City transportation officials are expecting to start taking bids on next summer’s project to restore two-way traffic to the 500 block of Main Street, with construction to start in the fall of 2013, on what could be a $21 million effort.

The latest project, part of the larger $40 million Cars Sharing Main Street initiative to return vehicle traffic to the downtown spine for the first time in decades, involves the portion of Main Street from Tupper Street to Mohawk Street. It will likely be done simultaneously with converting Pearl Street back to two-way traffic, which will require new lights, road markings and other features.

The $21 million cost of the 500 block will be covered through a range of sources, including $15 million from the U.S. Department of Transportation and $2 million from the city.

There’s also more than $10 million in state funding to continue the initiative further down the street.

The expected start of work next fall would come just as contractors wrap up the conversion of the 600 block of Main Street, which is starting now. That timing is deliberate, so as not to lose momentum, said Buffalo Public Works Commissioner Steven Stepniak.

Plans call for starting the 600 block work around the “portal” where Metro Rail goes underground before winter weather arrives this year and then to resume in the spring as soon as the weather permits.

“We’re excited about the 600 block. Everything’s going on schedule,” Stepniak said at the monthly board meeting this week of Buffalo Place, the nonprofit downtown business improvement district centered around Main Street.

The cost of the 600 block reconstruction, estimated at nearly $8 million, will be paid through $2.5 million in city and other local money, with the rest coming from the federal government.