Honestly, the real urban bench weight capacity usually refers to the seat, not the backrest. A typical well-built park bench has a bench backrest weight limit that is far lower than the official park bench load rating for the seat. Most public benches are designed for a total distributed load of about 500 to 1,000 pounds on the seat itself. That means two or three adults plus bags, with the weight spread evenly across the seating surface.
The backrest is a different story. When a couple of people sit on the backrest, you are putting roughly 300 to 450 pounds of concentrated load on a part that was only designed for leaning — usually a top rail or a few slats. Add movement, leaning back, or a little bounce, and the leverage can easily double the stress at the mounting points. Some heavy cast-iron or thick steel benches will shrug that off. But many modern aluminum, wood-slat, or surface-mounted benches can bend, crack, loosen, or pull their anchors out over time.
So the short answer? A quality urban bench may temporarily support a couple of people sitting on the backrest — maybe 300 to 400 pounds in real-world terms — but it is not rated or guaranteed for that. If you are going to do it, sit still, keep your weight evenly spread, and do not bounce. Otherwise, use the seat. If you want a reliable outdoor bench strength figure, check the manufacturer’s spec sheet or look for a load rating label.