1. The traffic is heavy.

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?)

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.)

Yes, in the context of traffic congestion, #2 ("The traffic is light") is the opposite of #1 ("The traffic is heavy"). When we say that traffic is heavy, it means that there are a large number of vehicles on the road, causing slow movement or congestion. On the other hand, when traffic is light, it implies that there are fewer vehicles on the road, resulting in smoother and faster flow of traffic.

Regarding #3 and #3-1, they convey a similar meaning. Both statements indicate that there is a high density of cars on the street, resulting in very little space between them. "Cars are bumper to bumper on the street" suggests that the cars are closely following each other with no space between them, while "The street is crowded with cars" implies that there are a large number of cars on the street, making it congested or crowded.