1. The traffic is heavy.

Question

2. The traffic is light.
(Is this(#2) the opposite of #1?)

3. Cars are bumper to bumper on the street.
3-1. The street is crowded with cars.
(Are both the same?)

Answer 1

Yes, 1 is the opposite of 2.

And, yes, 3 and 3-1 are basically the same. (3 implies more dense crowding than 2 does.)