ANTIOCH -- A project to widen Somersville Road is about to go from inching forward at a snail's pace into the fast lane.

The major thoroughfare on Antioch's western edge will be closed for about four months starting May 15, allowing it to be widened from two to four lanes. Traffic will be diverted down Contra Loma Boulevard, city officials said.

"The goal is to get it done as quickly as possible, so there's as little impact to local schools as possible," said Ron Bernal, Antioch's public works director.

Somersville will be closed from the Contra Costa Canal south of Buchanan Road to near James Donlon Boulevard by the entrance to Black Diamond Mines Regional Park.

Work is slated to be completed by Sept. 1. Signs have been put up on local roads to give drivers ample time to prepare, Bernal said.

The construction is being handled by Discovery Builders as part of a 2003 development agreement between the city and homebuilder for the Black Diamond Estates subdivision.

The improvements will include adding bike paths, a sidewalk on the west side of the road, a landscaped median and lighting.

"It won't be to the old county standard, and it will allow traffic to move much more freely, be safer, and improve access to Black Diamond Mines," Bernal said.

The construction will also include a traffic signal at James Donlon and Somersville, an improvement Black Diamond Estates residents have asked for at city meetings the past few years.

The project, which has had to overcome several disputes over the years over costs, includes constructing a wider pipeline and road crossing for Markley Creek.

While waiting for it to happen, Antioch has made short-term asphalt fixes on that stretch.

Antioch and Discovery, an affiliate of Concord-based A.D. Seeno Construction, agreed earlier this month to the 105 days of closure, allowing a December deadline to be met.

Also included in the pact was an accord that gives Discovery the option not to build underground pipes for its Tuscany Meadows planned subdivision on a Seeno-owned former Chevron property but rather use an open earth ditch until the project is developed.

Antioch also agreed not to contest annexation of that property into Pittsburg, but it can challenge the project. Discovery, in return, will not challenge Antioch's recent annexation to its northeast, and not challenge the city's recent denial of its The Pointe project.

Antioch will also pay Seeno about $446,000 from its water fund to resolve a water tank fee credit dispute, and receive $72,000 from Seeno for Markley Creek work.

Discovery officials did not return calls this week seeking comment.

For information and updates on the project, go to the city's website www.ci.antioch.ca.us or call 925-779-7050.

Contact Paul Burgarino at 925-779-7164. Follow him at Twitter.com/paulburgarino.