During peak traffic hours, the existing I-40/Donelson Pike (SR 255) interchange that provides access to Nashville International Airport (BNA)—and is the primary access between I-40 and downtown for Nashville’s highly populated Donelson and Antioch communities—experiences significant traffic volumes and delays. Spurred by Nashville’s robust growth over the last decade, this increased congestion has led many commuters traveling northbound along SR 255 to divert through airport property and utilize BNA’s discreet westbound I-40 entrance ramp to avoid the interchange. This diversion of higher-speed commuter traffic mixing with lower-speed airport traffic has created significant safety concerns as well as confusion for visitors unfamiliar with the airport’s ring-road system.
DDI in TN
Fewer crashes predicted on I-40
Fewer crashes predicted along SR 255
Last Impressions Count
At the request of BNA, the Tennessee Department of Transportation (TDOT) initiated a modification study for the reconfiguration of the vital interchange, which is not only the primary access point to the airport, but is also one of the last things that visitors departing from Nashville will see—making it important to leave a positive impression of the city.
A Fast-Track Schedule
In order to keep pace with improvements at Nashville airport, Gresham Smith was tasked with delivering the study in just four months as opposed to nine—a more typical time frame for a project of this complexity. To help meet the accelerated schedule, our team adopted a multidiscipline approach, utilizing traffic and roadway design staff in our Baton Rouge, Charlotte and Nashville offices.
Exploring the Options
The modification study involved our team exploring new interchange concepts for the location, including traffic roundabout options, directional interchange alternatives, and even a no-build option. Site challenges included crash rates up to three times the statewide average in the study area, rapidly increasing traffic volumes and changing traffic patterns, avoidance of the airport’s runway protection zone and accommodation of BNA’s $1.2 billion expansion plans.
An Effective Solution
Given these unique set of challenges, our team proposed a Diverging Diamond Interchange (DDI) as the most feasible and effective solution for the site. Selected by TDOT as the preferred alternative, the DDI’s unique configuration provides the ability to accommodate heavy-left turn movement from northbound SR 255 to westbound I-40 toward downtown Nashville. The cost-effective alternative will also accommodate a future planned interchange along I-40 at Harding Place, maintain crucial access to the airport during construction, and accommodate the airport's expansion plans.
As part of the project scope, Gresham Smith conducted a safety analysis utilizing the Interchange Safety Analysis Tool-Enhanced (ISATe) to predict how the DDI will impact safety within the study area. Along I-40, the safety analysis predicts the new diverging diamond will have 7 percent fewer crashes per year compared to a no-build alternative. Within the interchange along SR 255, the Highway Safety Manual’s crash modification factor predicts 33 percent fewer crashes are likely to occur, with 41 percent fewer fatal and injury crashes.
A True Team Effort
By working closely with TDOT, the Federal Highway Administration (FHWA) and other Gresham Smith staff, our team was able to deliver the interchange modification study—including FHWA approval—within the accelerated schedule. Gresham Smith is now moving forward into the design phase for the new DDI, with a projected completion date of 2023. The project represents only the fourth DDI in Tennessee and Gresham Smith's third in the state.
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