The story goes that while in London for a show at the Empire Pool, Wembley in 1972, promoters of the show started getting a bit worried as ticket sales were not what they could have been. The band, Shep, and Beatles associate Derek Taylor hit on the idea of a truck with the picture of a naked Alice and snake driving around London publicising the event. On June 29th the truck conveniently broke down, reputably after a member of the party "accidently" removed a part from the engine, in Picadilly Circus causing huge traffic jams and, of course, getting the naked Alice advert all over national television and on the front page of newspapers. Needless to say, the show sold out!
To be acurate photos actually show the truck broken down at Oxford Circus, not Picadilly Circus. That isn't to say it didn't "breakdown" twice but no shots of it at Picadilly Circus have surfaced. Also there isn't any evidence it was covered on national TV or on the front pages of any newspapers. Again, that isn't to say some paper somewhere didn't do that, but so far no evidence of it has surfaced. There IS the suggestion that the press were 'tipped off' in advance that something was going to happen and photographers were set up, waiting for it to arrive.
It's likely London local radio mentioned it in their traffic news at least, and maybe local London papers did run the story the next day, but there really doesn't seem to be any evidence the coverage would have been widespread, or quick enough, to sell out a slow selling show the size of Wembley, especially when the stunt happened only the day before the show.
Add to that the fact we have no way of knowing how many tickets had been sold for the show before the stunt (In 'Supermensch' Shep Gordon claims 50 but this seems unlikely after a previous sold out show at the Rainbow), or even if the show really sold out, and what you have is a supreme publicity stunt that has served Alice well for over 40 years.
Later in the day the truck went to the Chessington Zoo party and photos exist of the band posing in front of the billboard.
Interestingly in June 2014 gambling company Betfair tried exactly the same publicity stunt, this time using a truck with a giant octopus on the back... and what a surpirse it broke down in exactly the same place (Oxford Circus) as the Cooper billboard truck had.
As we're talking about billboards, below are photos of a few others used in Hollywood to promote various albums.