1. The traffic is usually heavy on the street on Sunday afternoon.
Is this correct? Or should "on" be changed into "in" in this case?
Would you help me with this problem?
The use of the word "on" would be correct in this context. Saying that "the traffic is usually heavy in the street" would imply that the traffic was a literal part of the street, which it is not. I would imagine this use of the term derives from the phrase "on top of (the street)", if that is of any help.