Interstate Access Requests

Posted by on Sep 17, 2019 in Road Safety, Transportation | No Comments

By: Erin Curry, P.E., R.S.P.

Did you know the state departments of transportation oversee all construction and maintenance of our Interstate System, but the federal government maintains oversight to interstate access?  Therefore, any change of access to the Interstate System requires approval by the Federal Highway Administration (FHWA).  I recently attended an FHWA workshop at the Tennessee Department of Transportation (TDOT) Region 3 office on Interstate Access Requests (IAR).   

We have prepared numerous IARs over the years for TDOT, including the soon-to-be reconstructed Exit 46, I-65 at Bear Creek Pike, Interchange in Maury County and recently reconstructed Exit 61, I-65 at Peytonsville Road in Williamson County.  The Peytonsville Road Interchange suffered a tragedy during construction when a fuel truck hit the newly constructed bridge, killing the driver and destroying the bridge.  https://youtu.be/53icZmy34vE

The workshop focused on types of interchanges, the FHWA Interstate Access Policy, design, traffic operations, and safety.  TDOT is in the process of changing several of our interchanges around the state, so I thought I would share with you the more popular types of interchanges and why they’re used.

Single Point Urban Interchange (SPUI):

  • Pros:
    • Compact footprint
    • Good operational efficiency
    • Easier coordination of signals along crossroad
    • Can better accommodate large trucks
  • Cons:
    • High cost due to large bridge structure
    • Not pedestrian friendly
    • Hard to expand in future
  • Notable Tennessee Locations:
    • Nashville, Briley Parkway @ McGavock Pike
    • Franklin, I-65 @ McEwen Drive
    • Jackson, I-40 @ N. Highland Ave (under construction)

Tight Urban Diamond

  • Pros:
    • Compact footprint
    • Good operational efficiency
    • Typical pedestrian crossings
  • Cons:
    • High cost due to large bridge structure
    • Increased right-of-way needs for left turns storage
    • Impacted truck turning efficiency
  • Notable Tennessee Locations:
    • Murfreesboro, I-24 @ Medical Center

Diverging Diamond Interchange:

  • Pros:
    • Excellent operational efficiency
    • Improved safety due to fewer intersection conflicts
      • 30-40% reduction in crashes
    • Comparatively less bridge cost
  • Cons:
    • Ample spacing to adjacent signalized intersections is critical
    • Exit-entrance movement not directly provided
    • Frontage roads pose compatibility challenges
  • Notable Tennessee Locations:
    • Sevierville, I-40 @ State Route 66
    • Alcoa, US129 @ Bessemer Street (1st DDI in Tennessee)
    • Spring Hill, I-65 @ Buckner Lane (under development)

We look forward to continuing to work with TDOT on Interstate Access Requests.  These projects utilize multiple Sain Associates teams – Transportation, Traffic, and GIS – and require close teamwork with TDOT. 

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