Tennessee Bridge Facts

Updated December 22, 2015

Current Bridge Status

There are 19,780 structures in the State of Tennessee that meet the definition of a public highway bridge under the National Bridge Inspection Standards. This number does not include the approximately 400 bridges owned and operated by Federal agencies.

Structurally Deficient (SD) Status - A highway bridge is classified as structurally deficient if one or more major structural components of the bridge are rated in "poor" condition (0 to 4 on the NBI rating scale). A bridge can also be classified as structurally deficient if its load carrying capacity is significantly below current design standards or if it crosses a waterway that frequently overtops the bridge during floods.

Functionally Obsolete (FO) Status - A highway bridge is classified as functionally obsolete if the design of the bridge does not meet current standards. Typically, this applies if the roadway width or the provided clearances of the bridge fall short of those desired for current traffic demand. Being functionally obsolete is generally considered to be less serious than being structurally deficient since the bridge may be in fair to good condition (structurally) and is perfectly safe to carry traffic.  

On-System Bridges are those maintained, owned and operated by the State of Tennessee. They include bridges on or over the Interstate Highway System, the State Route Highway System and miscellaneous highway bridges such as those in State Parks. These On-System Bridges currently number 8,317 structures of which 207 (2.5 %) are considered to be structurally deficient.
Off-System Bridges are those found on roads owned, maintained and operated by local governments including counties, cities and townships. These Off-System Bridges currently number 11,463 structures of which 759 (6.6 %) are considered to be structurally deficient. Looking at the entire population of highway bridges (irrespective of ownership), about 4.9 % of Tennessee Highway Bridges are currently Structurally Deficient.

The average age of all Tennessee highway bridges is 40 years old. This is slightly less than the National Average which is 42 years old based upon a 2013 analysis of National Bridge Inventory data. Bridges on the State Highway System (On-System) tend to be larger and slightly older than those on local highways.

The average age of an On-System Tennessee highway bridge is 43 years old compared with 38 years old for the average Off-System bridge.

Historical Improvements

The Tennessee Department of Transportation (TDOT), in cooperation with local governments, has worked to steadily improve the condition of Tennessee’s Highway Bridges over the last 20 to 30 years. In particular, there has been a focus at reducing the number of Structurally Deficient bridges. See the graph below:

Note that the percentage of Structurally Deficient Highway Bridges has been reduced from about 20%, in 1992, to less than 5% in 2015. This is a significant reduction achieved over the referenced 23 year period.

Even more impressive is that this improvement in overall highway bridge condition has been achieved on a “Pay as you Go” basis without building up any long-term debt for the Taxpayers of Tennessee.

Highway Bridge Funding

Historically, highway bridge funding is provided by the Federal Gas Tax as distributed by various Federal Highway Bills. The Federal Gas Tax has not been raised since 1993. Therefore, this funding source has been eroded due to inflation and due to the introduction of more fuel efficient vehicles. The Consumer Price Index (CPI) indicates that $100 of funding, in 1993 dollars, only has the buying power of $60.98 in 2015 dollars. This actually underestimates the effect of inflation since highway bridge construction costs have increased faster than the overall inflation rate.

The result, of this freeze in funding, means that most recent Federal Highway Bills have been short-term measures. In December 2015, Congress did pass a new, Highway Bill (The FAST Act) that provides approximately $305 billion for Nationwide Highway spending over the next five years. However, this bill did not raise the Federal Gas Tax and relies upon various funding sources.

Given the sometimes uncertain Federal Funding environment, Tennessee has supplemented Federal Funding with State Funds to support improvements to Highway infrastructure. For example, in addition to Federal Bridge funds (and matching State and Local funds), Tennessee also allocated approximately $38 million for repairs to bridges on the State Highway System in FY 2015-16. This was supplemented by approximately $9.5 million to fund the State Aid Bridge Grant Program to help local bridge owners with bridge replacement and rehabilitation projects.

However, State funding suffers from the same erosive forces that affect the Federal Gas Tax. The State Gas Tax has not been increased since 1989. The CPI indicates that $100 of funding, in 1989 dollars, only has the buying power of $52.36 in 2015 dollars. This may become a more important factor as the Tennessee highway bridge population continues to age and will need increased levels of funding to remain in serviceable condition in the future.

Overall, highway bridges are typically financed by a mix of Federal, State and local funding depending upon the specific nature of the project and the ownership of the bridge.