A plan to widen Interstate 5 in the Rose Quarter could produce thousands more tons of greenhouse gas emissions a year, according to a local think tank.

If enacted, the Oregon Department of Transportation (ODOT) Rose Quarter Improvement Plan will add two additional lanes to I-5 in the Rose Quarter area, equaling about 1.6 miles of new freeway lanes. According to analysis by Portland city planning think tank City Observatory, that could lead to 17.5 million additional vehicle miles—that is, the amount of miles driven by cars in a particular area—each year.

That 50 percent jump in miles driven on that section of I-5 could create between 4.7 and 7.9 thousand tons of new greenhouse gas emissions each year, City Observatory estimates.

City Observatory’s numbers greatly clash with ODOT’s own calculations.

In an environmental assessment released last month, ODOT predicted that the I-5 expansion would actually lower greenhouse gas emissions in the long run, because cars would be able to move through the Rose Quarter more quickly.

Freeway expansions generally do not succeed in reducing traffic or lowering greenhouse gas emissions because of a concept called “induced demand”—the observed phenomenon that when you create more space for cars, more cars will quickly fill that space.

Those who oppose the expansion, including No More Freeway Expansions organizer Aaron Brown, say ODOT didn’t back up its prediction with sufficient data, making it difficult to check the agency’s work.

“Our coalition would love to double check ODOT's arithmetic on these findings to confirm the agency isn't putting a thumb on the scale when conducting this analysis,” Brown told the Mercury after the report came out, “but we are unable to do so, since data ODOT used to conduct their traffic analysis are puzzlingly not included in the [environmental assessment] document.”

The City Observatory report is authored by Portland urban economist Joe Cortright. To gauge the I-5 expansion project’s impact, Cortright used a calculator created by researchers at the University of California, Davis (UCD), that uses data from previous freeway widening projects to make predictions about new expansions. UCD’s calculator is based on California data, so Cortright used findings from it to extrapolate what a freeway widening might mean for the Rose Quarter.

Cortright wrote that ODOT’s assessment "flies in the face of decades of experience and widely published research showing that, with great predictability, more freeway capacity generates proportionately more traffic, traffic congestion and pollution."

The public comment period for ODOT’s I-5 expansion project is now open.