Crowded usually means "lots of people" and heavy traffic usually means "lots of cars". Places are "crowded" like full restaurants or sold-out events. People get stuck in "heavy traffic" when they are trying to drive on busy streets.

crowded usually refers to people. heavy traffic refers to vehicles only. This street is crowded. (There are a lot of people on this street) There is heavy traffic on this street. (There are a lot of vehicles driving on this street)