Make sure to wait a couple of seconds before you hit each trashcan, they sometimes won't register if you run into them in quick succession.
You need to run into a Trashcan
to earn a Trashman Skill. These 4 types count, but the larger ones have to be hit fairly hard to count.
I have marked all the roads that have Trashcans on the sides in red, but basically this entire portion of Surfers Paradise
has Trashcans on the side of the road.