Four construction groups have been shortlisted by Georgia's transportation department to design and build much-delayed toll lanes along I-75 and I-575 in Cobb and Cherokee counties.

The aim of the 30-mile "Northwest Corridor" project is to reduce traffic congestion. The new lanes are expected to open in spring 2018. They would be reversible to accommodate the direction with the most traffic. The toll price, charged electronically, would rise and fall depending on congestion in the main lanes.

This is at least the third time the state has put the I-75/I-575 project out to bid. In 2005, it signed a contract for initial work, but eventually broke off that arrangement. It revised the project and put it out to bid as a public-private project last year, but canceled that bidding.

Starting Friday the four groups will receive new proposal requests about the project, which is expected to cost as much as $850 million. The bidding process will continue through early summer. A winner is expected to be announced in July.

Construction could start as soon as mid-2014, with the winning group expected to loan the state 10 to 20 percent of the project cost. The state is expected to chip in $500 million in gas taxes collected from motorists statewide.

The groups are: C.W. Matthews Contracting and the Michael Baker Corp.; Fluor-Lane; Georgia Transportation Partners including Bechtel Infrastructure, Kiewit Infrastructure South, Dewberry and Davis and STV Inc; and Northwest Express Road Builders including Archer Western Contractors, The Hubbard Group and Parsons Corp.

Also Thursday, the DOT board elected new Chairman Johnny Floyd, a former state representative, to a one-year term. Floyd has served on the board since 2008.