What is a Diverging Diamond Interchange?
A Diverging Diamond Interchange (DDI) is referred to as a 'crossover' type intersection by the Federal Highway Administration (FHWA) and its implementation is encouraged given its operational benefits. As noted on the FHWA website, it is called a 'crossover' type intersection since the left-hand side of the roadway is utilized "to facilitate unopposed left-turn movements. These designs can improve safety and reduce congestion at the intersection." FHWA has developed an informational brochure which can be viewed for additional information.
Why are we calling this project a “Modified” Diverging Diamond?
This interchange configuration proposed in Natick is unique in several ways. For this reason, the project team felt it was important to differentiate the Natick project from what may be found by someone who does an internet search for 'Diverging Diamonds'. The Natick project is different for several reasons:
- The project includes three independent bridges with a substantial separation between the Route 27 barrels with the exclusive shared-use-path bridge in-between. It is not uncommon to see traditional DDI's include one wide bridge where the roadway barrels are much closer and separated by a bicycle/pedestrian facility that would be protected via concrete barrier on either side.
- The increased separation between the barrels results in roadway curvature at both the 'crossover' points (entries and departures) which will require drivers to slow down. This geometry provides a traffic calming benefit. The target travel speeds along Route 27 through the interchange will be approximately 20 to 25 miles per hour.
- Because of the many private property constraints adjacent to the project, the Natick configuration is very ‘compact’ and situated within a relatively small footprint compared to more traditional DDI's.
- Given the context of the Natick project and input from the community, pedestrian and bicycle connectivity was identified as a major component to be included. The Natick configuration includes an extensive sidewalk and shared-use path (SUP) network which bridges a large gap that currently exists in the community. The project will provide walking, bicycling and transit opportunities that are currently severely lacking or non-existent. The intent is to provide a safe and accessible SUP which will encourage more trips by walking, bicycling and transit. The project will provide an excellent connection along Route 9 to the MathWorks campuses as well as the many other businesses along Route 9. The SUP will also connect to the Cochituate Rail Trail (CRT) which is a regional trail for the area that provides further non-motorized access to surrounding communities.
- Many traditional DDI's include a great deal of concrete and asphalt surfaces and many vehicle travel lanes. The Natick configuration only includes two through travel lanes in each direction along Route 27 and incorporates specific landscaping to soften the look of the interchange and to also provide shade opportunities for pedestrians and cyclists.
For the reasons above, the interchange project in Natick is referred to as a 'Modified' Diverging Diamond Interchange since it is not a typical DDI design. The Natick Modified DDI provides a context sensitive treatment where the design seeks to be a long-term solution for the community that strikes the appropriate balance of safety, operations, and connectivity.
The following 3D model shows the general configuration of the proposed interchange with the hope that it will provide a greater understanding of the intended improvements. Please keep in mind that the project is still in its early design stage and many components are still subject to change. For instance, the landscaping and many material choices still need to be properly vetted. Also, utility poles are not shown in the following video as much of that utility coordination is just being initiated.
Could MassDOT just replace the existing Rte 27 bridge over Rte 9 and leave everything else as-is?
This question has come up during the public process and it is a good question. Why spend the time and money on this interchange when it is just the bridge that needs to be replaced?
As part of the design process, we have learned many things about this interchange that we feel need to be addressed. If we were to replace the bridge and not address these needs, we feel the project would not serve the best interest of the community and the region.
Following are some thoughts on why it is important that this project take a holistic view of the area and be more than just a bridge replacement project:
Crashes – This interchange has a high crash experience, particularly along the on-ramps to Route 9. This crash history is indicative of a problem that should be addressed. Simply replacing the bridge would not address the crash and safety issues. The project will eliminate the ‘weaving’ condition that is currently experienced along both directions of Route 9 between the on/off ramps to Route 27. The project will reduce the number of on and off-ramps to and from Route 9. The project will provide deceleration and acceleration lanes along Route 9 to increase safety by eliminating the need to stop or yield at the at the end of the on-ramps, which currently occurs.
Multi-Modal Accommodations – The interchange does not provide a continuous pedestrian route and does not satisfy accessibility requirements. Also, there are no accommodations for cyclists. This results in a gap in the network for the Natick pedestrian and bicycle community. There are schools to the north of the interchange and students can currently be observed navigating the interchange even with the lack of appropriate facilities. There is a desire for improved accommodation which has clearly been voiced through the public outreach process. The project seeks to bridge the current divide that is created by the Route 9 interchange. The extensive network of sidewalk and shared-use paths will accomplish this.
Pedestrian/Cyclist Safety – Given the presence of pedestrian accommodations along Route 9, it is important to consider their safety when crossing the on/off ramps to Route 27, where drivers may be more focused on vehicle traffic. The project will install pedestrian-actuated warning systems at the crosswalks to heighten driver awareness of pedestrian crossings at these higher speed locations.
Bridge clearance – The Route 27 bridge clearance over Route 9 does not meet current MassDOT standards. Replacing the bridge 'in-kind' would perpetuate this non-compliance. The proposed interchange redesign will raise the profile of Route 27 to satisfy current clearance requirements.
Construction staging – To replace the bridge ‘in-kind’ still presents many challenges particularly when it comes to constructing the improvements while maintaining vehicle traffic through the work zone. To maintain acceptable traffic operations during construction a minimum of two lanes (one lane in each direction) would need to be provided. In order to maintain this lane configuration and minimize traffic impacts to the community, it would be necessary to build a temporary bridge ‘off-alignment’. So essentially a new, temporary bridge would be built adjacent to the existing bridge so that traffic can be shifted to the temporary bridge while the existing bridge is demolished and then rebuilt. Once the old bridge is replaced, traffic can be shifted back to the original alignment of Route 27 and then the temporary bridge would be removed, and the interchange would be restored. So, although replacing the bridge in-kind seems like a less involved solution, the option would have significant impacts to the area and involve a complicated, costly construction operation.
Given the reasons stated above, replacing the bridge ‘in-kind’ is not a prudent solution. The poor condition of the bridge is certainly a driving factor for the project, but the success of this project extends well beyond the mere replacement of a bridge structure. The project takes a holistic approach to solving a complex transportation problem and ultimately will result in improved safety, mobility and connectivity for Natick and the surrounding communities.
What other options were considered to improve the interchange?
The project area was initially evaluated from 2008 through 2014 as part of the initial project development. During the initial planning and project development phase, several interchange configurations were evaluated, including:
- Partial Cloverleaf Configurations (Two Alternatives)
- Frontage Road
- Single Point Diamond Interchange
- Partial Cloverleaf with Auxiliary Lanes on Route 9
The Partial Cloverleaf with Auxiliary Lanes on Route 9 was selected as the preferred alternative and advanced to a 25% Design. The project's progression later stalled in 2017 due to unfavorable feedback from the public at the Design Public Hearing, held on June 8, 2017.
In response to the unfavorable public feedback of the 25% design, an independent review of additional alternatives was pursued. Through that process several configurations were investigated to improve traffic safety, operations, and mobility for all users in the project area.
Through the independent design process two additional interchange alternatives were determined to be appropriate. Those consisted of the following:
- Modified Diverging Diamond Interchange
- Grade Separated Multilane Roundabout Interchange
Ultimately, traffic operations, safety concerns, right-of-way impacts, environmental impacts, accommodation of multi-modal travel, and construction costs were all factors that contributed to the choice of the recommended alternative, the modified diverging diamond interchange. For additional information about the concepts considered, refer to the June 12, 2019 – Public Information Meeting, referenced within the Documents & Meeting Materials portion of this web page.