HomeMy WebLinkAboutFinal Report September 2006 (2 MB)_201401311701070457
ROUNDABOUT IMPLEMENTATION REPORT
Final Draft
MILESTONE ROTARY
ORANGE STREET/ SPARKS AVENUE/ OLD SOUTH ROAD/
MILESTONE ROAD
NANTUCKET, MASSACHUSETTS
PREPARED FOR:
NANTUCKET PLANNING AND ECONOMIC DEVELOPMENT
COMMISSION
September 2006
06914.800
September 11, 2006
Nantucket Planning and Economic Development Commission
16 Broad Street
Nantucket, MA 02554
Attention: Mr. T. Michael Burns, AICP
Transportation Planner
Dear Michael:
Re: Roundabout Implementation Report -- Final Draft
Milestone Rotary
Milestone Road/ Orange Street/ Old South Road/ Sparks Avenue
Our Project No. 06914.800
Ourston Roundabout Engineering, Inc. has been retained by the Nantucket Planning and Economic
Development Commission to prepare a preliminary design for a roundabout to replace the Milestone
Rotary at the intersection of Orange Street, Sparks Avenue, Old South Road, and Milestone Road.
The preliminary design is shown as Figure 3 at the end of the report. It is based on an ellipse, with an
inscribed circle diameter (ICD) varying between 140 and 148 feet. The roundabout is located such
that a pedestrian area can be accommodated between Sparks Avenue and Old South Road.
Sidewalks are located around the rest of the roundabout as well, and bicycle lanes are shown on
Orange Street that terminate at the roundabout. The size and location of the roundabout would impact
the lands between Old South Road and Milestone Road and along the east side of Orange Street.
All entries of the roundabout flare from one to two lanes. The Orange Street entry is configured as a
left plus a through/right turn lane as per the existing rotary. This is duplicated at Old South Road,
which has been widened from a single-lane entry. The Milestone Road entry is configured as a
left/through plus a right turn lane as per the existing rotary. This is duplicated at Sparks Avenue, which
also has been widened from a single-lane entry. This configuration enables the exits to be single-lane,
like the existing rotary, and keeps the ICD small. To educate about correct lane use, the truck apron is
shaped so that motorists entering from Milestone Road or Sparks Avenue and making a left turn will be
“spiralled out” to the outer lane of the circulatory road so they are not trapped in their lane and forced
into making an unsafe lane change inside the roundabout.
The preliminary design addresses the geometric deficiencies with the existing rotary as follows:
There is currently no lateral deflection for motorists entering the rotary from Orange Street,
and little lateral deflection for those entering from Sparks Avenue and Milestone Road. The
Orange Street and Milestone Road entries have been realigned, and a larger central island
introduced to increase deflection for all motorists. The design of the roundabout will all but
eliminate the possibility of a high-speed injury collision. The fastest possible speed through,
in the absence of other traffic and ignoring all signs and pavement markings, will be under 30
mph.
110 Scotia Court, Unit 41 Phone: (905) 686-6402 www.ourston.com
Whitby, ON L1N 8Y7 Canada Fax: (905) 432-7877 mail@ourston.com
The lack of deflection on the Orange Street and Milestone Road entries means that these
motorists tend to enter the rotary at higher speeds, and sometimes in platoons. The
introduction of YIELD control on all entries, and the increased deflection, will slow traffic down
and give all motorists a more equal opportunity to enter the roundabout.
Congestion is being experienced on the single-lane Sparks Avenue and Old South Road
entries to the rotary during peak times in the summer. The roundabout will significantly
lessen delays and queues in the short term compared to the rotary. In the longer term, it will
generally maintain low delays and queues except for a few critical time periods. The worst-
case conditions for motorists in the future are expected to be during the 2014 PM peak hour
on the Sparks Avenue left/through lane, with an average delay of 75 seconds per vehicle and
a 95th percentile queue of about 20 vehicles. These values are relatively high for a
roundabout, and are about the same as conditions currently being experienced by motorists
entering the existing rotary from Old South Road during peak times. This is a function of the
design trade-offs associated with the size and location of the roundabout to minimize property
impacts. All other delays and queues will be lower than these values. Future delays and
queues will be much higher along Sparks Avenue and Old South Road if the existing rotary is
to remain.
Pedestrian and bicycle travel through the rotary is difficult. The roundabout includes
sidewalks and pedestrian crossings on all four legs, rather than just Sparks Avenue and Old
South Road, and bicycle lanes and terminations on Orange Street.
There are numerous access and parking conflicts on the southwest corner, between Sparks
Avenue and Old South Road, including some motorists cutting through the front of the parking
lot. The outer curb of the roundabout provides an opportunity to consolidate access to the
parking lot to two locations and make cutting through less likely. Cut outs for driveway
access have also been provided in the Orange Street and Old South Road splitter islands.
