Roads in north Kent will "seize up" without another Thames crossing, a Kent councillor has said.
Cllr Keith Ferrin said the Dartford crossing was "almost at capacity"
Keith Ferrin said traffic projections showed north Kent would become "solid with traffic" if another crossing was not built.
He highlighted the need for a crossing as Kent and Essex councils commissioned a £100,000 feasibility study.
The structure would be east of Dartford and could be either a rail or road link, Kent County Council (KCC) said.
Mr Ferrin said most of the traffic going through Kent was heading for north of the Thames, and a second crossing would ease the load on the M25 Dartford crossing, where the bridge and tunnel were almost at capacity.
He said the Dartford crossing was set to become the choke point with traffic volumes continuing to grow.
The new structure would provide a link between the A2 in Kent and the A12 in Essex, he added.
KCC said it would be "sea-side of Dartford and somewhere in the estuary" but the exact crossing points were still not known.
Suggestions have included that the crossing could run parallel to the existing bridge and tunnel at Dartford, or cross the water at Gravesend, Sheppey or Grain.
The study being commissioned is expected to take between 18 months and two years.
Mr Ferrin said: "It's a precursor to a full study which only the government can afford, because a full study we think would cost over £1m in itself."
"Traffic projections for the quite near future show that unless there's another Thames crossing, the whole of north Kent will seize up, solid with traffic."
London councils have already considered a crossing between Newham and Greenwich, which went to a public inquiry in 2005. That crossing was given planning permission by the two councils in 2004.