Signal System Upgrades at Intersections on Tennessee’s I-24 Smart Corridor Reduced Overall Crashes by Up to 30 Percent.
Project Evaluated ITS Enhancements for a Smart Corridor between Nashville and Murfreesboro.
Tennessee, United States
Evaluating the Impacts of I-24 Smart Corridor Strategies
Summary Information
Tennessee Department of Transportation (TDOT) launched the I-24 Smart Corridor Project, which covers the area between Nashville and Murfreesboro. It includes I-24, the parallel SR-10, and connector routes between them. The goal of the project was to increase travel time reliability, increase mobility, reduce crash concentrations, and develop agency coordination. The project included various strategies including signal system upgrades, integrated emergency pull-offs, ramp extensions, connected vehicle infrastructure, new radar detection system (RDS) stations, dynamic message signs, lane control signs (LCS), variable speed limits (VSL), and integrated corridor management. These strategies were deployed between 2018 and 2025. Additional future components included ramp metering and choice lanes. The objective of this study was to identify performance measures and the necessary data to determine the impacts of the strategies deployed as part of the I-24 Smart Corridor.
METHODOLOGY
Data used for the project included traffic speed, volume, occupancy, travel time, and crash information collected by TDOT and third-party vendors. Project evaluation was based on the performance measures listed below.
-
Crash rate (total and by injury and collision type) and secondary crash rate,
-
Travel time, buffer time and buffer index (the extra time travelers need to ensure on-time arrivals),
-
Vehicle miles traveled (VMT), delay and cost of delay per VMT, and
-
Compliance rate with variable speed limits
Before and after periods were defined based on the specific enhancements and their implementation dates. A before-and-after analysis, using statistical methods and benefit-cost analysis, was conducted to evaluate the impacts.
FINDINGS
- After the signal system upgrades, three out of four cities (Nashville, Smyrna, and Murfreesboro) showed statistically significant reductions in rear-end and sideswipe crashes. Overall crash reductions ranged from 20 to 30 percent.
- After the implementation of VSL and LCS, the overall number of crashes declined by 18.1 percent. Rear-end and sideswipe crashes decreased by 14.0 and 3.2 percent, respectively. Secondary crashes dropped by 13.8 percent and the rate of secondary crashes declined from 7.72 to 6.61 percent.
- Ramp extensions in the westbound direction led to an increase in average speed at two specific exits, with 19.2 and 12.3 percent improvements respectively during peak hours. The corresponding Buffer Time Index at those ramps decreased by 58.9 percent and 15.3 percent.
- VMT increased by three percent in the eastbound direction and 10.3 percent in the westbound direction.
- In the two years following VSL/LCS implementation, average travel time eastbound increased from 23.9 minutes to 25.3 minutes and then remained stable. Average travel time westbound increased from 26 minutes to 32.5 minutes and then remained stable.
- TDOT calculated a return of $1.13 for every dollar invested, assuming a 10-year service period and a seven percent annual discount rate.
- The annual crash savings were estimated at $10.1 million, while the net annual return after operations and management costs about $1.2 million.
