HomeMy WebLinkAboutFour Corners Intersection Evaluation (1.2 megs)_201401311323280403T:\10508.00\docs\memos\10508TechMemo.doc
Transportation
Land Development
Environmental
S e r v i c e s
101 Walnut Street
P. O. Box 9151
Watertown, MA 02471-9151
617 924 1770
FAX 617 924 2286
Memorandum To: Andrew Vorce, AICP, Director
Michael Burns, Project Manager
Nantucket Planning & Economic
Development Commission
2 Fairgrounds Road
Nantucket, MA 02554
Date: June 23, 2008
Project No.: 10508.00
From: Joseph Magni, P.E. - Project Manager
Matthew Hayes, P.E. – Project Engineer
Erin Thompson, E.I.T. – Traffic Engineer
Re:Four Corners Intersection Evaluation
Nantucket, Massachusetts
INTRODUCTION
The Town of Nantucket, through its Board of Selectman, has retained Vanasse Hangen Brustlin, Inc
(VHB) to evaluate the intersection of Prospect Street, Atlantic Avenue, Sparks Avenue and Surfside
Road, known locally as Four Corners, to identify existing deficiencies, develop and analyze a variety
of alternatives, and detail our recommendations. The Nantucket Planning & Economic Development
Commission (NP&EDC) is programming state and federal funds through the Transportation
Improvements Program for Fiscal Year 2009 to implement the preferred design alternative chosen
from the concepts presented herein.
The intersection was most recently studied as part of the Mid-Island Traffic Study1 performed by
Greenman-Pedersen, Inc (GPI) in 2005 for the NP&EDC. For the purposes of this technical
memorandum, data collection efforts performed as part of this the study has been utilized.
EXISTING CONDITIONS
Surfside Road from the south and Atlantic Avenue from the north intersect in the mid-island area.
Prospect Street intersects this roadway from the west and Sparks Avenue forms the easterly leg of
the intersection. Prospect Street and Sparks Avenue are offset by approximately 165 feet. All
1 Traffic Study & Strategy for the Mid-Island Area, Greenman-Pedersen, Inc, July 8, 2005
approaches consist of a single multi-purpose
lane and are under STOP-control. The
Nantucket High School is located on the
southeast quadrant of the intersection. The
telephone control cabinets (Inset 1) is also
located on this corner and approximately 115’
south of Sparks Avenue is a large Elm tree
(Inset 2) that must be retained and protected
from an historical perspective. The high school
driveway is located just south of the Elm tree.
There is currently signage identifying the area
as a 20mph school zone. Inset 1 – Telephone Control Cabinets
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Inset 2 – Elm Tree, looking north on Surfside Road
The main entrance to the Nantucket Cottage Hospital is located approximately 300 feet west of the
intersection along Prospect Street. The southwest and northeast quadrants of the intersection are
under ownership of the hospital. A private residential home is located on the northwest quadrant of
the intersection with driveway access on Atlantic Avenue. There are several utility poles close to the
edge of road, narrowing sidewalks at various locations (Inset 3).
As previously reported, the intersection
experienced 13 crashes from 2000 through
2003. These crashes included a pedestrian and
bicyclist injury but generally consisted of rear-
end and angle type collisions. More recent
data is now available from the Massachusetts
Highway Department (MassHighway) and has
been reviewed. There were 3 reported crashes
during the 2004-2006 time period. Two of these
crashes were rear-end type and the other was
an angle crash. It is worth noting that there
have been no known fatalities reported at this
location. Rear-end and angular collisions can
be expected at this intersection due to the
offset nature of Prospect Street to Sparks
Avenue, sight distance constraints, and
minimal roadway widths.
Inset 3 – Utility pole on southwest corner
DESIGN ALTERNATIVES
As early as 1982, Bruce Campbell Associates proposed the realignment of Prospect Street to
eliminate the off-set intersection configuration. At the time abutters and the community rejected the
notion of relocating residents for transportation improvements. With much of the abutting land now
under the ownership of the local hospital and school district, the Board of Selectman is anticipating
that modifications to the intersection will be embraced. Three design alternatives were considered
and are described below. Reference is made to concept plans enclosed with this memo. The first two
alternatives are roundabouts. A description of the various components and design features of a
roundabout can be found on the following page.
