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Traffic Congestion
Public transit is perceived by the general populace as an effective method of combating traffic congestion. In a survey conducted by the city of Colorado Springs in 2011, 77 percent of respondents believed that congestion relief was a "Very or somewhat important" benefit from public transit. These results were replicated in a 2012 survey conducted by the Natural Resources Defense Council in 2012, where 73 percent of respondents said that traffic congestion relief was a "Very or somewhat convincing" argument to expand funding to public transportation. The Lane Transit District also found that the most popular rationale for supporting a 2012 expansion to its bus network was congestion relief.

The actual relationship between traffic congestion and public transportation is much more complicated, with no established academic consensus on the issue. Several studies have found that public transportation has no substantial effect on traffic reduction, whereas other studies have found a positive relationship between congestion reduction and public transportation investment. One potential cause for the mixed results is the heterogeneity of the cities and their public transportation being analyzed. In a 2015 literature review conducted by Beaudoin et al, the researchers found that the type of transit and the city being analyzed impacted the conclusion reached.

The largest factor limiting public transportations impact on traffic is induced demand, the phenomenon where increasing supply simultaneously increases consumption. In the context of traffic congestion, the concept of induced demand would suggest that any driver who opts for public transportation in place of automobiles would be replaced by another driver. This theory is supported by a 2018 study which found that "a 10% increase in transit capacity is associated with a 0.4% increase in auto travel."