My next highest priority would be expanding Metro. Metro itself should be expanded everywhere, including the Red Line to Germantown and Clarksburg. There should be hundreds of stations in the Washington-Baltimore region, as well as around the country.
Additionally, we should make a high-speed rail and mass transit infrastructure supplemented with transit oriented development. I believe this would help solve a lot of problems simultaneously, like congestion, global warming, equity, and public safety. Chinese President Xi, an engineer, developed an immense high-speed rail network in just a decade. America should be able to as well.
In Maryland, this would include building a high speed rail line with exactly 5 stations:
Hagerstown -> Frederick -> Gaithersburg -> Bethesda -> DC
This would immediately cut down congestion along I-70 & I-270 and can be expanded into a larger regional network, including a line across the Potomac river towards Dulles and beyond, as well as going across the state to BWI Airport and even across the Chesapeake and into Ocean City.