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From Massachusetts Highway Design Guide, Exhibit 6-27
x Alternative 1 (120’ diameter roundabout) – The circle is large enough to accommodate a
WB-50 design vehicle while maintaining a large center island for landscaping with a 7’ truck
apron. (See Figure 1) All approaches have splitter islands capable of providing sufficient
pedestrian refuge however crosswalks have only been proposed on two approaches: Sparks
Avenue and Surfside Road. In order to have crosswalks on all approaches, a sidewalk
easement would be necessary on the northwest corner or the splitter island on Atlantic
Avenue would need to become mountable for larger vehicles. This alternative shifts traffic
further from the Elm tree and maintains the current telephone boxes. Three houses owned
by the hospital, including 1 Surfside Road (Inset 4), would need to be relocated, with two
more being within the 10’ setback zoning requirements. Several utility poles would have to
be relocated.
Inset 4 – 1 Surfside Avenue, from Prospect Street
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x Alternative 2 (100’ diameter roundabout) – This alternative is similar to the Alternative 1
except the circle diameter has been reduced. (See Figure 2) The vehicular levels of service are
not significantly impacted by the reduction of the diameter, but the queue on Surfside Road
will increase due to the elimination of the slip lane. Because the overall circle diameter is
smaller, truck maneuvers become more difficult. The center island is much smaller with a
14’ truck apron. The splitter islands would have to be all or partially mountable to
accommodate larger vehicles. This reduces the amount of protection for pedestrians as they
cross the roads. Two houses owned by the hospital would be required to be relocated and
the porch of 61 Prospect Street (Inset 5) would need to be removed. It is likely that house
will be within the 10’ zoning setback requirement. This option also shifts traffic away from
the Elm tree and maintains the telephone boxes. Two utility poles would have to be
relocated.
Inset 5 – 61 Prospect Street
x Alternative 3 (4-Way Stop) - This alternative, as shown in Figure 3, is similar to the concept
presented in the Mid-Island Study. Prospect Street and Sparks Avenue are realigned to form
a more traditional all-way stop-controlled intersection. The original concept (Figure 4) was
deficient in that it did not accommodate larger vehicles and the slip lane on Surfside Road
was not long enough to be beneficial. In order for northbound right turns to have sufficient
access to the slip lane by not being block by the queue of the left and through traffic on
Surfside Road, the lane would need to be extended to Vesper Lane. To avoid the Elm tree,
the slip lane has been located on the school property. The parking lot entrance on Surfside
could be relocated to Sparks Avenue with no loss of parking spaces. It is unclear as to where
the current school crosswalk (Figure 6) on Surfside Road at Vesper Lane should be relocated.
Prospect Street and Sparks Avenue have been realigned more severely than shown in the
original concept to facilitate turning movements by larger vehicles. This alignment requires
the need to relocate two houses owned by the hospital on Prospect Street and the potential
construction of a retaining wall for the home at 77 Sparks Avenue. Crosswalks on all four
approaches can be provided. Approximately four utility poles would require relocation but
the Elm tree and telephone boxes would be untouched. Using the traffic volumes projected
in the Mid-Island Study, this alternative is expected to operate poorly with significant driver
delay.
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Figure 6 – Looking south on Surfside Drive
Other alternatives were dismissed:
x Sparks Avenue realignment – Sparks Avenue approaches Surfside Road/Atlantic
Avenue at an acute angle that makes vehicle turning movements difficult. By realigning
the roadway to form a traditional ‘T’ intersection with Surfside Road/Atlantic Avenue
vehicle movements would be easier. However, there would be minimal separation to
the intersection with Prospect Street. Vehicle movement turning counts identify a large
proportion of traffic from Prospect Street heading to Sparks Avenue. A vehicle would
have to make a right off Prospect Street and then wait for a gap in traffic before turning
left onto Sparks Avenue. With narrow lanes and the short distance between locations,
the intersection would quickly grid-lock during peak travel periods.
x Roundabout with 3 approaches - The school property has a generous lawn on the
southeast corner that was considered for the placement of a roundabout. Atlantic
Avenue would be realigned to form a ‘T’ intersection with Prospect Street. Through and
left-turning traffic from Atlantic Avenue traffic would enter the traffic stream on
Prospect Street and together they would enter the roundabout as a single approach. This
alternative was discarded for several reasons. The circle has to accommodate the WB-50
design vehicle and the right turn from Surfside Road northbound to Sparks Avenue is
complicated by the location of the Elm tree. The circle would end up close to the
realigned intersection of Prospect Street and Atlantic Avenue, leaving insufficient room
for Atlantic Avenue traffic to combine with Prospect Street prior to entering the circle.
x Signalization – The intersection does meet several vehicular volume warrants and has
potential to reach the school crossing criteria. However a traffic signal is undesirable on
the Island. Vehicle turning movements would remain difficult without significant
roadway widening and/or realignment. To obtain acceptable levels of service, exclusive
turn lanes on several approaches would be required.
