SEATTLE - Everyone feared it would be a nightmare, but the Monday commute on Interstate 5 turned out to be a dream. But transportation officials said Tuesday's commute could be a nightmare if drivers don't stick to their alternate routes and transportation.

Transportation officials breathed easier on Monday as about half the average number of motorists drove Monday morning on a mile-long stretch of the highway's northbound lanes south of downtown, where a 19-day construction project is under way.

Washington Department of Transportation engineers estimated 3,300 vehicles an hour traveled through the construction area during the morning commute.

"So far, so good," said Paula Hammond, the state's interim transportation secretary.

The Monday evening commute was fairly light as well and flowed "pretty smoothly," despite an evening Seattle Mariners baseball game at Safeco Field, just south of downtown, Transportation Department spokesman Travis Phelps said.

"Drivers did their part and avoided construction on I-5," he said.

Even a couple of stalled vehicles and one minor collision in the work zone caused only minor backups in the late afternoon, Phelps said.

But while Monday's commute appeared to be a mostly easy one, transportation officials urged drivers not to assume that traffic during the rest of the project would be as smooth.

The big worry is that people who stayed home today will see that the commute wasn't too bad, causing a crush of extra drivers on area highways during the next few days. They've dubbed this the "boomerang effect."

"We are light today, but if a lot of people change their mind about their route and come back to I-5 we could have some congestion tomorrow," Hammond said. "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."

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

"We still need half of the drivers that normally use northbound I-5 to take an alternate route or ride the bus, car pool - anything that they can do to help us," said Laura Johnson with the Department of Transportation.

Construction work began late Friday night on one of the most heavily used stretches of I-5 in the state. Crews are repairing and replacing expansion joints and repaving sections of the road.

Monday morning, thousands of workers switched their schedules, took vacation, worked from home or hopped on buses or trains rather than risk getting caught in backups some transportation officials feared could stretch as long as 30 miles.

It was standing room only on some of Sound Transit's Sounder commuter trains. The first train of the day shuttled nearly 2,200 passengers from Tacoma to Seattle, more than double last week's average, said Linda Robson, a spokeswoman for the regional transportation agency.

All told, nearly 5,900 people rode five Sounder trains from the south into the city - including an extra train added for the construction project. Last week, the average ridership on four trains from the south was just over 3,600, Robson said.

King County Metro's water taxi from West Seattle across Elliott Bay to downtown set a record with 191 passengers on the third sailing of the morning, compared to 55 on a typical morning run.

Many of Metro's buses were fuller than usual, especially before 7 a.m., and while they stayed full throughout the morning commute, the agency reported no significant delays.

Nearly 140 buses Metro diverted off I-5 averaged delays of less than 10 minutes, and no standby buses were needed, said Kevin Desmond, the agency's general manager.

Traffic reporters for local television and radio stations said cars were traveling around 45 mph Monday morning, the posted speed limit through the construction area.

Phelps said traffic in the area was moving at about 40 mph around 5:30 p.m., close to the peak of the evening commute.

Drivers avoiding I-5 ran into more congestion than usual on suggested alternate routes, including U.S. 99 and Interstate 405.

Transportation officials were hoping Monday's smoothness doesn't prompt masses of drivers to give up telecommuting, mass transit or whatever they were doing besides driving on I-5.

"We are worried that drivers are going to get a little complacent and say, 'Hey, this wasn't that bad,' ... and everyone's going to hit the road Tuesday or Wednesday," Phelps said.

Construction on I-5 is scheduled to continue through Aug. 29.

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