AP Traffic from the Beltway, left, merges with traffic from I-270, center, during evening rush hour in Potomac. (Manuel Balce Ceneta)

WHEN GOV. Larry Hogan (R) last year proposed a $9 billion blueprint to widen the Beltway, Interstate 270 and the Baltimore-Washington Parkway by building more than 100 miles of toll lanes — potentially one of the most audacious public-private partnerships in the nation — the project was attacked as politically opportunistic, a boondoggle that would wreck neighborhoods and harm the environment. Nonetheless, Mr. Hogan’s plan has advanced, impelled by one undeniable fact known to hundreds of thousands of daily commuters who crawl along those roadways day after soul-crushing day: Suburban traffic long ago outstripped highway capacity.

It will get much worse in the coming decades. The Washington region is expected to grow by 1.5 million people by 2045, to nearly 7 million. No matter how much new investment is poured into transit — we hope it’s a lot— demand for new roadways will soar. If you doubt that, think of Uber, Lyft and self-driving cars, and consider this: Car usage nationally is expanding at roughly twice the rate of population growth.

That’s one reason Mr. Hogan’s plan, or some scaled-down version of it, makes sense, depending critically on the fine print of any deal the state strikes with a private-sector company that would design, build, finance and operate the roads in return for decades of toll revenue. Supporting evidence can be found in the Virginia suburbs, where 55 miles of toll roads have been built since 2012, with 40 or so more coming online in the next four years. There, commuters (a third of them Marylanders) make more than 100,000 daily trips in the high-occupancy toll lanes on the Beltway and Interstate 95, where pricing varies according to demand to keep traffic flowing at 65 mph. An additional 10,000 or so pay daily to use the HOT lanes on Interstate 66 inside the Beltway.

Critics of Mr. Hogan’s proposal, still being shaped by state officials, point to Virginia’s high rush-hour tolls. But no Marylanders would be forced to pay tolls on the four new lanes the governor has proposed; existing lanes on all three highways would remain free. And on Virginia’s I-66, where high tolls have sparked outrage, critics forget that the drivers who must pay them — those without passengers during rush hour — were completely barred from the road before tolling was introduced.

Still, Maryland officials must proceed cautiously. Unlike Virginia, which put upfront public funds into the project — and got an excellent deal in return — Mr. Hogan has ruled out any state investment. That trade-off would likely mean higher tolls and a scaled-back project — for instance, one lacking tunnels, bridges or other features that would avoid razing businesses, houses and apartments along Maryland’s portion of the Beltway, which runs in a narrow corridor through dense neighborhoods. And Maryland officials, eager to maximize toll revenue, seem reluctant to follow Virginia’s example of letting carpoolers drive free in the toll lanes. That could compound already heavy usage by solo drivers in a pattern known as “induced demand” — not a good outcome for the environment or for traffic.

If Maryland is smart about striking a deal, it could shift much of the risk of building, operating and financing a major infrastructure project to a private firm. The devil, as usual, is in the details.