Evening rush-hour traffic on the Capital Beltway in Silver Spring. (Sarah L. Voisin/The Washington Post)

IT IS FOLLY to consider projected growth in the Washington region’s population — more than 1 million new jobs and people by 2045 — and conclude that the existing, badly congested road network will be adequate. Improving and expanding mass transit options will be essential. But so will improving and expanding major highways.

Yet local elected officials and some activists are inviting Marylanders to descend into such folly as the state prepares to move ahead with Gov. Larry Hogan’s proposal for a public-private partnership to widen the region’s two most important commuter arteries: the Capital Beltway and Interstate 270. They imagine that transit alone can accommodate those million-plus new residents — never mind that transit ridership is declining or that vehicle miles traveled are soaring.

If suburban residents think traffic is bad now, let there be no delusions about the future without major road improvements: It will be unimaginably worse, no matter how many tens or hundreds of millions of dollars are invested in better subway, bus and commuter-rail service. That will deepen the day-destroying commuting travails of hundreds of thousands of area residents.

Make no mistake: This region should invest in better transit. Metrorail ridership is plummeting partly because subway service became less reliable (though it now is on a gentle upswing). Metrobus passengers are opting out because payment systems are balky, routes obsolete and travel times sluggish. Without service upgrades, those riders won’t come back.

In fact, Maryland has been making transit strides under Mr. Hogan, a Republican who was instrumental in greenlighting the Purple Line light rail project and in finding hundreds of millions of dollars from the region to improve Metrorail. Local elected officials in Montgomery and Prince George’s counties who oppose his plan to widen the Beltway and I-270 conveniently never mention that. In demanding better transit but no major road improvements, they ensure a future defined by failure and economic anemia.

Maybe in a perfect world, no one would drive cars and no new roads would be needed. We don’t live in that utopia. The fact is that vehicle miles traveled are increasing faster than the rate of population growth in this country. If politicians think that can be reversed, or that Uber and Lyft will somehow become obsolete, they haven’t talked to a young person lately.

Yes, widening the Beltway and I-270 may entail razing a few dozen homes (and compensating the owners). That is the cost of improving the daily commute of hundreds of thousands of commuters.