Photo courtesy of State House News Service.

With roughly 600,000 new residents expected to join the state by 2040 — the equivalent of adding another Boston-sized city — traffic congestion will likely get worse unless action is taken to address it, the head of a state transportation commission said Wednesday.

“With respect to the highway system in particular, the commission expects that keeping congestion at reasonable levels will rely more on the efforts to manage the demand for existing capacity than increasing its supply,” Steve Kadish, who chaired Gov. Charlie Baker’s Commission on the Future of Transportation, said at a Transportation for Massachusetts event delving into the panel’s findings.

“While selective highway improvements will be necessary, the era of major highway construction is long gone,” Kadish said. “Congestion mitigation and better transit must go hand-in-hand because neither building nor managing congestion by itself will work.”

As Kadish spoke, just before 9 a.m., the roughly five-mile trip from Somerville to the downtown Boston venue where the event was held, the Massachusetts Continuing Legal Education auditorium, would have taken a driver 33 minutes, or a little more than 9 miles per hour for a route that’s mostly highway.

A 12-mile drive from Braintree was projected at 47 minutes, and the 22 miles from Framingham would have taken just over an hour.

At the same time, Red Line commuters were grappling with the crowding and after-effects of earlier delays caused by a disabled train taken out of service at JFK/UMass and a signal problem at Davis. Disabled trains and signal problems are common occurrences.

The report, unveiled last Friday, makes 18 broad long-term recommendations, including to “prioritize” investment in public transit, provide better mobility options in rural areas, develop a strategy to support autonomous vehicle development, and enable electric vehicle charging infrastructure across the state.