From the conceptual design work and the development of the preliminary design, a safe and efficient
roundabout is possible at this location. The roundabout will be able to accommodate current traffic
volumes and some measure of future growth, have a reduced potential for collisions, and better serve
pedestrians and bicyclists than the existing rotary. We therefore recommend replacing the Milestone
Rotary with a roundabout.
Yours truly,
OURSTON ROUNDABOUT ENGINEERING, INC.
Philip Weber, P.Eng.
Senior Project Manager
/pw
110 Scotia Court, Unit 41 Phone: (905) 686-6402 www.ourston.com
Whitby, ON L1N 8Y7 Canada Fax: (905) 432-7877 mail@ourston.com
TABLE OF CONTENTS
PAGE
1.0 INTRODUCTION .............................................................................................................................................1
1.1 BACKGROUND AND PROBLEM DEFINITION ................................................................................1
1.2 ROTARIES AND ROUNDABOUTS...................................................................................................2
2.0 THE ROUNDABOUT DESIGN........................................................................................................................4
2.1 CONCEPT DEVELOPMENT.............................................................................................................4
2.2 DESCRIPTION OF PRELIMINARY DESIGN....................................................................................4
2.3 CAPACITY ANALYSIS ......................................................................................................................5
2.4 DEFLECTION....................................................................................................................................6
2.5 OTHER DESIGN CHECKS ...............................................................................................................8
3.0 CONSTRUCTION ISSUES............................................................................................................................10
3.1 ILLUMINATION ...............................................................................................................................10
3.2 SIGHTLINES AND LANDSCAPING................................................................................................10
3.3 CONSTRUCTION STAGING ..........................................................................................................10
3.4 PRELIMINARY COST ESTIMATE ..................................................................................................10
4.0 CONCLUSIONS ............................................................................................................................................11
4.1 PROBLEM EVALUATION AND CONCLUSIONS ...........................................................................11
4.2 RECOMMENDATION......................................................................................................................11
FIGURES
FIGURE 1 The Milestone Rotary, Looking West.................................................................................................1
FIGURE 2 Rotary Being Replaced with Roundabout, Kingston, New York ........................................................3
FIGURE 3 Milestone Roundabout – Final Preliminary Design.........................................................End of report
FIGURE 4 Milestone Roundabout – Signing....................................................................................End of report
FIGURE 4 Vehicle Path Radii (from the FHWA Guide).......................................................................................7
FIGURE 5 Example of Too Little Entry Path Deflection......................................................................................8
FIGURE 6 Milestone Roundabout – Critical Deflection Checks.......................................................End of report
FIGURE 7 Milestone Roundabout – Design Vehicle Right Turns ....................................................End of report
FIGURE 8 Milestone Roundabout – Design Vehicle Left Turns.......................................................End of report
FIGURE 9 Example of Entry Path Overlap.........................................................................................................9
FIGURE 10 Milestone Roundabout – Landscaping and Property Impacts ........................................End of report
APPENDICES
APPENDIX A CAPACITY ANALYSIS OUTPUT
This report was prepared with funding from the Massachusetts Highway Department
and the Federal Highway Administration.
1.0 INTRODUCTION
1.1 BACKGROUND AND PROBLEM DEFINITION
Ourston Roundabout Engineering, Inc. has been retained by the Nantucket Planning and Economic
Development Commission to prepare a preliminary design for a roundabout to replace the Milestone Rotary
at the intersection of Orange Street, Sparks Avenue, Old South Road, and Milestone Road.
The rotary serves motorists traveling between the downtown area, Nantucket Memorial Airport, Mid-Island,
Siaconset, and Madaket. It operates as a typical rotary with tangential entries under YIELD control, with the
exception of the Sparks Avenue approach which intersects at 90 degrees under STOP control. The Orange
Street and Milestone Road legs have entries that flare from one to two lanes. The other two legs have
single-lane entries. All exits are single-lane. The inscribed circle diameter (ICD) of the rotary is about 120
feet. Refer to Figure 1.
FIGURE 1
The Milestone Rotary, Looking West
Photo: Pictometry International
The idea of replacing the rotary with a roundabout had its origins in the Traffic Study & Strategy for the Mid-
Island Area dated July 8, 2005. The study identified several geometric deficiencies at the rotary and
recommended that consideration be given to replacing it with a roundabout. Some of the problems with the
rotary include:
There is currently no lateral deflection for motorists entering the rotary from Orange Street, and
little lateral deflection for those entering from Sparks Avenue and Milestone Road. This allows the
possibility of high-speed collisions with circulating traffic. In particular, Orange Street is aligned
straight through the intersection, creating a condition where an entering motorist could crash into a
circulating motorist at high speed at nearly a 90 degree angle. This collision type almost always
results in injuries.
