Short answer: leave at least 36 inches, or about 915 mm, of clear space between urban benches if that gap is part of an accessible pedestrian route. That is the minimum clear width required by ADA Standards for Accessible Design for accessible routes. But 36 inches only really works for straight pass-through travel. If you expect a wheelchair user to turn, pause, or park next to a bench, plan for more room.
For two benches facing each other, aim for a 60-inch, or 1525 mm, diameter turning space between them. This gives a wheelchair user enough room to approach, turn around, and back up without repeated maneuvering. If space is tight, 48 inches is a reasonable comfortable minimum for most straight travel and passing situations, but it is not a full turning space.
When benches are placed side by side, leave at least 36 inches between them if that gap is meant to be an accessible route. In busy plazas, park paths, or transit stops, 42 to 48 inches is better because it lets a wheelchair and a walking pedestrian pass without conflict. Also remember that many wheelchair users prefer to sit beside a bench rather than transfer onto it. Provide a level clear floor space of at least 30 by 48 inches, or about 760 by 1220 mm, next to the bench. Keep that space free of trash cans, bike racks, planters, signs, and other obstacles.
In practical terms, use 36 inches as the absolute minimum, 48 inches as a comfortable standard, and 60 inches if you want full turning access between urban benches.