ORINDA -- New electronic message signs inside the Caldecott Tunnel are confusing and slowing down westbound drivers headed into Oakland on Highway 24.

Caltrans officials say the worst backup and confusion occurred during the Wednesday morning commute, after the five ceiling signs in the third bore were turned on for two weeks of testing.

The testing continues through the end of the week, continuing to present a potential distraction.

When ready for use later this year, the signs will flash information about traffic accidents, stalls, closed lanes or other conditions inside the third bore, where a 1982 gas tanker collision and fire killed seven people and led to changes in American tunnel safety design.

"Anything that is new and that drivers aren't accustomed to is going to take a little time to get used to. There is a learning curve," said Caltrans spokeswoman Ivy Morrison. "We appreciate people's patience for this necessary testing of signs that enhance drivers' safety."

Caltrans began testing the evening of Sept. 24, with messages explaining it was a test.

On Wednesday morning, puzzled motorists slowed, making the westbound Caldecott commute worse than usual.

Caltrans switched to a less confusing message to "drive safely" and the traffic flow appeared to improve, Morrison said.

Some motorists questioned the tests and the value of the signs themselves. "What the blazes will the signs do other than distract?" tweeted Jerome Pandell.

Morrison said during regular use, the signs will be on only when needed to give safety and traffic flow information.

"The signs are intended to distract people and get their attention about essential safety information," she said. "They will be off the rest of the time."

In case of a fire inside the tunnel, the signs will steer commuters to evacuate in new escape passageways excavated between the third and new fourth bore.

In response to complaints that the test messages are scrolled rather than fixed, Morrison said scrolled messages must be tested to ensure that all sections of the signs function properly.

The third bore opened in 1964.

The $402 million fourth bore, expected to open by the end of the year, also has electronic message signs that are being tested but the public can't see those signs.

Caltrans must finish testing on a variety of safety systems before the fourth bore is ready to open.

About 160,000 vehicles a day pass through the Caldecott Tunnel.

Reporter Elisabeth Nardi contributed to this story. Contact Denis Cuff at 925-943-8267. Follow him at Twitter.com/deniscuff.