Mayor Walsh’s big, but incomplete, plan for Long Island

The Long Island bridge was closed in October 2014 and later demolished. Matthew J. Lee/Globe Staff/File

MAYOR MARTIN J. WALSH’S plan to rebuild the bridge to Long Island and create a new addiction recovery campus there may be a sound idea — even an inspired one. But the administration needs to do its due diligence before embarking on a project of that scale and ambition, fully exploring alternatives and making a sharper case for an investment that could top $100 million. Long Island has a history of serving the vulnerable. For years it hosted a homeless shelter and addiction recovery programs. Then, in October 2014, the city abruptly condemned the rickety bridge connecting the island to the mainland.

Over the next few years, officials focused on relocating displaced programs in and around Boston. But now that the transition is complete, the question of what to do with Long Island — where the city still pays to maintain its unused buildings — is front and center. There are all sorts of possibilities; with its sweeping views, it could be a prime recreation spot, served by ferries. But Walsh, a recovering alcoholic who once led meetings for fellow addicts on the island, envisions a brand new suite of recovery services to complement what’s already on the mainland.

For years, he said in a recent City Hall interview, Long Island offered addicts a unique opportunity to escape the temptations of the city and heal — and it could do so again. “The island is special,” he said, “because it can be viewed as a therapeutic community where you’re out of the chaos of your life. . . . If you’re an alcoholic, you need to get out of that barroom, if you’re a drug addict, you need to get out of that crack house, if you’re a heroin addict, you need to get out of that place where you shoot [up].”

It’s a compelling vision — and a welcome one, in many respects, from a mayor who is sometimes criticized for thinking small. But given the price tag — $40 million to $100 million for the bridge and untold sums for fashioning a new campus on the island — vision isn’t enough. Taxpayers need a clearer sense for how the city would pay for the campus — and what projects would be sidelined to make it happen. The opioid crisis is a big priority for the city, but it’s not the only one.

We also need to know if there are less expensive sites for a recovery campus. The mayor acknowledges the city hasn’t done an analysis of alternatives. City taxpayers need to know there is sound medical research supporting the value of an isolated treatment facility, not just the mayor’s hunch. And if Long Island is the best site, the city should also conduct an analysis of whether the recovery campus could be served by boat.

Walsh is quick to point out that past attempts to site recovery facilities in the heart of the city have run into stiff neighborhood opposition. That’s true. But several facilities have sprouted nonetheless. If another, cheaper option in Boston wouldn’t work, perhaps a facility outside the city would — and maybe it could attract the sort of state funding that’s unlikely to materialize for a rebuild of the city-owned Long Island Bridge.

The mayor’s grand vision for Long Island may be worth pursuing, in the end. But there’s a logical next step that needs to be taken: to show it’s the best way for Boston to invest in recovery.