Motorists could be forced to pay tolls on one of London's busiest main roads under costly plans to replace a flyover with a tunnel.
Transport for London is to carry out a feasibility study into replacing the A4 Hammersmith flyover with a 'flyunder'.
Boris Johnson announced a detailed investigation into the cost of the scheme during his official visit to the US.
The mayor saw a similar project in Boston where a six-lane elevated highway was buried in a series of tunnels.
But the Central Artery Tunnel Project, known as the Big Dig, was a financial scandal running years behind schedule.
The cost of the work spiralled out of control from $2.8bn to $22bn, making it America's most expensive new road.
Critics called it America's 'Greatest Highway Robbery.'
Boris Johnson also announced studies into the cost of creating new tunnels at four other sites - the A13 at Barking Riverside, the A3 in Tolworth, the A316 at Chalkers Corner and the A406 at New Southgate.