I find Bubble Gum Ally ways both cool and really repulsive at the same time. Here are three examples.
The Gum Wall in Post Alley, Seattle is literally covered in old chewing gum which people have stuck on it. Tradition dictates that the sticking of gum on wall started in the 90's while patrons waited in line for buying theater tickets.
Another example is Bubblegum Alley, San Luis, Obispo which even has its own bit of bubble gum street art. This bubble gum street art piece is a 15 foot high and 70 foot long alley which is lined with used bubble gum left by passers-by.
Finally there is also The Maid Rite Sandwich Shop in Greenville, Ohio. Somehow customers started sticking used gum on the exterior of the shop and never stopped.
There are been lots of attempts to get people to dispose of their gum as seen in trends below. None really seem to work and I know for sure in London that most of it ends up on the street.
Using my geography degree for a second, to me the soft management approach seems to works the best. Let people think they are still rebelling and sticking on walls free style - when in effect this is a positive approach stopping the whole city from getting covered in the darn sticky stuff.