookshire Avenue) with a local street (Cherokee Drive). The
Brookshire Avenue approaches at this intersection experience significant queuing and delay
during peak periods, particularly when school is in session and this intersection is reaching a
point where a higher level of intersection control (i.e., signalization) will be necessary in order to
improve its operation and reduce the extensive queuing and delay. In lieu of signalizing the
intersection, further analysis to confirm the feasibility of the conversion of this intersection to a
mini -roundabout is recommended. In addition to enhancing the operation of the intersection, a
mini -roundabout at this location may provide a traffic calming benefit as well.
• i
Efficiency & Adaptability
Quality of Life, Infrastructure & Parks
A total of $450,000 in Measure R Local Return is proposed for the FY 2020-2021 Capital
Improvement Program in account no. 56-16831 for the implementation of roundabouts within
the City.
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