The state’s top transportation official said Friday that all 18 spotlights on the northbound approach to the Tobin Bridge will be replaced, following an inspection after one of the light fixtures fell into the roadway.

After the inspection, the state removed three of the lights that showed early signs of the corrosion that likely caused the other to fall, motivated by what Secretary of Transportation Richard A. Davey called “an abundance of caution.”

The state also took down four other fixtures that appeared secure but had burned-out bulbs that could not be replaced because the covers were rusted shut, Davey said. The 10 left will remain in place until the state can complete a replacement for all 18 in the coming year, likely at a cost of less than $100,000, he said.

Davey emphasized that the problem was isolated and that the flawed fixtures were not the primary lights on the vast Mystic River bridge. Instead, they were a set of spotlights illuminating the viaduct carrying US Route 1 northbound traffic out of Boston — emerging from underground below Charlestown’s City Square — and onto the lower deck of the Tobin Bridge.

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“I just want to reassure the public that the bridge is safe,” Davey said. “Given our inspection last night, we believe this light that fell was in fact the only one that had such a substantial wear that it would fall.”

The fallen light, which weighs between 10 and 15 pounds and is roughly the size of a coffee-table book, was discovered by a state trooper on the side of a travel lane near the shoulder at about noon Thursday. It did not appear to have hit any vehicles, officials said.

That very light had been flagged and photographed for an inspection report last September, with state bridge engineer M.P. Griffin noting corrosion and a crack where that fixture was attached to a vertical support pier, possibly due to age and salt air. The problem was rated “6 m-p,” a code meaning it was not considered an immediate safety hazard but should be monitored and replaced or repaired when money is available, officials said.

“This particular light was noted as needing to be observed, but not to the extent we thought it would fall,” Davey said, calling it a reminder “not only that we have to be vigilant but that we have to continue to invest in our transportation system.”

The 18 light fixtures and the viaduct itself date to the early 1990s, after the tangle of elevated ramps blotting out City Square were submerged in what was known as the CANA (Central Artery North Area) project, a companion to the Big Dig. The Tobin itself is 62 years old.

The so-called CANA viaduct is inspected every other year, as are all state bridges and elevated highways not rated structurally deficient, a list of about 500 deteriorated bridges inspected more frequently. The process is thorough, Davey said, with the viaduct inspection taking 58 hours and the full Tobin inspection lasting more than a month. They will be fully reinspected again next year.

In addition to flagging the lights, Griffin’s 2011 report noted that conduits — narrow steel pipes — carrying utility wires over northbound traffic were loose as a result of worn or broken support straps. Those were repaired Friday morning, officials said.

Davey said the fallen light was completely unrelated to the eight-foot, 110-pound light that fell in the Thomas P. “Tip” O’Neill Jr. Tunnel in February 2011, triggering an inspection that resulted in the need to replace thousands of potentially flawed fixtures throughout the Big Dig tunnel system. The state did not reveal that problem to the public for more than a month, and the speed and nature of that disclosure as well as the internal response embroiled Davey’s predecessor and the department in controversy.

Davey said he was unhappy the light fell but pleased with the response. “Our team went out and immediately did the inspection. I’m pleased that we’re here talking with the public and letting them know what we’re doing about it,” he said. “One thing that we continue to maintain at the governor’s behest . . . is complete transparency with the public, given what occurred last year.”