Kate Ackerson, PE, Senior Project Engineer, Kleinfelder
It is no secret to transportation design professionals that city streets can be some of the most challenging spaces to reconstruct. Settlers of Greater Boston did not build streets with the intention to support modern vehicles, traffic patterns and populations that are present in the region today. Instead, cities and towns are left with restricted rights-of-way (ROW) that limit their ability to provide some of the most basic infrastructure, like fire hydrants and utility poles, without obstructing accessible routes for pedestrians. Narrow ROW, non-compliance with national and state accessibility regulations, and the need to retain existing ecological features result in head-scratching predicaments for engineers, transportation planners, and city partners alike.
But there is a solution. It involves the transformation of these streets through the removal of substandard sidewalks, while simultaneously meeting accessibility requirements; creating safe spaces for pedestrians and cyclists; and increasing tree canopy to reduce the urban heat island effect. Sprinkle in some green stormwater infrastructure, too. This brings us to the concept of "shared streets design."
What are shared streets?
Shared streets in residential areas are typically implemented on existing low-volume, low-speed roadways and function as a public space shared by all modes of transportation. Shared street designs create an accessible pedestrian route within the street and level connections between homes and the roadway by removing the typical grade-separated curb line.
Traditionally, the design approach for narrow, non-ADA compliant residential streets is to install curb bump outs around pinch points. This, however, typically requires a loss of existing parking in neighborhoods where it may already be very limited. Often in urban settings, a Massachusetts Architectural Access Board (MAAB) variance is also required for one side of the street as many existing streets lack a design option that meets Commonwealth regulations on both sides. Image 1 shows typical existing conditions of urban streets that lack adequate accessibility due to existing pinch points caused by features like trees and utility poles. Image 2 shows the traditional design approach that often leaves one side of the street inaccessible, and one side of the street impacted by loss of parking.
Image 1 shows existing conditions of Suffolk Street in the City of Cambridge, which lacks accessible
sidewalks due to pinch points caused by features like trees and utility poles. Inaccessible sidewalks
limit the ability to plant new trees and install more greenspace, thereby preventing the reduction of impervious areas.
Image 2 shows a rendering of a traditional roadway design on Suffolk Street. This approach includes installing a proposed
bump out around existing pinch points, which results in the loss of parking and can leave one side of the street inaccessible.
In contrast, shared streets use a variety of design elements to create a common space used by all modes: pedestrians, low-speed vehicles, and cyclists. The accessible pedestrian space becomes the roadway itself and complies with acceptable cross-slopes like a traditional sidewalk. Existing sidewalks are largely replaced by new trees with enlarged soil volume, planting areas, and green stormwater infrastructure. The design maintains access for emergency and maintenance vehicles, and compliant access to private driveways, pathways, and other access points along the back of the former sidewalk. Image 3 is a rendering of a proposed shared street that shows the transformation of the sidewalk and other elements into a shared roadway space.
Image 3 is a rendering of a shared street design on Suffolk Street. Areas that were once impervious
are replaced by new trees and plantings to maximize the street’s urban forest canopy. An accessible
pedestrian route has been created within the street, providing compliant access to both sides, and
level connections between homes and the roadway have been created.
To help mitigate surface water flooding, the roadway can be designed with an inverted crown to direct runoff away from abutters and into the middle of the street. Replacing existing hardscape materials with vegetated soils, low-lying plantings, and new trees helps store surface runoff and promotes the health of existing and new canopy.
Where is this happening?
The city of Cambridge has successfully implemented a shared street design on Longfellow Road that allows the street to be accessible for people of all ages and abilities, the maintenance of existing trees and the maximization of green space.
Similarly, through the Port Infrastructure Improvements Project, the city of Cambridge and Kleinfelder are working together to redesign several residential streets in the historic Port neighborhood into shared streets. Traffic studies, including vehicle volumes and speed data, have been collected throughout the neighborhood and reviewed for consideration of future shared street layouts. Today, many residents of the Port neighborhood use the existing streets as if they were already functioning as shared streets; people walk and children play in the middle of the street due to the constrained ROW and substandard sidewalks. Various design elements, such as different asphalt coloring treatment and signage, will be implemented to alert users that they are entering a shared street.
Residents in the Port are also experiencing the burden of high temperatures as the area has been recognized as an urban heat island due to the limited tree canopy and high density of impervious surfaces. High Solar Reflectance Index (SRI) paint coatings will be applied to these shared streets to not only serve as a visual cue to users that these streets are unique, but will also help reduce heat due to their lighter, reflective properties. The shared streets design also aligns with the city’s Urban Forest Master Plan, which serves as a guide to the citywide commitment to protecting and maintaining the existing urban forest canopy and continuing to increase the tree canopy to reduce the urban heat island effect. Shared street designs will allow for the maintenance of existing trees and the installation of new tree plantings, with accessible routes being maintained. The city’s Resilient Cambridge initiative will also be considered in the designs to further the city’s goals to make it a more sustainable environment and improve the quality of life for residents.
Through traffic studies, feedback from various city departments and key stakeholders including a working group of Port neighbors, the team is working to overcome limitations presented by the Port’s existing challenging infrastructure. Shared streets design provides an innovative solution that can positively change the way Cambridge uses its streets.
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