Ourston Roundabout Engineering, Inc. September 2006 Roundabout Implementation Report – Final Draft
06914.800 Milestone Rotary, Nantucket, MA
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The lack of deflection on the Orange Street and Milestone Road entries means that these motorists
tend to enter the rotary at higher speeds, and sometimes in platoons. Under conditions of high
demand they can dominate the rotary, making it difficult for other motorists to enter. This condition
would exist at the Sparks Avenue entry were it not for the STOP control and high circulating flow
from the previous entries.
Congestion is being experienced on the single-lane Sparks Avenue and Old South Road entries to
the rotary during peak times in the summer. The resulting delays and queues are expected to
increase with further development and traffic growth. The 2014 traffic forecasts from the Traffic
Study & Strategy for the Mid-Island Area are for a 3 percent increase in traffic per year over 10
years, or 34 percent in total.
Pedestrian and bicycle travel through the rotary is difficult. Most of the demand is between Orange
Street and the newly-completed bike path along the east side of Old South Road. Because there is
no pedestrian crossing of Milestone Road most pedestrians and bicyclists cross Sparks Avenue
and Old South Road, through the parking lot on the southwest corner. If these users are traveling
from the bike path to the downtown area they must cross Orange Street (where there is no
pedestrian crossing) or ride against traffic on the sidewalk on the west side.
There are numerous access and parking conflicts on the southwest corner, between Sparks
Avenue and Old South Road, including some motorists cutting through the front of the parking lot.
Some consolidation of access would be useful in conjunction with a roundabout.
The average daily traffic (ADT) through the rotary is estimated from the 2004 traffic counts in the Traffic
Study & Strategy for the Mid-Island Area at about 25,000 vehicles per day. This is for the month of July,
which is considered to be the busiest month. The highest hourly volumes are during the PM peak hour at
about 2,700 vehicles per hour.
The Milestone Rotary is experiencing on average 6 collisions per year. Approximately 5 percent involve
injury and 5 percent involve bicyclists. Apparently the collision rate of 0.57 per million vehicles entering is
below district and state averages for the same range of intersection volumes.
1.2 ROTARIES AND ROUNDABOUTS
Rotaries are a common means of traffic control in the northeast, having been installed in Massachusetts,
New York, New Jersey, and several other states starting in the 1940’s. They generally fell out of favor by
the 1960’s because of operational problems, and many were replaced with traffic signals or grade separated
interchanges.
The operational problems with rotaries include:
One or more entries that are tangential to the central island. This means that entering motorists
experience little or no lateral deflection, and thus in the absence of traffic control (or if they ignore
the traffic control), they can enter the rotary at speed. This is evident on three of the four entries to
the Milestone Rotary. Fortunately this highly probable collision type does not seem to be
manifesting itself as a recorded collision problem, but the potential is there.
Ourston Roundabout Engineering, Inc. September 2006 Roundabout Implementation Report – Final Draft
Inconsistent traffic control. Some rotaries operated with YIELD or STOP control for circulating
motorists, rather than entering motorists. This meant that under high traffic flows circulating traffic
could back up and block traffic from entering, eventually locking the entire rotary. This is not the
case with the Milestone Rotary, as all entries are under YIELD (or STOP) control.
06914.800 Milestone Rotary, Nantucket, MA
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Inefficient use of space to achieve capacity. Rotaries generally achieve higher capacities by
increasing size. This allows more traffic to circulate at the same time or be stored in the circle, or
increases weaving distances to allow more time for motorists to change lanes to find their exit.
Unfortunately, making the rotaries larger also led to higher speeds and more frequent and serious
collisions. The Milestone Rotary is too small to exhibit these characteristics, but it could benefit
from a re-design to increase traffic capacity.
Figure 2 shows a large rotary that has now been replaced with a roundabout in Kingston, New York. Note
the tangential entries, similar to the Milestone Rotary. The smaller roundabout is proving to be safer and
have higher capacity than the former rotary.
FIGURE 2
Rotary Being Replaced with Roundabout, Kingston, New York
Photo: New York Department of Transportation
Well-designed roundabouts do not experience the problems of rotaries because entries are always under
YIELD control, because there is deflection on the approaches to slow motorists before they enter, and
because it is safe to accommodate pedestrians and other users due to slow traffic speeds and consistent
conditions for motorists.
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2.0 THE ROUNDABOUT DESIGN
2.1 CONCEPT DEVELOPMENT
We initially developed two roundabout concepts to “explore the design space”. This is where concepts are
developed to explore capacity, safety, and cost trade-offs. Usually the costs involve grading or property
impacts.
