Do wider roads attract more cars?
After we published our story about TDOT’s plan to widen Interstate 75 and I-24, more than one of our followers was less than optimistic about what the project would do for traffic at the split. They brought up the concept of induced demand, the idea that if you increase the supply of something, or lower its cost, more people will want it. In the case of highway expansion, will adding more lanes attract more drivers?
A highly cited study from 2011 found what the authors dubbed “the fundamental law of road congestion.” Using interstate data across more than 200 metropolitan areas, they landed on a striking conclusion: There’s a nearly one-to-one relationship between additional interstate capacity and additional interstate usage.
In other words, any expansion of interstates leads to an equal increase in cars on the road. The authors suggested a few reasons why vehicles might fill all of the interstate available to them.
Bigger interstates attract more commercial drivers, such as long-haul truckers
Larger highway networks induce households to drive more
Better driving conditions lead to population growth in metro areas
But other studies have shown that increasing road capacity has a smaller effect, or no effect, on induced demand. In a research article from 2002, the authors noted how the complicated web of factors leading to highway usage (such as new commercial developments attracting drivers and lower gas prices) can make it hard to tell whether highway expansion alone brings more cars to the interstate.
According to TDOT’s website, the I-75/I-24 project is partially meant to meet existing demand by increasing road capacity. But it’s also to fix “operational deficiencies” where cars merge on and off the interstate, per the project description.
And so far, outcomes are promising. After the first phase of construction was completed, drivers saw a 12% increase in travel speed during peak traffic times in 2022.
At least one Chattamatters follower thinks we’re answering the wrong question altogether. Instead of asking whether widening the interstate split will improve traffic, he thinks we should be asking: “What could the hundreds of millions of dollars for this ‘fix’ have done for other transportation needs in our city”?
- Answered by William Newlin, william@chattamatters.com