A case study of how traffic would change if a nominal toll were added to Girard Point Bridge in Philadelphia illustrates the trade-offs: PennDOT could raise significant revenue, but traffic time would increase — as would emissions.

The study, done for the Delaware Valley Regional Planning Commission and funded with a grant from the U.S. Department of Transportation, estimated a $1 toll on the bridge would bring in about $30,500 per day and $46,700 per day with a $2 toll during morning peak hours. Annually, those tolls would be $11.1 million and $17 million, respectively…

“We’re talking about how to make a good compromise among different stakeholders,” Sean Qian, a civil and environmental engineering professor at Carnegie Mellon University and co-author of the study, said. “PennDOT, I do see their perspective because it’s important to have extra revenue to fix the infrastructure. Especially for Pennsylvania, it’s a big challenge.”