The first concept was a roundabout with an inscribed circle diameter (ICD) of 180 feet. This is considerably
larger than the existing rotary in order to develop enough deflection for the Milestone Road to Orange Street
movement using the existing alignment of Milestone Road. The main advantage of this concept was more
vehicular capacity due to the larger size of the roundabout and wider entry possible on Sparks Avenue. The
main disadvantage was that property would be required between Old South Road and Milestone Road, and
at the parking lot between Sparks Avenue and Old South Road.
The second concept had a smaller ICD of 140 feet, somewhat larger than the existing rotary. In this case
deflection was achieved for the Milestone Road to Orange Street movement by re-aligning part of Milestone
Road from about 250 feet back from the edge of the roundabout. The main advantage of this concept was
less property required overall. However, there would be very high peak hour queuing on the Sparks Avenue
approach in the future because only a single-lane entry could be developed. There would also still be
property impacts to the parking lot between Sparks Avenue and Old South Road.
Both concepts developed deflection for the Orange Street entry by re-aligning the approach, which meant
minor property impacts on the east side of Orange Street.
Based on discussions with the Nantucket Planning and Economic Development Commission, it was
determined that the second concept was favored, but with modifications that would eliminate impacts to the
parking lot between Sparks Avenue and Old South Road.
2.2 DESCRIPTION OF PRELIMINARY DESIGN
The preliminary roundabout design is based on an ellipse rather than a circle, with an ICD varying between
140 and 148 feet. This has been done in order to attain better overall deflection (a topic discussed later).
The roundabout has been moved to the east compared with the second concept so that a pedestrian area
can be accommodated between Sparks Avenue and Old South Road. This has again resulted in property
impacts between Old South Road and Milestone Road. Deflection for the Orange Street and Milestone
Road approaches has been achieved similar to the second concept.
The preliminary design is shown as Figure 3 at the end of the report. The approximate limits of construction
from the middle of the intersection are:
250 feet along Orange Street excluding the bike lanes.
120 feet along Sparks Avenue.
230 feet along Old South Road.
400 feet along Milestone Road.
The presence of an outer curb for the entire roundabout provides an opportunity to consolidate access to
the parking lot between Sparks Avenue and Old South Road to two locations and make cutting through less
likely. Cut outs for driveway access have been provided in the Orange Street and Old South Road splitter
islands. These cut outs should be the same material as the splitter islands, but be mountable curb instead
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of barrier curb. The preliminary design also includes the location of sidewalks and pedestrian crossings on
all four legs, and bicycle lanes and terminations on Orange Street.
All entries of the roundabout flare from one to two lanes. The Orange Street entry is configured as a left
plus a through/right turn lane as per the existing rotary. This is duplicated at Old South Road, which has
been widened from a single-lane entry. The Milestone Road entry is configured as a left/through plus a right
turn lane as per the existing rotary. This is duplicated at Sparks Avenue, which also has been widened from
a single-lane entry. This configuration enables the exits to be single-lane, like the existing rotary, and keeps
the ICD small. To educate about correct lane use, the truck apron is shaped so that motorists entering from
Milestone Road or Sparks Avenue and making a left turn will be “spiralled out” to the outer lane of the
circulatory road so they are not trapped in their lane and forced into making an unsafe lane change inside
the roundabout.
Figure 4 shows recommended signing for the roundabout. The design of the map-type diagrammatic signs
is based on sign design guidelines from the UK, and has been used on a number of roundabout projects in
the US.
2.3 CAPACITY ANALYSIS
A capacity analysis of the roundabout was undertaken using the computer program RODEL.1 The program
is based on research from a comprehensive study undertaken in the United Kingdom in the late 1970s of
the entry capacities of roundabouts at 86 public road sites. From the research very robust empirical
formulas for capacity prediction were developed from direct measurement.2 The formulas relate the
capacity of a roundabout entry to the circulating flow past that entry, and the effect of 6 geometric design
parameters: ICD, entry width, road half width, effective flare length, entry radius and entry angle. Following
the initial research the methods were validated to confirm the suitability of the parameters, the most recent
on 35 roundabouts in 1997 which concluded that no changes to the original formulas were necessary. Our
early experience is proving the UK formulas to be valid for US roundabouts and conditions.
The capacity analysis was carried out using the 2004 traffic counts and 2014 traffic forecasts in the Traffic
Study & Strategy for the Mid-Island Area. The entries were checked on an individual lane basis, since
RODEL assumes equal lane utilization rather than the left plus through/right or left/through plus right turn
configurations of the preliminary design. A table summarizing the analysis, and selected RODEL outputs,
are included in Appendix A.
