SEATTLE -- The Transportation Department says the big Interstate 5 construction project in Seattle is five days ahead of schedule. But traffic congestion may still get worse.

Based on the pace of the project, transportation officials planned to move into the third phase on Thursday, and crews shut down another northbound lane just after 8 p.m. Wednesday night in preparation for the switch.

But late Wednesday night, officials decided to hold off the switch until Thursday night and reopened the lane for the Thursday morning commute.

"We basically bit off a little more than we can chew," said Megan Soptich with the Department of Transportation. "We pushed our crews really hard today, and we're concerned that we won't be able to get the work fully completed by tomorrow (Thursday) morning."

The 19-day project is on track for completion in 14 days, by August 24th. Since Friday night crews have been replacing expansion joints on a mile-long stretch of the northbound lanes between the West Seattle bridge and Interstate 90.

Transportation officials were pleasantly surprised that the traffic congestion on the first three commuting days was not as bad as feared. They had warned for months that the work could back up traffic all the way to Tacoma. Commuters listened, and many have been choosing alternatives.

Officials had worried about what they call the "boomerang effect," where drivers would become complacent after seeing the easy first few weekday commutes, and everyone would go back to their old routines.

They fear it could still happen, especially with the early closure of the additional lane.

"It's only going to take one day where people choose to go back to their old habits for us to have a mess out here," said Paula Hammond, the state's interim transportation secretary.

On a normal weekday about 126,000 drivers use the stretch of I-5 that is being repaired.

If the repair work is completed early, the contractor will receive $100,000 for each day finished ahead of schedule.

For More Information:

Check our online traffic section for continuing coverage of the I-5 construction project, get your questions answered by KOMO traffic reporter Jenni Hogan, and check travel times, live cameras and more:

http://www.komotv.com/traffic