Balfour Beatty has been awarded a £321m contract to upgrade parts of the M25.
The construction group will increase capacity on 28 miles of the motorway by converting sections of the hard shoulder into a traffic lane with variable speed limits.
The joint project, with Skanska UK, will take place between junctions five and seven and 23 and 27 in Kent, Surrey, Essex and Hertfordshire. The work is expected to start this month and end by February 2015.
The company said the upgrade will also see parts of the central reservation converted into a concrete barrier to "restrict the impact of incidents and reduce delays for motorists".
Balfour Beatty said it had also won a contract in the United States to build a power plant in Delaware. The Garrison Energy Center is also expected to be finished in 2015.
Ian Tyler, the chief executive, said: "As the industry continues to shift to cleaner-burning natural gas, we anticipate a significant number of gas-fired power plant opportunities, especially in North America over the next decade."