Builders have started work on a mega-project to widen the Seward Highway from Tudor to Dowling roads and elevate the highway to make room for Campbell Creek Trail underneath.The $40 million job will lead to more than two years of traffic disruption, including a detour off the highway for southbound travellers beginning in mid-July and likely continuing through the winter.

But afterward, the expansion should cut down on rush hour traffic jams on the highway, and ease the flow on other crosstown routes like Elmore and Dowling roads as well, said state project manager Chong Kim.

The Seward Highway is the most trafficked road in the state. There were 76,000 vehicles per day, on average, when vehicles were counted for this project in 2005, Kim said. By 2035, 92,000 cars and trucks are expected daily.

The federal government is mostly paying for the expansion, he said.

The project consists of:

• Expanding the highway from four to six lanes.

• Building new highway and frontage road bridges over Campbell Creek and connecting the trail underneath.

• Adding pathways along the frontage roads.

• Realigning interchanges at Dowling Road and Tudor Road to match the expanded highway.

This is the first phase of a three-part project to widen the highway to six lanes all the way to DeArmoun Road, said Kim, of the Alaska Department of Transportation.

The builders plan to accomplish the two-mile, Tudor-to-Dowling section by summer 2014.

They'll first finish Homer Drive, then divert southbound highway traffic to it at mid-summer this year, Kim said. That will clear the southbound lanes for construction.

The speed limit will drop from 65 to 55 mph for the detour, but there will be the same number of lanes and traffic is expected to flow freely, Kim said.

"We're sacrificing Homer Drive to provide the same level of service."

All but two of the Homer Drive businesses can use the Old Seward Highway for access; the state will build access from International Airport Road for them, he said.

When they're ready to work on northbound lanes next summer, transportation officials probably will detour city-bound traffic to the completed southbound lanes, Kim said.

Raising the highway and both frontage roads about eight feet to extend Campbell Creek Trail underneath is a major piece of the project, and will create a long-awaited trail segment. The trail, one of two that cross the heart of town from east to west, is bisected and disconnected by the highway and its frontage roads.

Until now, there's been only an unofficial, cramped trail along the creek under the highway. Kim estimated it at five-feet high.

When the road project is done, there will be a 10-foot-high lighted space under the highway, and equally high tunnels under the frontage roads, he said.

"It's the most talked about accessory in the trail system," said Lori Schanche, a city project manager who deals with trails.

"This will be a jewel when they get it done," said Damian Schroder, a GIS developer who rides his bicycle from home near Tudor Elementary School to work west of the highway on International Airport.

Kim said the bridges over Campbell Creek will cost $15 million to $20 million.

The highway will gradually rise beginning 400 feet on either side of the bridge, Kim said. The bridge itself will be 146 feet long, he said.

Project officials will hold an open house from 7 to 9 p.m. Thursday at the Dimond Center Hotel.

Open house: Scheduled at the Dimond Center Hotel (700 E. Dimond Blvd.), 7 to 9 p.m. Thursday.

Road closure timeline:

• Homer Drive, a frontage road west of the highway, is closed from Tudor Road to north of International Airport Road beginning Wednesday.

• In mid-July, southbound lanes of the Seward Highway will close between Tudor and Dowling roads. Traffic will divert to Homer. This will lasts all winter.

• In summer of 2013, work begins on northbound lanes of the Seward Highway and on Brayton Drive frontage road east of the highway.

• Summer of 2014, project completed.