With reference to gridlock on the city streets, the answer may be in the hands of ABP (who seem to gain financially from the cruise industry).

When there are cruise ships at the City and Mayflower terminals, all passengers are sent into the city centre to board either via the gate at Southern Road or at the other end the Western Docks by the Holiday Inn. If there are also cruise ships at the Eastern Docks (dock gate 4) then all the passengers are heading the same way!

Why doesn’t ABP allow the cruise passengers for the Western Docks to enter/leave the port via First Avenue at Millbrook (which is only a short hop from the bottom of the M27) and then the congestion will be in the docks area for those cruise ships.