New York City adopted congestion pricing earlier this month, hoping to reduce traffic congestion and encourage greater use of public transportation by charging drivers a fee to enter Manhattan.

With fewer vehicles on the island, will the new policy have an environmental impact by reducing emissions? A researcher at The University of Iowa Tippie College of Business has studied trucking emissions in cities with similar congestion pricing policies and found it doesn’t always lead to more environmentally friendly results.

Ann Melissa Campbell, a professor of business analytics who studies the transportation industry, says trucks that enter the congestion zone tend not to leave the restricted area once they’re in it because they would have to pay the fee again to re-enter. Instead, they stay in the zone for extended periods of time to make all their deliveries or pick-ups in one trip. So many trucks in the zone at once produce emissions that minimize any air quality improvement.

Campbell analyzed data from Berlin, Stockholm, and Milan, among other cities that have congestion pricing policies. Her other findings include…

• larger fees have more impact than smaller fees on limiting vehicle access to the congestion zone
• the fees have a bigger impact in larger cities than in smaller cities, to the point they may be a particularly bad choice for smaller cities because the impact is minimal for the amount of driver inconvenience.