The honest answer is: it depends on the bench, but many street benches are built with a subtle textured grip rather than a completely smooth, slippery finish.
If you look at powder-coated metal benches, the coating usually has a fine orange-peel texture. It is not rough like sandpaper, but you can feel a light drag when you run your hand over it. That texture helps with grip and keeps the bench from looking too glossy. Wooden street benches can feel smoother, especially if the slats are sanded and sealed. However, many outdoor wooden benches are grooved, ribbed, or lightly brushed to shed water and give the seat a more secure feel. Recycled plastic benches often look smooth from a distance, but up close they usually have an embossed or matte surface to prevent slipping. Concrete benches are almost never glass-smooth; they are typically honed, sandblasted, or acid-etched, which gives them a fine gritty texture.
A completely smooth seat looks clean and modern, but it can become slippery when wet, frosty, or dusty. A textured grip is more practical for public outdoor use because it provides better traction in rainy weather and hides small scratches and wear. The downside is that textured surfaces can trap a little more dirt in the grooves, so they may need occasional washing.
If you are checking a specific bench, the easiest test is to touch it. A glossy, glass-like seat is smooth. A slightly pebbled, matte, or brushed seat is textured. In public spaces, you will usually find a matte or lightly textured finish on the seat, even when the frame looks polished, because municipalities and designers prioritize slip resistance and long-term durability.
So if you are choosing a street bench or just curious about one, expect most outdoor benches to have at least a light textured grip on the seating surface. Fully smooth finishes exist, but they are less common on benches exposed to weather and heavy public use.