From what I've gathered, the plastic add-on's are most at risk for leverage attacks compared to the sleeves, since those can't be popped as easily.
And then there is also just dumb installations, like this classic video from Philadelphia: https://www.youtube.com/watch?v=aCYP0sZX5u0 -- though the Philadelphia Bike Coalition said that it wasn't that bad.
These Simple Devices Turn Every Sign Post Into A Bike Rack | Fast Company
One of the challenges of urban cycling is finding a place to park your bike. A simple design from U.K.-based Smartstreets might help: The Cyclepark, made of two metal loops, hooks around existing lampposts and street signs to provide extra bike parking on every block.
“By making use of what’s already there, you’re not adding obstacles on the pavement for pedestrians,” says Andrew Farish, one of the designers of the Cyclepark. “And the less stuff you have on the street, the better the street looks.”
People already tend to lock bikes to sign posts and railings when they can’t find a bike rack, but without something more substantial to latch onto, it’s easy for bikes to fall over or even be lifted over the pole and stolen. It’s also often illegal to use a street post if there isn’t an official rack.
The Cyclepark is a little like the circular racks that some cities have added to old parking meters, but street posts are even more ubiquitous than meters, making it easier to create a broad network of parking. In New York, for example, there are 250,000 posts for lighting alone, and countless other street signs. “This stuff is already there,” says Farish. “Often something like a signpost literally does one thing. They have to have a sign there that explains what parking is, or what a single yellow line means, so they’ve got this post and literally all it’s doing is holding up a tiny little eight-by-eight-inch sign. By converting that into a bike park, you’ve released new value from that infrastructure.”
The brightly colored Cycleparks can also double as markers for bike routes around a city, leading cyclists down a particular path. They’re cheaper to install than a typical rack, since the process takes around 10 minutes and there’s no need to jackhammer holes into the pavement (there’s also likely less need for permitting and consultations, since the change is so minor).
Farish says the racks are a way to improve the look of a city by reducing clutter. “It’s a rare opportunity to give a visible upgrade to a street,” he says, explaining that much of the work that cities do often goes unnoticed by the public. “It’s really a way of encouraging cycling and doing it a way that doesn’t compromise the street’s aesthetic.”
