The following report by Team 4 investigator Jim Parsons first aired on Channel 4 Action News at 5 p.m. on March 2, 2006.

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A Team 4 investigative report revealed that western Pennsylvania has more than 500 bridges with weight limit signs on them -- which means they can't hold the load they were designed to carry.

Now, the city of Pittsburgh has a plan to fix its worst bridge -- but it's still years away.

Just how weak is the McArdle Road Bridge on Pittsburgh's South Side?

Consider this: It has the same posted weight limit -- three tons -- as the one-lane wooden bridge on Creek Road in Hanover Township, Washington County.

And six months ago, that bridge collapsed when an overweight truck rumbled across.

It has since been rebuilt.

"We are keeping a close eye on it," Costa said.

Costa said the McArdle Road Bridge cannot withstand what Team 4 captured on video -- overweight vehicles, like a school bus and a delivery truck, driving across.

"If we have a problem with trucks continuing to use that bridge, we will have to shut it down -- make it one lane, one direction. The other option is to close it down altogether until we can undergo reconstruction," Costa said.

But that won't happen for two more years.

"Right now, the bridge is under design. The design work should be done and bids ready for reconstruction should be late 2007, early 2008," Costa said.

Unfortunately, there is no such plan to make repairs to the Greenfield Bridge over the Parkway East, which has been dropping chunks of concrete now for the past couple of years.

"There's no plans to do anything with the Greenfield Bridge until at least 2010," Costa said.

Until then, city engineers will continue to rely on a net and a temporary roof over the Parkway to protect motorists from falling concrete.

They were erected two years ago after a piece of debris fell from the bridge and crashed through a car windshield, critically injuring the driver.

"Structurally, it's safe. It's just the columns and piers. Every once in a while, concrete has a tendency of breaking loose, and then we knock it down before it goes onto the net or bridge," Costa said.

McArdle Road is the worst city-owned bridge in Pittsburgh, but the weakest bridge owned by the Pennsylvania Department of Transportation in the area is Boulevard of the Allies Bridge in Oakland.

That is scheduled for replacement this spring.