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TRAFFIC OPERATIONS ANALYSIS
This memorandum utilizes the projected 2014 volumes from the Mid-Island Study based on 2004
traffic counts for the analysis. MassHighway does not typically accept traffic counts more than three
years old. However a review of the count data provided by MassHighway shows little growth on
various parts of the island since 2003. The 2014 project volumes assumed a 3% per year growth rate
and therefore are considered to be conservative and have not been adjusted.
The capacity analyses for the roundabout alternatives were performed using aaSIDRA2. Stop-
controlled alternatives were analyzed utilizing HCS20003. Both software programs are recognized
and accepted by MassHighway for performing unsignalized intersection capacity analysis.
Level-of-Service Criteria
Level-of-service (LOS) is the term used to denote the different operating conditions which occur on a
given roadway segment under various traffic volumes loading. It is a qualitative measurement of the
effect of a number of factors including roadway geometry, speed, travel delay and freedom to
maneuver. LOS provides an index to the operational qualities of a roadway segment or an
intersection with letter designations ranging from A to F. LOS A represents the best operating
condition, and LOS F represents the worst operating condition.
The level-of-service designation is reported slightly differently for signalized and unsignalized
intersections. For signalized intersections, the analysis considers the operations of all traffic entering
the intersection and the LOS designation is for the overall operations at the intersection. For
unsignalized intersections, the analysis assumes that the traffic on the mainline is not affected by
traffic on the side streets. Therefore, LOS designations are determined for the critical movements at
the intersection, which are typically the turning movements. The evaluation criteria used to analyze
the study intersection are based on the Highway Capacity Manual4. It has been common practice
and recommended by the Transportation Research Board to utilize the signalize intersection criteria
for roundabouts. Criteria are shown in the following table.
Unsignalized Intersection Roundabout
Level of Service Delay Range (sec/veh) Control Delay (sec/veh)
A 0-10 < 10
B > 10-15 > 10 - 20
C > 15-25 > 20 - 35
D > 25-35 > 35 - 55
E > 35-50 > 55 - 80
F > 50 >80
Source: Highway Capacity Manual 2000, Transportation Research Board, Washington, DC, 2000.
Capacity Analyses Discussion
Three different peak periods were reviewed for all selected alternatives: weekday morning, weekday
evening and Saturday mid-day. The results are summarized in the table on the next page.
Both roundabout alternatives, under 2014 design year, are expected to operate at excellent levels of
service. Prospect Street may experience queue lengths over 200 feet during peak travel times and
under Alternative 2, Surfside Road could have queues reaching 300 feet. Alternative 3 reports much
higher queues and significantly higher vehicle delay.
2 Signalized & Unsignalized Intersection Design &Research Aid, aaSIDRA 3.2.0 1455 Version; Akcelik & Associates Pty Ldt.
Greyhorn, Victoria, Australia; 2007.
