Take the low road in Allston

Eastbound Mass Pike traffic viewed from the Allston footbridge off Cambridge Street.Pat Greenhouse/Globe Staff

Elevated expressways are toxic for cities. Massachusetts spent two decades and $15 billion fixing the damage that an elevated Central Artery did to downtown Boston.

But all that money and toil taught us nothing. The Massachusetts Department of Transportation is poised to order up a $1 billion new stretch of elevated highway in Allston.

Yes, a decaying Interstate 90 viaduct needs to come down. But the department should rebuild the highway at grade — translation: on the ground — not overhead. This would save $100 million or more, and would be cheaper to maintain, while leaving open the option of building pedestrian paths, bike lanes, and green spaces across I-90. The state, the City of Boston, and Harvard University — which owns the acreage to be freed up by the I-90 overhaul — should foster a dense new residential and employment center and reconnect parts of town that have been chopped up by highway lanes for half a century.

If we don’t make smart investments now, future generations will wonder: What on earth were they thinking?

Two outside parties have proposed plausible schemes to rebuild I-90 at grade. MassDOT should commit to the cheaper of these alternatives — the one proposed by the transportation group A Better City — and then lean on Harvard and the city to help build transit, environmental, and open-space improvements around it.

Instead, the state is following the path of least resistance: replace a costly old hunk of anti-urban infrastructure with a costly new hunk of anti-urban infrastructure.

One reason is that nobody’s in charge of making sure Allston as a whole turns out right. Neither Harvard nor Boston Mayor Marty Walsh has taken command of the discussion. Meanwhile, MassDOT and its leader, Secretary Stephanie Pollack, clearly don’t want responsibility for answering lots of gauzy urban-planning problems. Instead, the state is defining its own task narrowly and tamping down hopes that the highway overhaul will yield piles of money for ancillary projects.

Pollack and her boss, Governor Charlie Baker, may not want to be in the place-making business in Boston. But, like it or not, they are. The I-90 overhaul is simply too big a deal to be a transportation project alone. The choices that MassDOT makes now — not just how to configure the highway, but also when to build West Station, a proposed transit hub in Allston — will determine how the area develops for many years.

On the upside, there’s already plenty of brainpower at work on this issue. In the last few years, an army of planning do-gooders — everyone from a classroom of Northeastern undergraduates to major architecture firms to a team of prominent volunteer designers — have taken a shot at reimagining the Allston project area.

But these independent groups are operating with little official direction and no clear sense of budget constraint — and with no assurance that the powers that be are paying any attention to their work. MassDOT itself did fund a planning study on behalf of the City of Boston, but the city is waiting on Harvard to solidify its plans, and Harvard’s planning happens at a glacial pace.

Harvard, to its credit, recently upped its financial commitment to West Station. It also named a new president, Lawrence Bacow, who gained a reputation as a visionary at Tufts — and could do wonders to move things along in Allston. (Hint, hint.)

Yet the state and the city could do more, too. In other situations, Baker and Walsh have stepped in decisively when they weren’t required to. They worked together to lure General Electric to Boston two years ago. To help ease Amazon’s expansion in the Seaport, and the jobs that will come with it, the Baker administration announced Tuesday the state would kick in $20 million for infrastructure improvements.

Progress only happens when everyone steps up. The state needs to take a more expansive view of its role in Allston, stop pushing for an elevated highway, and commit to the transit improvements that community groups, mobility advocates, and nearby employers are all demanding. And if Harvard, the city, and the state can all agree on a broader action plan for the area, they can have a productive conversation about who pays for what.

Now’s the time to dream big in Allston. Our hopes should be high, but the highway belongs on the ground.