@octavzlatior: "He is walking on the street." sound absolutely fine to me. You should submit a report for your answer to be accepted next time.
I can't figure out when I should use "auf" and when I should use "an". Can someone clarify this for me? Thank you.
With Laufen' (verb of action) shouldn't the two-way preposition, auf, be akkusativ?
The choice of dative or accusative after two-way prepositions has nothing to do with the verb as such.
It's to do with whether you are describing a location, or the destination of motion.
Here, there is motion; but the walking takes place only on the street. The street is this the location of that motion, not the destination or endpoint or target of motion which starts somewhere else.