“It just repels you,” said Hayes Morrison, Somerville’s director of transportation and infrastructure, shuddering at the thought of walking beneath the eroding span, which carries the McGrath Highway (Route 28) over surface streets.

Of course, the 1950s planners who built the overpass paid little heed to the people who might walk, bike, or reside in its shadow. Urban neighborhoods were meant to be leapfrogged by suburban commuters; road builders fixated on blasting away bottlenecks.

Now that the McCarthy is falling apart, even the state Depart­ment of Transportation agrees it is an overbuilt vestige and has promised to take it down. But the contractors who mobilized beneath it recently are not there to dismantle it. ­Instead, the state is reinforcing the McCarthy, spending $10.9 million to keep it standing for a decade or more.

Transportation officials say that will buy them the time needed to develop replacement plans and secure $70 million for a network of landscaped surface streets and sidewalks, with crosswalks, improved inter­sections, well-timed traffic lights, and bicycle lanes.

Advocates for removing overpasses call it throwing good money after bad. But the state’s top highway official said it would be a mistake to remove the McCarthy without being ready to follow up with re­designed surface roads.

“It’s not as simple as just remov­ing the overpass,” said Frank DePaola, the Department of Transportation’s highway ­administrator. “We’d end up with much worse traffic congestion.”

A similar course is planned next year with the Bowker Overpass, the elevated highway that splits the Back Bay from Kenmore Square and casts a shadow over Frederick Law ­Olmsted’s Emerald Necklace. More fraught than the ­McCarthy, the Bowker connects Storrow Drive with the Fenway and Longwood Medical Area, and spans the Massachusetts Turnpike and railroad tracks, as well as the park and the neighborhood.

So the state will invest $10 million in that structurally deficient span, as well, buying time to analyze traffic and contemplate whether to rebuild the Bowker in part or in full.

Those who want to see the McCarthy and Bowker come down worry the repairs will ­allow the state to drag its feet on more substantial work.

“We want to see these projects move to ‘shovel ready’ and not just shored up and then left behind,” said Jackie Douglas, executive director of the ­LivableStreets Alliance , a nonprofit that promotes walking, biking, and mass transit.

Winning widespread support for such projects can be challenging, judging from two other campaigns to address decay­ing vestiges of mid-20th century planning.

Consider Jamaica Plain’s Casey Overpass, which channels the Arborway over Forest Hills. In March, the state said it would raze the overpass and ­replace it with surface streets, following months of meetings with a local committee. Neighbors who believe the community is being railroaded have vowed to fight the project.

In Charlestown, the city’s ­attempt to “boulevardize” the Rutherford Avenue corridor along the neighborhood’s western edge has caused continued hand-wringing over how dramatic the change should be and whether it would backfire and worsen traffic congestion, even after four years of meetings.

Each case is laced by competing undercurrents that have long coursed through local history, with Boston both a seat of innovation and a place resistant to change.

Forty years ago, the region was a pioneer in rejecting highways as the best way to revitalize stagnating cities. Governor Francis W. Sargent disavowed his prohighway past and rejected the Southwest Expressway and the Inner Belt. The Southwest Expressway would have barreled an interstate straight through Jamaica Plain and Roxbury to downtown Boston; the Inner Belt would have cut through Roxbury, the Fens, Brookline, Cambridge, and Somerville with an eight-lane highway.

Sargent also canceled plans for other urban highways and called for massive investment in transit and commuter rail ­instead.