Four Corners Intersection
The purpose of this project is to utilize Town of Nantucket’s Complete Streets Policy (PDF) and the 2006 Massachusetts Highway Department’s Project Development and Design Guide and other Highway Division standards and criteria to design and construct a modern roundabout at this location, as recommended in the Nantucket Master Plan and the NP&EDC’s Regional Transportation
The existing intersection (also known as the “Four Corners” intersection) consists of an awkward configuration consisting of an approximately 75 foot offset alignment of the Sparks Avenue and Prospect Street approaches, which creates a double turning movement responsible for considerable confusion, congestion / delays, and accidents. The four single lane approaches are under an all-way stop control with Surfside Road, Sparks Avenue and Prospect Street approaches providing 11-foot travel lanes and Atlantic Avenue only providing 10-foot lanes. Sidewalks are provided along both sides of Surfside Road and Sparks Avenue, along the east side of Atlantic Avenue and along the south side of Prospect Street. A marked crosswalk is provided on the east leg of the intersection across Sparks Avenue with school-crossing signs. Utility poles are located close to the exiting roadway edges and there a utility control cabinet located on the High/Middle School property at the southeast comer of the intersection. Also located in front of the High/Middle School property on Surfside Road is a significantly sized elm tree. This tree is located directly behind the existing curb line, approximately 100 feet south of Sparks Avenue.
The intersection is primarily surrounded by single family residential structures, with the exception being the Nantucket High School and Middle School located in the southeast area of the intersection. Recent analysis shows there have been a total of 6 crashes from 2009 through 2013. These crashes generally consisted of angle and rear-end type collisions. These types of crashes can be attributed to the offset intersection geometry, sight distance constraints, and minimal roadway widths. The intersection was found to have averaged less than two (2) reported motor vehicle crashes per year over the five-year review period with no fatalities. This crash rate is below the MassDOT statewide and Highway Division District 5 averages for an unsignalized intersection.
Improvements to this intersection were originally recommended in the Mid-Island Area Plan by the Nantucket Planning and Economic Development Commission (NP&EDC) in June 2003, and later further studied as part of the Traffic Study and Strategy for the Mid-Island Area in July 2005. A roundabout alternative was ultimately supported by the NP&EDC and included in the Regional Transportation Plan following an analysis completed by Vanasse Hangen Brustlin, Inc (VHB) in March 2010.
Since a modern roundabout alternative would have significant right of way impacts to abutting property owners, primarily the Nantucket Cottage Hospital, an additional alternative was explored by Vanasse & Associate, Inc. in June 2016 as part of the improvements to the Hospital. This alternative reconfigures the intersection to function as a single-lane "mini" modern roundabout with improved pedestrian and bicycle access. This alternative also provides a right-turn slip lane to accommodate Atlantic Avenue right turns to South Prospect Street for passenger cars and buses. This slip lane is necessary to accommodate right turns from Atlantic Avenue to South Prospect Street due to the restricted existing roadway configuration approaching the intersection. The size of this roundabout is significantly smaller than a single lane modem roundabout, with a 38-foot diameter center island and a 16-foot travel lane for a total inscribed circle diameter of 70 feet. To accommodate larger design vehicles, the center island and median delta islands at the approaches to the roundabout would be constructed of mountable materials (i.e., cobblestone or belgium block) and pavement markings to allow trucks and buses to maneuver through the roundabout. Trucks (WB-50) could be accommodated on all movements except right turns from Surfside Road to Sparks Avenue and from Atlantic Avenue to Prospect Street; and left turns from Prospect Street to Atlantic Avenue. It is expected that trucks could avoid these movements via alternative travel routes through the implementation of appropriately placed signage. Large vehicles would occupy most of the circle when turning, limiting access to the roundabout to other entering vehicles and significantly reduce the capacity of the mini roundabout.
This Option would advance the latest “mini-roundabout” alternative as it would have reduced property impacts to both the School and Hospital properties.
- 2018 VHB Functional Design Report – in development
- 2016 Alternatives Analysis (PDF), a 2016 report by Vanasse & Associates prepared for Nantucket Cottage Hospital
- Appendix 1 (PDF)
- Appendix 2 (PDF)
- 2010 Alternative Analysis (PDF), a 2010 VHB report of an additional roundabout "Alternative 4”
- 2009 Peer Review (PDF), a 2009 peer review by Ourston Engineering of the 2008 VHB report
- Four Corners Intersection Evaluation (PDF), a 2008 VHB report of existing conditions with an evaluation of alternatives to improvement overall function.
The purpose of these projects is to utilize Town of Nantucket’s Complete Streets Policy and the 2006 Massachusetts Highway Department’s Project Development and Design Guide and other Highway Division standards and criteria to design and construct a modern roundabout at the following three (3) intersections, as recommended in the Nantucket Master Plan and the NP&EDC’s Regional Transportation Plan (click links below for additional information on each location):
- Surfside Road at Bartlett Road
- Four Corners (Sparks Ave, Surfside Rd, Atlantic Ave, and Prospect St)
- Old South Road at Fairgrounds Road
- “A primary benefit is the reduction of vehicle speeds in and around the roundabout. Roundabouts improve pedestrian crossing opportunities, providing mid-block refuge and the ability for pedestrians to focus on one traffic stream at a time while crossing with or without crossing guards.” – Federal Highway Administration, Safe Roads for a Safer Future
- “In settings with large numbers of children, lowering vehicle speed has great potential for injury prevention. Pedestrian crashes involving a child most often result from the child’s error, thus slower speeds give motorists more time to react and can lessen injuries when crashes do occur.” [Retting, Ferguson, & McCartt, 2003] – National Center for Safe Routes to School, Safe Routes to Schools Guide
- General benefits according to the National Center for Safe Routes to Schools:
- Lower motor vehicle speeds and increased yielding behavior
- Fewer conflict points
- Higher visibility of pedestrians in the crosswalk
- Lower exposure to motor vehicles because of the shortened crossing distance
- Simpler crossing due to the splitter islands, which provide mid-crossing refuge and allow the pedestrian to focus on traffic from one direction at a time