Five men from the Fathers 4 Justice pressure group caused chaos by blocking most of the main routes into London, a court has heard.
Southwark Crown Court heard that the men, dressed as "superheroes", scaled gantries across busy carriageways displaying banners in February 2004.
The rush-hour protests caused huge tailbacks, the court was told.
The men deny one count of conspiracy to cause a public nuisance by hanging "material" from the gantries.
'Danger to motorists'
The charge claims that this material "interfered with the free and safe passage of road traffic".
Jonathan Stainsby, from Devon; Graham Manson, from Isle of Dogs, east London; Paul Robinson, from Hampshire; Darryl Westell, from Nottingham; and Mattaus Huber, from Dagenham, Essex, also deny four counts alleging they caused a danger by obstructing the roads.
They are accused of blocking the southern approach to Blackwall Tunnel, the A13 Canning Town flyover, the east-bound M4 carriageway and the A4 at Shepherd's Bush.
Anthony Wicken, prosecuting, told the court the day-long protests by the men - dressed as Superman, Batman and Spider-Man - also obscured vital traffic directions and other information.
"An essential part of a purpose of gantries is to warn motorists of the dangers ahead, of restrictions in force and so on," he said.
"It must be obvious that if the dissemination of that information is impeded by banners... then clearly a danger exists to motorists. We submit that is a criminal offence." He told the jury that they should not judge the organisation but insisted there could be no doubt they had been part of a conspiracy. The case was adjourned until Wednesday.