Honestly, it depends on the bench, but most standard street benches give you about 16 to 20 inches of real seating space from front to back. That is the range I would expect if you are looking at a typical outdoor park bench, transit bench, or commercial street bench.
Slatted park benches often run closer to 15 to 17 inches deep. Heavier public benches made from concrete, metal, or thicker wood slats usually sit around 18 to 20 inches deep. Backless benches and plaza-style benches can feel even deeper because there is no backrest, and the platform itself may measure 20 to 24 inches or more. However, the usable sitting area is often smaller once people sit naturally and leave a little gap behind their knees.
From an ergonomic standpoint, a seat depth of about 17 to 18 inches tends to fit most adults comfortably. It supports your thighs without pressing into the back of your knees. If a bench is shallower than about 15 inches, taller people can feel like they are sliding off. If it is much deeper than about 20 inches with a backrest, shorter people may struggle to sit back and still bend their knees comfortably. That is why many commercial outdoor benches are designed with a seat depth of roughly 17 to 19 inches.
So if you are trying to measure these street benches, go from the front edge of the seat straight back to the front face of the backrest or the rear edge of the seat. For most standard outdoor benches, you should expect around 16 to 20 inches of actual seating space from front to back. If comfort or accessibility is a priority, look for a bench around 18 inches deep with a supportive backrest, and avoid designs that are either too shallow or overly deep.