The following average delays and levels of service (LOS) can be expected by 2014 during the summer peak
periods:
On the Orange Street entry, about 45 seconds per vehicle during the PM and Saturday peak
periods in the through/right turn lane, or LOS ‘E’. All other average delays would be less than 15
seconds per vehicle, or LOS ‘A’ to ‘B’.
On the Sparks Avenue entry, about 75 seconds per vehicle during the PM peak period (LOS ‘F’),
and 45 seconds during the Saturday peak period (LOS ‘E’), in the left/through lane. All other
average delays would be less than 20 seconds per vehicle, or LOS ‘A’ to ‘C’.
1 RODEL 1 – Interactive Roundabout Design, Rodel Software Ltd. and Staffordshire County Council, United Kingdom (Licensed to
Ourston Roundabout Engineering, 2000).
2 R.M. Kimber, The Traffic Capacity of Roundabouts, LR942, TRL, 1980.
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On the Old South Road entry, up to 50 seconds per vehicle during the PM peak period in the
through/right turn lane, or LOS ‘E’. All other average delays would be less than 20 seconds per
vehicle, or LOS ‘A’ to ‘C’.
On the Milestone Road entry, about 35 seconds per vehicle during the PM peak period in the
left/through lane, or LOS ‘D’. All other average delays would be less than 15 seconds per vehicle,
or LOS ‘A’ to ‘B’ .
Thus the highest average delay is expected to be during the 2014 PM peak hour on the Sparks Avenue
entry, at about 75 seconds per vehicle. This is relatively high for a roundabout, and is about the same as
delays currently being experienced by motorists entering the existing rotary from Old South Road during
peak times. This high peak hour delay is a function of the design trade-offs associated with the size and
location of the roundabout to minimize property impacts. The first concept would have resulted in more
vehicular capacity but at the cost of greater property impacts, particularly along the east side of Orange
Street.
The highest 95th percentile queue is expected during the 2014 PM peak hour on the Sparks Avenue entry,
at about 20 vehicles. Again this is fairly high, and may affect driveway access along Sparks Avenue during
these occasional time periods. The second-highest 95th percentile queue is expected on the Orange
Avenue entry during the 2014 PM peak hour, at about 15 vehicles.
These are future peak hour values. Average delays and queues will be lower until such time as the forecast
2014 traffic volumes are reached. In fact, little delay and queuing should be evident with a roundabout until
about 2010.
The existing rotary was also modeled with RODEL. However, the results should be used with caution
because the geometric parameters of the rotary are outside the normal operating range of the program. It is
certain that future delays and queues will be much higher along Sparks Avenue and Old South Road if the
existing rotary is to remain. As a worst-case, the analysis shows that during the 2014 PM peak period on
the Sparks Avenue entry to the rotary, average delays will likely be over 8 minutes per vehicle, with 95th
percentile queues of well over 100 vehicles. A table summarizing the analysis is in Appendix A.
It should be noted that during peak times pedestrians and bicyclists in the crosswalks of a roundabout (or
even a rotary in some cases) will experience low delays because they can cross between vehicles queued
at the yield line.
2.4 DEFLECTION
Several checks were conducted through the conceptual design process, and finalized during the preliminary
design. The most important is entry deflection, which is considered a “proxy” for safety. The more
deflection an entry has, generally the more safe it should be. This is because deflection will slow motorists,
making them more likely to yield on entry and less likely to cause entry-circulating collisions. However,
there are limits. An entry that is over-deflected can cause rear-end or loss of control collisions.
Deflection is calculated by determining worst-case or fastest-path speeds for each entry. A fastest-path
speed is the fastest possible speed through a roundabout. It is only possible in the absence of other traffic
and ignoring all signs and pavement markings, and studies have shown less that 30 percent of the driving
population is capable of driving a fastest-path. Thus it is a conservative measure.
Figure 4 illustrates the radii corresponding to various fastest-path checks from the FHWA publication
Roundabouts: An Informational Guide.
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FIGURE 4
Vehicle Path Radii (from the FHWA Guide)
R1 = Entry path radius
R2 = Circulating radius
R3 = Exit path radius
R4 = Left turn radius
R5 = Right turn radius
Source: Roundabouts: An Informational Guide
A fastest-path check is carried out by drawing a curve that represents the straightest possible path through
a roundabout. The path should be offset 5 feet from the face of curb or 3 feet from a painted centerline, and
is best represented by a smooth spline from entry to exit. For example the R1, R2, R3 spline comes within 5
feet of the curb at the entry, 5 feet of the central island, and 5 feet of the curb at the exit. The minimum radii
of the arcs along this spline correspond to the R1, R2, and R3 radii. Of these, the R1 entry path radius is
usually the most critical. On skewed intersections sometimes it is the R5 or right turn radius.
