The environmental group that's been pushing for a free public transit system said Thursday that if Mayor Michael Bloomberg doubled the congestion pricing fee, free subways and buses could be a reality.

The Institute for Rational Urban Mobility says if the city were to charge drivers $16 dollars to enter most of Manhattan at all times, there'd be enough revenue that the subways and buses could be free. The mayor wants to charge drivers $8.

The group says a $16 toll would bring about $3 billion in revenue — which would be enough to cover all transit fares paid in the city in a typical year.