That's a great question that anyone who's walked through a city has probably wondered about! From my experience observing and dealing with municipal bins, there's no single universal percentage, but there are clear practical limits and consequences.
Most standard public trash cans are designed to function properly when filled to about 75-80% of their total volume. This isn't just about the lid closing; it's about safety and efficiency. When a bin reaches this point, the contents are still below the rim and the lid can swing shut completely, containing odors and preventing litter from being scattered by wind or animals.
The real problems start when bins are filled to the brim or beyond. Once the waste level reaches the very top of the container (100% full), the lid rests on top of the trash. It might appear closed from a distance, but it's not sealed. A slight breeze or a nudge from a person or animal can easily pop it open. This is the most common "failure" point.
When people continue to pile waste on top of a full bin, creating a mound, the lid is physically prevented from closing. It will sit at an angle, often held open by the overflow. This is when you see the classic scene of garbage spilling onto the sidewalk.
However, the "won't close properly" issue can also be mechanical. Constant overfilling puts immense stress on the hinges and hydraulic closing mechanisms found on some modern bins. For a pedal-operated bin, overfilling makes it very hard to depress the pedal, and the lid may slam shut violently. For bins with a swinging lid, heavy, over-packed trash can strain the hinges, causing them to bend or break over time. A bin with a damaged hinge will never close right again, even when empty.
So, while the visual breaking point is an overflowing mound, the functional breaking point for proper closure is actually right at the rim. The best practice for everyone is to stop using a bin once the waste is level with the top edge. If you encounter a completely full bin, it's better to find another or hold onto your trash temporarily rather than contributing to an overflow that creates a mess, a health concern, and potential damage to public property. Proper use ensures the bins work as intended for everyone.