Find yourself wondering whether your headlights are out, only to discover they've been working the whole time?

It might be because the plastic covering the bulbs has become clouded or yellowed.

In fact, at an average of 11 years old, the typical vehicle on the road today is generating only 22 percent of the light that it did when its headlights were new, according to a new study by AAA.

"We were really shocked by the amount of deterioration in light output that we saw," said Greg Brannon, AAA's director of automotive engineering. "It's a major safety concern out on the roadways today."

The automotive safety and insurance firm is advising owners of older vehicles to get their lights replaced to achieve maximum performance.

Restoration helps and is advisable if vehicle owners can't afford to replace their lights, which can cost several hundred dollars. But restoration, which costs less than $80 professionally, doesn't return the lights to their original output.

The problem is that new headlights have a coating applied to their plastic casings to protect them from sunlight – and that coating wears off over time, causing the casing to go from transparent to translucent.

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"Picture putting a light bulb in a container that has a lampshade pulled over it," Brannon said. "That's what we're talking about."

Cars that experience a lot of sunlight are most vulnerable. For example, a car parked mostly outside in Florida or Arizona will likely begin to deteriorate in about three years. Other vehicles begin to deteriorate in about five years, Brannon said.

Part of the problem is that many drivers don't notice the issue since their lights degrade gradually over time.

Options for rehabbing headlights, based on two popular sedans:

Total replacement with parts from automaker: $331 to $427; achieves 100 percent performance.

Total replacement with certified aftermarket parts: $131 to $259; achieves 90 percent performance.

Total replacement with non-certified aftermarket parts: $104 to $190; achieves 83 percent performance.

Professional restoration: $77; achieves 70 percent performance.

Do-it-yourself restoration: $21; achieves 70 percent performance.

Source: AAA tests

Follow USA TODAY reporter Nathan Bomey on Twitter @NathanBomey.