I get this question a lot, and the honest answer is: it depends on the bench. Not all urban benches are designed with wet-weather grip in mind, but many modern ones are.
Most quality urban benches now use a textured finish or naturally slip-resistant material to reduce the chances of someone slipping after rain. For example, powder-coated steel slats often have a slightly rough, matte surface that stays grippier than old gloss paint. Recycled plastic lumber and grooved hardwood or thermowood benches also give your shoes something to bite into, even when they are damp. Some designs add anti-slip strips or rubber inserts right where people sit and stand.
That said, smooth metal, polished stone, and glossy concrete benches can definitely become slick when wet. If water pools on the surface and there is no texture or drainage gap, treat it with caution.
So how do you check a bench quickly after rain? Look for a matte or sandy texture, visible grooves, drainage slots, or added grip strips. Run the sole of your shoe across the surface if you can do so safely. If it feels slippery dry, it will probably feel worse wet. Also remember that algae, moss, or wet leaves can turn even a textured bench into a slip hazard, so maintenance matters as much as the material.
Urban planners and outdoor furniture manufacturers increasingly specify slip-resistant coatings for public seating because wet conditions are normal outdoors. The best designs are tested for grip in rainy weather, not just in dry conditions.
Bottom line: many urban benches do have a non-slip surface when wet, but it is not a guarantee across every city or park. When in doubt, choose a bench with visible texture and avoid the shiny, smooth ones after a downpour.