That's an excellent and surprisingly practical question! It gets right to the heart of why our streets sometimes look messy despite having trash cans. The short answer is that there isn't a single magic number for depth, as wind behavior is complex. However, based on engineering principles and urban studies, a depth of at least 30 to 36 inches (about 75 to 90 cm) is widely considered effective for standard public bins.
The logic behind this is simple: it's about creating a "still air" zone. When wind flows over the opening of a can, it creates turbulence that can lift and carry lightweight items like napkins and plastic bags. A deeper can means the trash settles further from this turbulent opening, making it much harder for gusts to reach down, grab something, and pull it out. Think of it like a well; you can't easily reach the water at the bottom.
But depth is only one part of the solution. The design of the opening is equally crucial. A smaller, hooded opening or a vertical slot, perhaps with a swinging door or a deflector, is far more effective than a wide, open mouth. These designs disrupt the wind's path, preventing it from getting a direct line to the contents inside.
Furthermore, proper maintenance is non-negotiable. The best-designed, deepest bin in the world will still create litter if it's overfilled. When trash piles up above the rim, it's completely exposed and becomes an easy target for the wind. Regular emptying is just as important as the initial design.
Finally, strategic placement matters. Positioning bins in the wind shadow of a building, a wall, or even dense shrubbery can dramatically reduce the wind force they experience. So, while aiming for a depth of around three feet is a great baseline for manufacturers and city planners, combining that with smart design, consistent upkeep, and thoughtful placement is the true secret to keeping litter where it belongs.