Traffic congestion could cost residents and businesses in the East of England £2bn a year by 2021, a report claims.
Delegates at a transport summit in Newmarket were given a Transport Economic Evidence Study (TEES).
This aims to calculate the costs of traffic congestion and identify where it is having the greatest impact.
It concludes congestion across Essex, Bedfordshire, Cambridgeshire, Norfolk, Hertfordshire and Suffolk will grow from £1bn in 2003 to £2bn by 2021.
Costs are on track to more than double unless radical joint action is taken, TEES reports.
Congestion hotspots causing 85% of damage to the regional and national economy have been identified.
The benefits of targeted road improvements are discussed but a slowing of growth is predicted when constraints on economic productivity have been relieved.
Better management of demand for road travel and additional rail capacity will increasingly do more to reduce the cost of congestion in the long term, the report by consultants Steer Davies Gleave said.
Richard Ellis, chair of EEDA, said: "We now have the economic evidence to support the East of England's case for continued investment in the transport infrastructure.
"Investment in transport is critical to supporting the regional and national economy but equally working smarter and managing demand will be key to unlocking economic growth.
"It is now time for all of us to get behind our regional transport campaign to lobby for a more reliable, effective and safer transport network for the benefit of all that live, work and invest in the East of England."
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