A simple engineering error led to the $1 million repair of the eastbound state Route 16 offramp to Sprague Avenue, top state highway officials told legislators Thursday.

Transportation Secretary Paula Hammond told members of the state Legislature’s Joint Transportation Committee that failures in the state’s oversight of the project allowed the misplaced off-ramp to be built.

“We slipped up on this one,” said Hammond, who was joined at the meeting in Redmond by Deputy Secretary Dave Dye and Olympic regional administrator Kevin Dayton.

After the meeting, Dayton said he had completed his draft investigation into the mistake – which left the offramp too tall to link up with a bridge leading to Sprague Avenue. He’ll forward the report on to Hammond.

“This should never have been an error,” Dayton said. “Someone should have designed it right and someone should have reviewed and caught it.”

Dayton gave this recap of what happened:

In the 2001 design plan, eastbound state Route 16 was to be two lanes and the Sprague offramp was designed accordingly.

In 2007, new traffic standards extending to 2030 were adopted and a third general purpose lane was added to meet higher traffic forecast.

“We revised the profile for the ramp and didn’t do it correctly,” Dayton said.

The Sprague Avenue offramp has two “gores” – the places on the highway where the exit lane separates from the mainline. One is denoted by painted lines; the other is a barrier.

Dayton explained that the elevation of the lane at both gores should have been the same. Instead, the barrier was 18 inches higher.

“They didn’t hold the elevations.” Dayton said. “That was the engineering error. If it had been held there would have been no problem at all.”

The mistake might have been spotted during a review had the Sprague offramp exit stayed a part of the eastbound state Route 16 Project, Dayton said.

Instead, sometime in 2007, the exit – complete with its engineering error – was moved to the westbound state Route 16 project, he said.

The move was done at the City of Tacoma’s request that the Sprague Avenue exit open next year when the westbound project is scheduled to be done in 2011, he said. Otherwise, the exit would have remained closed until 2014.

Someone in the design office in 2007 did see the elevation problem and noted it in eastbound plan review.

Dayton said the staffer expected the elevation differences to be caught during the eastside review project.

Hammond told the committee that the design of the project took from 1998 through 2007. During that time, she said, two key people, including the project manager who was promoted, were transferred, leaving a gap in oversight.

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