It marks the final phase of the A602 improvement works, between Watton-At-Stone and Tonwell, including the A119 junction and the section between Heath Mount School and the Stoneyhills junction.
Substantial earthworks are required and a new bridge over the Dane End tributary is to be constructed.
The improvements will help to ease congestion issues and will ensure the road has sufficient capacity to serve planned growth in residential and commercial property development in the county.
Interserve Construction expects to start work this month with completion due by early 2022.
Hertfordshire County Council selected Interserve using through its Eastern Highways Alliance Framework 2.
Phil Bibby, Cabinet Member for Highways and Environment, said: “The A602 is a key route between the A1(M) and the A10, as well as an important road for local traffic, so the improvements we’re planning will make a real difference.
“The tight bends and traffic turning right both cause delays and fixing these problems will help traffic flow more freely and safely.
“With around 100,000 new homes expected in Hertfordshire by 2031, we’re expecting an increase in the number of vehicles on the road, so it’s vital that we invest in key schemes to improve the road network.”
Wayne Howell, Divisional Director, Interserve said: “Our appointment to improve the A602 in Hertfordshire is a further addition to the portfolio of highways we are delivering on behalf of local authorities including recent appointments in Suffolk and Thurrock.”