PHOTO: Design documents show the design of the new interchange on I-65 / Photo courtesy of the city of Spring Hill
BY ALEXANDER WILLIS
After the city of Spring Hill was awarded a $25 million grant from the state, in what Mayor Rick Graham called “one of the biggest news items since Saturn [General Motors plant] came into Spring Hill,” the Transportation Advisory Committee held a presentation Monday night, breaking down the details of the monumental project.
“This interchange will create a gateway into Spring Hill, result in major improvements to our local roadways and give us what’s needed to attract a wide range of new corporate office investment so that more of our residents can work where they live,” Graham said. “This is great, great news… what a perfect Christmas present.”
The interchange is planned to be constructed on I-65 between State Route 396 (Saturn Parkway) and I-840, extending Buckner Road out east to connect with I-65, just north of Summit High School.
The design of the new interstate is planned to be a diverging diamond interchange (DDI), which sees drivers on the overpass briefly drive on the left side of the road, before lanes move back over to the right side.
This unique design allows for there to be only two traffic signals with short cycle lengths, reduces the amount of conflict points in which drivers must turn left into traffic, and costs significantly less than a standard interchange to construct. It’s location will also serve to directly connect Spring Hill to I-65, greatly reducing work commute time for many Spring Hill residents.
A relatively new design for an interchange, the DDI design hadn’t seen any use outside of France prior to 2009, with only 117 across the world over.
“The primary purpose and need for the interchange is to address the regional mobility solution,” said assistant city administrator Chuck Downham during the meeting. “It doesn’t take anybody long to figure out that if Saturn Parkway backs up, Spring Hillians have no way to get basically to the interstate, and to their places of employment. It becomes an arduous task of trying to navigate the secondary roads, and state highways and so forth.”
View below for a brief description on how a diverging diamond interchange functions
Beyond significantly improving traffic flow and mobility, the new interchange could even save lives.
Currently, one of the most common access points to I-65 for Spring Hill residents is Saturn Parkway, which itself is accessed by either Port Royal Road or Kedron Road – neither of which have any traffic signals. Drivers coming off of I-65 to return home are often forced to use Saturn Parkway, where turning left onto either Kedron Road or Port Royal Road can often prove to be a lengthy ordeal.
In fact, during that same meeting, Rajor Trucking owner Toby Kemp asked the committee if they would consider placing traffic signals on Saturn Parkway and Kedron Road, calling it a “very dangerous intersection,” saying his truck drivers are often stuck waiting to turn left for more than ten minutes at a time.
Downham revealed that the section of I-65 between Sr-396 and I-840 saw an average of 76,500 drivers daily in 2017, which has seen hundreds of crashes over the years, some even fatal.
“We’ve had 251 crashes along I-65 between 840 and Saturn Parkway in recent years,” Downham said. “Some of those have involved, very tragically and unfortunately, fatalities.”
Another major benefit of the new interchange, Downham explained, is the draw it brings in terms of economic development.
Research conducted by the Economic Research Development Group (ERD Group), an economic research firm, found that a well-designed interstate can not only attract swaths of new businesses and development to a community of any standing, it “can significantly alter the economic trajectory of a declining community.” The impact can be so significant, ERD Group has found, that intersections can even “lead to increased activity in adjacent markets,” meaning Columbia and Thompson’s Station could see some economic growth as well.
The $25 million grant that will help push this project along comes from the U.S. Department of Transportation Better Utilizing Investments to Leverage Development (BUILD) program, which itself is funded by the Transportation, Housing and Urban Development Appropriations subcommittee. The BUILD program awards competitive grants to communities in Tennessee and the United States at large to help fund highways, public transportation systems, bridges and freight railroads.
Cities across the United States competed for this competitive grant, with the city of Spring Hill being the only city in the state to be awarded the funds, as well as only one of 12 cities in the entire country to receive the maximum amount of $25 million.
The cost for the interstate project itself is estimated to be approximately $29 million, with an additional $19 million for Buckner Road and Buckner Lane extensions, making the project as a whole cost around $48 million.
Despite the high price point, Downham explained that the eventual cost of not constructing the interchange, which measures variables like vehicle expenses, travel time costs and economic impacts, would eventually cost upwards of $2.8 billion over the years. As the project only costs approximately $48 million, this gives the project a benefit to cost ratio of 55.52 to 1.
The deadline for city staff to reach an agreement with the Department of Transportation and the Federal Highway Administration is September 20, 2020.