Adding road capacity is fruitless, another study finds

By Chris McCahill

As cities grow and traffic increases, road capacity investments offer diminishing returns and even make traffic worse, according to a recent international study. Looking at 24 cities across the globe, researchers found that for every one-percent increase in road capacity, average traffic speeds drop 0.014 percent. “This decrease in speed may be a result of the demand induced due to an increase in the supply of roads,” they explain. Public transportation doesn’t suffer the same consequences.

For this study, researchers at Imperial College London and the National University of Singapore leveraged the largest publicly available set of traffic data, compiled from traffic sensors by a team at ETH Zürich. Using the data, they modeled relationships between road occupancy (traffic density) and traffic flow in each city, noting that while traffic flow can be highly unpredictable at higher occupancy levels, it tends to plateau and decrease.

Traffic occupancy versus flow in Tokyo (left) and Toronto (right). Source: Anupriya et al., 2023.

The researchers used their models to estimate the impacts of added capacity on average traffic speeds and found that, if anything, they have a small negative effect. They explain:

“Our results suggest that capacity expansions do not lead to substantial changes in the average travel speed in the network. Thus, building more roads in major urban areas may create more congestion, pollution and collisions. Moreover, such policies may also increase the other wider negative consequences of vehicular travel such as global warming and climate change as they allow more mobility for urban residents.”

They note that some targeted capacity expansions are inevitable, but other congestion management strategies like operational improvements, congestion pricing, and parking management could be more beneficial.

Finally, they add that while roadways become less efficient as the density of users increases, literature overwhelmingly suggests the opposite is true for rail and bus networks. “Thus,” explain the authors, “public transport services are potentially more productive and cost-efficient compared to vehicular travel in dense city centres.”

This study, while novel in its approach, only adds to the evidence and general understanding of induced demand, which more transportation agencies are recently coming to terms with.

Photo Credit: Omar Ramadan via Pexels, unmodified. License