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ANALYSIS: Data suggest high quality rail service would reduce I35 congestion

Implementing a high quality rail service in Central Texas would speed up all modes of travel in the region, including driving on I-35, according to a recent master's thesis. The study’s traffic simulation model revealed that more than 4 million trips between Austin and San Antonio would shift from private cars to the train, reducing the number of vehicles on the road by more than 20 percent.

Not only would a train allow riders to bypass traffic, it would also improve traffic flow on one of the nation’s most congested highways, something TxDOT has struggled to achieve for decades. Even for people who do not want to ride on a train, the existence of an alternative would speed up their commutes.

The findings from Budow’s model are consistent with multiple traffic studies analyzing the effects of new public transportation on traffic volumes, which record significant reductions in congestion after the construction of rail (Bhattacharjee 2012, Anderson 2013, Fageda 2021). This is because train services increase the overall capacity of the corridor, rather than increasing vehicular capacity, which just works to induce more driving.

The difference between adding capacity through train service and adding capacity through expanding a roadway lies in its effects on land use. Studies show how expanding roadways leads to more car-reliant development, which adds more people to the road and eventually results in worse congestion (Noland 2001, Yu and Zhou 2021, Lee 2018). Real life examples of this theory can be seen all around the world and even in our own backyard. The Katy Freeway in Houston, which was expanded to 26 lanes in 2008, now experiences worse congestion than it did before the expansion.

Adding capacity through train service, however, reduces the demand for driving by giving an alternative. Train stations act as a magnet for development, centering more people and businesses in walkable areas of cities, such as downtowns, reducing the need for car-based transportation.

Budow hails from Baden-Württemberg, a state in Germany that relies on car manufacturing for its economy but also has a dense passenger rail network. He wanted to make it clear that adding train services does not contradict investments in car-based infrastructure, but rather compliments it.

“Developing a train network does not mean that you cannot or should not use your car. It is an alternative, not a replacement,” he said. “Implementing it appropriately, everyone will benefit from higher capacity in the transportation infrastructure.”


Anderson, M. (2013). Subways, Strikes, and Slowdowns: The Impacts of Public Transit on Traffic Congestion.

Bhattacharjee, S., & Goetz, A. R. (2012). Impact of light rail on traffic congestion in Denver. Journal of Transport Geography, 22, 262–270.

Fageda, X. (2021). Do light rail systems reduce traffic externalities? empirical evidence from mid-size European cities. Transportation Research Part D: Transport and Environment, 92, 102731.

Lee, S. (2018). Transport policies, induced traffic and their influence on vehicle emissions in developed and developing countries. Energy Policy, 121, 264–274.

Noland, R. B. (2001). Relationships between highway capacity and Induced Vehicle Travel. Transportation Research Part A: Policy and Practice, 35(1), 47–72.

Yu, H., & Zhou, Y. (2021). Highway spending and induced vehicle emissions: Evidence from the US states. Resource and Energy Economics, 65, 101245.

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