The Department for Transport has published the results of an investigation into linking existing roads between the two cities, via Bedford and Milton Keynes.
It said upgrading and addressing gaps in the network could improve journey times by up to 30 minutes.
But sustainable transport campaigners said the plan was "disappointing".
The study said the "knowledge economy" relied on workers with highly specific skills working in close proximity to each other, which is harder to achieve with poor transport links.
It also anticipated that congestion along the existing network could be "expected to intensify" by about 40% by 2035 leading to increased journey times.
But Bridget Fox, from the Campaign for Better Transport, said the plan did not cater to cyclists or bus users.
"Oxford and Cambridge have pioneered transport policies based on providing alternatives to car dependency," she said
"To build new road connections drawing more traffic into the area risks undermining their achievements."
The Oxford to Cambridge Expressway Strategic Study was launched last year and is one of six studies aimed at addressing challenges facing the UK's road network.
Transport Minister John Hayes said the planned Â£15bn road upgrades were aimed at "creating jobs and opportunities".
A government study has said a "brain belt" expressway between Oxford and Cambridge could boost the UK economy.