A balanced design minimizes the differences between the various R1 values. The R3 exit path radius is not
a critical check unless the value is overly small, as a motorist’s ability to accelerate from the R2 position to
the exit area rarely exceeds the calculated exit speed owing to the R3 path. R4 is governed by the size of
the central island and generally operates at the lowest speed.
Contrary to expectation, a tight entry radius and straight alignment of the entry into the central island does
not always equate to a low entry path radius, as evident in Figure 5. Entry radius comes from the geometric
design, while entry path radius comes out of the composition of the design (arrangement of entry alignment
and circle) and is a result of the fastest-path through a roundabout.
For the preliminary design, we measured the following R1 values, plus critical R5 or right turn radius from
Milestone Road to Orange Street:
Orange Street R1 value of 214 feet, or 27 mph.
Sparks Avenue R1 value of 158 feet, or 24 mph.
Old South Road R1 value of 210 feet, or 27 mph.
Milestone Road R5 (right turn) value of 239 feet or 28 mph.
Thus the fastest possible speed through the roundabout, in the absence of other traffic and ignoring all signs
and pavement markings, is under 30 mph. Figure 6 at the end of the report shows the deflection checks for
these movements.
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FIGURE 5
Example of Too Little Entry Path Deflection
Photo: Ourston Roundabout Engineering
2.5 OTHER DESIGN CHECKS
The roundabout can accommodate a WB-50 design vehicle. Judging by the size of the current rotary this is
the largest truck able to use the intersection. A truck apron is provided for vehicle over-tracking. It should
be noted that trucks will need to use both lanes of the entry when:
Entering the roundabout from Sparks Avenue.
Turning right from Old South Road to Milestone Road.
This is similar to a truck swinging wide to make a right turn at a conventional intersection, and is common at
many roundabouts. Figures 7 and 8 at the end of the report show right turns and left turns for the WB-50
design vehicle through the preliminary design.
Another important design check, applicable to multi-lane roundabouts, is that of avoiding path overlap on the
entries or exits. The natural path of vehicles can be determined by assuming motorists stay within their lane
up to the yield line. At the yield line, they will maintain their natural trajectory into the circulatory road and
through to the exit. If the roundabout geometry tends to lead vehicles into the wrong lane, this can result in
path overlap. Figure 9 shows an example of entry path overlap. Path overlap can occur at roundabout exits
as well.
The preliminary design was checked to ensure a low probability of entry path overlap, particularly on the
Orange Street and Old South Road entries where two vehicles can enter the roundabout and circulate side
by side.
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FIGURE 9
Example of Entry Path Overlap
Source: Wisconsin Department of Transportation Facilities Development Manual
A final design check was to ensure that any pedestrian refuge areas on the splitter islands are at least 8 feet
wide, to accommodate a bicyclist or person pushing a stroller. The splitter island on the Sparks Avenue leg
is too small to accommodate pedestrians, and so pedestrians must cross to the outside of the splitter island
as they do with the existing rotary.
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06914.800 Milestone Rotary, Nantucket, MA
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3.0 CONSTRUCTION ISSUES
3.1 ILLUMINATION
With reconstruction of the rotary the opportunity should be taken to improve the illumination of the
intersection. There are currently two luminaires on the central island of the rotary. Generally the preferred
practice at roundabouts is to illuminate them from the perimeter in, rather than from the central island out,
and place luminaires to light conflict areas, pedestrian crosswalks and important signs from the front. The
luminaires should depart from a straight line along the edge of the road at the roundabout to identify it to
motorists from a distance at night.
3.2 SIGHTLINES AND LANDSCAPING
Minimum stopping sight distance of the yield line of a roundabout is required for approaching motorists. As
with other intersections, minimum sightlines should also be preserved between entering motorists and
pedestrians about to step into the crosswalks on the upstream or downstream legs of the roundabout.
Landscaping within these sightlines should be limited in height to under 3 feet.
Landscaping should be limited in height in the central island to preserve sightlines for circulating motorists.
However, it is good practice to have an area of the central island higher than 3 feet to block through
sightlines and provide a terminal vista to approaching motorists. The size of this area is dependent on
sightline requirements for circulating motorists. Figure 10 at the end of this report shows the area in the
central island of the preliminary design in which landscaping of any height may be implemented.
3.3 CONSTRUCTION STAGING
Ideally when a roundabout is constructed at an existing location the entire intersection is closed to traffic. It
will likely not be possible in this case to close the entire rotary for the reconstruction process. Therefore the
best course of action may be to close the Sparks Avenue leg of the intersection for the duration of the work,
as traffic could be redirected to Orange Street, and build the roundabout in two stages. A suggestion is to
build the east side of the roundabout first, using the two lanes southbound on Orange Street and the parking
lot on the corner between Sparks Avenue and Old South Road as a temporary widening to detour traffic.
