Article and sidewalk

Hi,

I already submitted this question, but I think it disappeared because I didn't see it for awhile.

My question was about article before sidewalk.

'You should never spit on a sidewalk' vs "You should never spit on the sidewalk'

'Mopeds should not be on a sidewalk' vs 'Mopeds should not be on the sidewalk'

I wanted to confirm if either article is correct in these constructions, I think the difference is that 'a sidewalk' is one of many sidewalks in the city, but 'the sidewalk' is one giant area where people walk. So it depends which I have in mind.