State officials said today that workers have removed three light fixtures on the Tobin Bridge because of concerns about the wear on them after a fallen fixture was found Thursday on the bridge’s lower deck.

“I think it reminds us not only that we have to be vigilant, but we have to continue to invest in our transportation system,” said Transportation Secretary Richard A. Davey.

Another four fixtures were removed because the screws were rusted shut on the plastic lid that covers the lightbulb and the bulb could not be replaced, said Transportation Department spokeswoman Cyndi Roy. But there were no concerns about the structural integrity of those four, she said.

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There are a total of 18 lights of the same type on the bridge. Ten were left in place, she said.

Roy also said that three conduits — steel pipes that carry the wires to the lights — showed some wear and their structural supports had worn down. Workers were putting up new supports this morning, she said.

The fixture, which weighs between 10 and 15 pounds and was described as the size of a pumpkin, fell about 15 feet from a bracket underneath the Tobin’s upper deck onto an approach ramp to Route 1 in Charlestown. A state trooper spotted the fixture around noon Thursday. It wasn’t clear when it came down, the Globe reported today. Officials said corrosion appeared to be the cause.

“I’m surprised it fell and thankful it didn’t fall on any car or even cause an accident,” said Davey.

He said all of the lights will be replaced in the next year at a cost of about $100,000.

The bridge undergoes biennial inspections. It was last inspected in September and was due to be inspected next year, officials said.