The Area 7 contract extension is forecast to be worth an extra £60m a year to the joint venture, which also includes Halcrow and Colas.
Under the contract, A-One+ will carry out a range of operational and routine maintenance services and deliver improvements on the motorways and trunk roads until June 2016.
The HA has already worked with A-one+ to deliver substantial savings as part of the retrofit of Asset Support Contract conditions to existing MAC contracts.
The extension will enable the agency to develop and test new approaches to achieving even greater efficiencies.
Andrew Wyllie, Chief Executive of Costain, said: “This contract further contributes to the strength of our order book, which comprises approximately 90% repeat business.”