Once the east side is completed, traffic could be switched over in order to build the west side of the
roundabout.
3.4 PRELIMINARY COST ESTIMATE
The reconstruction of the Milestone Rotary to a roundabout is expected to cost approximately $475,000.
This includes road base, asphalt, curb work, and re-alignment of the Orange Street and Milestone Road
approaches within the limits of construction, signs and markings, utility pole relocations, illumination,
drainage, landscaping of the central island, traffic control, engineering and contingencies. The estimate
does not include the acquisition of property.
Frontages along which property impacts can be anticipated are also shown in Figure 10 at the end of this
report. There will be impacts between Old South Road and Milestone Road, and on the east side of Orange
Street. However, there may be the opportunity to sell some excess land on the west side of Orange Street
adjacent to the roundabout entry.
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4.0 CONCLUSIONS
4.1 PROBLEM EVALUATION AND CONCLUSIONS
The preliminary roundabout design should address the geometric deficiencies with the existing rotary as
identified in the Traffic Study & Strategy for the Mid-Island Area. The details are as follows:
There is currently no lateral deflection for motorists entering the rotary from Orange Street, and
little lateral deflection for those entering from Sparks Avenue and Milestone Road. The Orange
Street and Milestone Road entries have been realigned, and a larger central island introduced to
increase deflection for all motorists. The design of the roundabout will all but eliminate the
possibility of a high-speed injury collision. The fastest possible speed through, in the absence of
other traffic and ignoring all signs and pavement markings, will be under 30 mph.
The lack of deflection on the Orange Street and Milestone Road entries means that these motorists
tend to enter the rotary at higher speeds, and sometimes in platoons. The introduction of YIELD
control on all entries, and the increased deflection, will slow traffic down and give all motorists a
more equal opportunity to enter the roundabout.
Congestion is being experienced on the single-lane Sparks Avenue and Old South Road entries to
the rotary during peak times in the summer. The roundabout will significantly lessen delays and
queues in the short term compared to the rotary. In the longer term, it will generally maintain low
delays and queues except for a few critical time periods. The worst-case conditions for motorists in
the future are expected to be during the 2014 PM peak hour on the Sparks Avenue left/through
lane, with an average delay of 75 seconds per vehicle and a 95th percentile queue of about 20
vehicles. These values are relatively high for a roundabout, and are about the same as conditions
currently being experienced by motorists entering the existing rotary from Old South Road during
peak times. This is a function of the design trade-offs associated with the size and location of the
roundabout to minimize property impacts. All other delays and queues will be lower than these
values. Future delays and queues will be much higher along Sparks Avenue and Old South Road
if the existing rotary is to remain.
Pedestrian and bicycle travel through the rotary is difficult. The roundabout includes sidewalks and
pedestrian crossings on all four legs, rather than just Sparks Avenue and Old South Road, and
bicycle lanes and terminations on Orange Street.
There are numerous access and parking conflicts on the southwest corner, between Sparks
Avenue and Old South Road, including some motorists cutting through the front of the parking lot.
The outer curb of the roundabout provides an opportunity to consolidate access to the parking lot
to two locations and make cutting through less likely. Cut outs for driveway access have also been
provided in the Orange Street and Old South Road splitter islands.
4.2 RECOMMENDATION
From the conceptual design work and the development of the preliminary design, a safe and efficient
roundabout is possible at this location. The roundabout will be able to accommodate current traffic volumes
and some measure of future growth, have a reduced potential for collisions, and better serve pedestrians
and bicyclists than the existing rotary. We therefore recommend replacing the Milestone Rotary with a
roundabout.
Ourston Roundabout Engineering, Inc. September 2006 Roundabout Implementation Report – Final Draft
06914.800 Milestone Rotary, Nantucket, MA
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SurfsideMadaket
APPENDIX A
CAPACITY ANALYSIS OUTPUT
Appendix A
Milestone Roundabout, Nantucket, MA
RODEL 1 Capacity Analysis
September 11, 2006
Page 1 of 6
TABLE A
Summary of Existing Rotary Operational Performance
2004 Traffic Counts 2014 Traffic Forecasts
Entry Lane Avg. Delay
(s/vehicle) LOS
Max.
Queue
(vehicles)
Avg. Delay
(s/vehicle) LOS
Max.
