I should say only about 20 ft. But it would be quite difficult to cultivate and there might be a lot of damage from passers by.
I’d stick to grassy central reservations if I were you
Aunty, you are quite right, but I think that gardening outside those grassy reservations might be possible! I think you could do it with Dowding-style no-dig beds, since digging would be out anyway. They would probably need to be raised beds, and if the lanes between raised beds are wide enough, you might be able to set up toll booths, thereby turning a profit. That only works if lanes are parallel to traffic lanes of course.
Problem-solver, that's me.
You're welcome.