3 Highway Capacity Software, HCS2000 Version 4.1f; McTrans, University of Florida; 2003.
4 Highway Capacity Manual 2000; Transportation Research Board, Washington, D.C.; 2000.
Capacity Analysis Summary – 2014 Volumes Alternative 1 Alternative 2 Alternative 3 V/C* Delay** LOS*** 95th Queue(1)V/C Delay LOS 95thQueueV/C Delay LOS 95th Queue(2)Weekday Morning Prospect Street - LTR 0.58 6.1 A 161 0.61 7.2 A 178 0.84 32.88 D 245 Sparks Avenue - LTR 0.56 6.3 A 129 0.54 6.7 A 143 0.86 36.24 E 263 Surfside Road - LT 0.42 3.5 A 66 0.74 9.5 A 282 0.64 20.45 C 122 Atlantic Avenue - LTR 0.26 4.5 A 51 0.28 4.9 A 55 0.45 15.95 C 75 Overall 0.58 5.1 A 0.74 7.6 A 29.16 D Weekday Evening Prospect Street – LTR 0.74 13.9 B 271 0.78 16.5 B 303 1.00 77.57 F 506 Sparks Avenue – LTR 0.57 6.9 A 135 0.55 7.3 A 147 1.00 77.56 F 490 Surfside Road – LT 0.39 3.5 A 64 0.70 8.7 A 250 0.66 27.60 D 145 Atlantic Avenue – LTR 0.47 5.8 A 101 0.47 6.6 A 112 0.73 31.79 D 177 Overall 0.75 7.5 A 0.79 10.1 B 59.57 F Saturday Midday Prospect Street – LTR 0.65 8.7 A 203 0.69 10.3 B 226 0.89 39.65 E 292 Sparks Avenue – LTR 0.55 6.2 A 123 0.53 6.5 A 136 0.90 43.33 E 308 Surfside Road – LT 0.52 3.6 A 83 0.76 9.4 A 312 0.60 20.21 C 112 Atlantic Avenue – LTR 0.32 4.6 A 64 0.34 5.0 A 69 0.52 18.08 C 92 Overall 0.67 5.7 A 0.76 8.4 A 33.99 D * V/C -- Volume-to-capacity ratio. ** Average Intersection delay, expressed in seconds per vehicle. *** LOS -- Level-of-Service. (1) 95th queue expressed in feet (2) Calculated using methodology outlined in Tian and Kyte, Transportation Research Record:Journal of the Transportation Research Board, No. 1988.Transportation Research Board of National Academics, Washington D.C. 2006
Date: June 23, 2008
Project No.: 10508.00
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CONCLUSION AND RECOMMENDATION
The existing configuration of the intersection and the components of the three design alternatives are
compared in Exhibit 1. Based on our review of the data (i.e., existing geometry, traffic volumes,
vehicle crashes, pedestrian activity, crash data, and utility constraints) we recommend Alternative 1
as the preferred alternative. Although it requires the relocation of 3 residential structures and
potential zoning waivers of two additional structures, it is our opinion that the 120’ diameter
roundabout will ensure a design that will accommodate vehicle and pedestrian traffic for many
years to come and at the same time fit into the character of Nantucket.
Vanasse Hangen Brustlin, Inc.Safetyaffic Congestion (Level-of-dates Large Vehicles (WB-LandscapingmTreelephone Control CabinetsExisting Structures (# of Houses)ck Zoning Requirement #of Properties)Relocation (# of Poles)on CostImprovement OptionsDescriptionAdvantagesDisadvantagesPedestrianSReduce TrafService)Accommoda50)Areas for LaAvoids ElmAvoids TelepRelocate Ex10' Set-backViolation (#Utility Pole RConstructioDo Nothing Retain existing/off-set geometry and all-way stop control at intersection.No Cost Unsafe and poor level of service.&&&&NoneAlternative 1 120' RoundaboutCircular, raised, often landscaped island that Additional feature intersection on Impacts to Hospital Property, High Costpforms a hub for traffic to flow around it, with splitter islands used to channelize traffic and provide pedestrian protectionNantucket;creates potential landscaping area, good level of service, increased safety.py g&(3Houses)&(3 Properties)&(5 Poles)HighAlternative 2 100' RoundaboutCircular, raised, often landscaped island that forms a hub for traffic to flow around it.Additional feature intersection on Nantucket; creates potential landscaping area, good level of service, increased safety.Impacts to Hospital Property, High Cost, Splitter Islands not used for pedestrian protection&(2Houses)&(1 Porch)&(4 Poles)HighAlternative 3RealignProspect StreetEases TurningPoor level of service;Alternative 3Traditional 4-way Stop with Northbound Slip-LaneRealignProspect Streetand Sparks Avenue to form traditional intersection geometryEases TurningMovements for larger vehicles; Reduces risk of right-angle crashesPoor level of service;impacts to Hospital and School Properties; retaining wall for private abutter&&&(2Houses)&(3 Poles)Moderate= Positive Impact/Yes= No Impact/Maybe&= Negative Impact/NoT:\10508.00\ssheets\10508matrix.xlsExhibit 1 Four Corners IntersectionComparison of Alternatives