That's an excellent and practical question about urban design! The short answer is: there's no universal, legally mandated standard size for trash can openings across all cities, but there are strong common practices and design principles that create de facto standards in many municipalities.
Most urban trash cans you encounter in public spaces like parks, sidewalks, and transit stations typically have openings ranging from 20 to 30 centimeters (about 8 to 12 inches) in diameter for round openings, or similar dimensions for rectangular slots. This size isn't arbitrary. It's carefully engineered to serve multiple purposes. Primarily, it aims to discourage the disposal of oversized items like household bags of garbage, boxes, or construction debris, which should be handled through bulk pickup services. A restricted opening helps keep the can available for smaller, everyday litter like food wrappers, coffee cups, and bottles from pedestrians.
The design also considers operational efficiency for sanitation workers. An opening that's too small frustrates public use, leading to littering beside the bin. One that's too large defeats the purpose of containing waste and makes the cans easier targets for rummaging by animals or people, creating messes. You'll often see slightly different opening styles—some are vertical slots, others are horizontal, and some have hinged or baffled lids—all aiming to balance easy public access with controlled waste intake.
While there's no single global standard, many city public works departments follow guidelines from organizations like the Waste Management Association or adopt successful designs from other cities. The next time you're in a city, take a look at the trash cans. You'll likely notice their openings are quite similar in size, a quiet testament to this shared design logic aimed at keeping our sidewalks cleaner and our waste systems running smoothly.