Queue
(vehicles)
AM Peak Hour
Orange St Right/Thru
Orange St Left
11
8
B
A
1
1
23
12
C
B
4
1
Sparks Ave 19 C 4 214 F+ 73
Old South Rd 20 C 4 205 F+ 74
Milestone Right
Milestone Rd Thru/Left
11
11
B
B
1
1
22
23
C
C
4
4
PM Peak Hour
Orange St Right/Thru
Orange St Left
31
13
D
B
7
2
323
40
F+
E
111
8
Sparks Ave 55 F 14 514 F+ 178
Old South Rd 38 E 10 316 F+ 123
Milestone Right
Milestone Rd Thru/Left
13
19
B
C
2
4
28
111
D
F+
6
38
Saturday Peak Hour
Orange St Right/Thru
Orange St Left
35
11
D
B
5
1
556
20
F+
C
172
2
Sparks Ave 35 D 5 699 F+ 201
Old South Rd 27 D 4 381 F+ 119
Milestone Right
Milestone Rd Thru/Left
13
13
B
B
1
1
24
26
C
D
3
3
Notes: The geometric parameters of the existing rotary are outside the normal operating range of RODEL.
Results should be used with caution. The rotary was modeled using a capacity factor of 0.80 on
Sparks Avenue and Old South Road.
LOS is based on All-Way STOP criteria as per the Highway Capacity Manual.
Appendix A
Milestone Roundabout, Nantucket, MA
RODEL 1 Capacity Analysis
September 11, 2006
Page 2 of 6
TABLE B
Summary of Proposed Roundabout Operational Performance
2004 Traffic Counts 2014 Traffic Forecasts
Entry Lane Avg. Delay
(s/vehicle) LOS
Max.
Queue
(vehicles)
Avg. Delay
(s/vehicle) LOS
Max.
Queue
(vehicles)
AM Peak Hour
Orange St Right/Thru
Orange St Left
7
5
A
A
1
0
10
7
A
A
1
1
Sparks Ave Right
Sparks Ave Thru Left
7
9
A
A
0
1
8
17
A
C
1
3
Old South Rd Right/Thru
Old South Left
9
7
A
A
1
0
16
8
C
A
3
1
Milestone Right
Milestone Rd Thru/Left
8
8
A
A
1
1
13
13
B
B
2
2
PM Peak Hour
Orange St Right/Thru
Orange St Left
11
7
B
A
2
1
47
13
E
B
16
2
Sparks Ave Right
Sparks Ave Thru Left
8
13
A
B
0
2
11
76
B
F
1
19
Old South Rd Right/Thru
Old South Left
12
7
B
A
2
0
8
48
A
E
1
13
Milestone Right
Milestone Rd Thru/Left
8
10
A
A
1
2
16
35
C
D
3
11
Saturday Peak Hour
Orange St Right/Thru
Orange St Left
12
7
B
A
2
1
45
10
E
A
9
1
Sparks Ave Right
Sparks Ave Thru Left
8
13
A
B
0
1
10
43
A
E
1
6
Old South Rd Right/Thru
Old South Left
10
7
A
A
1
0
22
10
C
A
3
1
Milestone Right
Milestone Rd Thru/Left
8
8
A
A
1
1
14
14
B
B
2
2
Notes: RODEL output is provided for the shaded conditions only.
LOS is based on All-Way STOP criteria as per the Highway Capacity Manual.
Appendix A
Milestone Roundabout, Nantucket, MA
RODEL 1 Capacity Analysis
September 11, 2006
Page 3 of 6
Orange St/Sparks Ave/Old South Rd/Milestone Rd
RODEL 1 Output, 50th Percentile Confidence Level
2004 Traffic Count, PM Peak Hour
Sparks Avenue, Right Lane
Sparks Avenue, Through/Left Lane
Appendix A
Milestone Roundabout, Nantucket, MA
RODEL 1 Capacity Analysis
September 11, 2006
Page 4 of 6
Orange St/Sparks Ave/Old South Rd/Milestone Rd
RODEL 1 Output, 50th Percentile Confidence Level
2004 Traffic Count, PM Peak Hour
Old South Road, Right/Through Lane
Old South Road, Left Lane
Appendix A
Milestone Roundabout, Nantucket, MA
RODEL 1 Capacity Analysis
September 11, 2006
Page 5 of 6
Orange St/Sparks Ave/Old South Rd/Milestone Rd
RODEL 1 Output, 50th Percentile Confidence Level
2014 Traffic Forecasts, PM Peak Hour
Sparks Avenue, Right Lane
Sparks Avenue, Through/Left Lane
Appendix A
Milestone Roundabout, Nantucket, MA
RODEL 1 Capacity Analysis
September 11, 2006
Page 6 of 6
Orange St/Sparks Ave/Old South Rd/Milestone Rd
RODEL 1 Output, 50th Percentile Confidence Level
2014 Traffic Forecasts, PM Peak Hour
Old South Road, Right/Through Lane
Old South Road, Left Lane