explanation, since this one might be more confusing than most:

Traffic congestion does indeed waste gas. However, any place worth driving to is going to have congestion--driving without congestion is easy, fast, and comfortable, so people generally won't take other options until roads become congested. Thus, congestion actually reduces gas usage overall, because it is only once areas become congested that people stop driving places.

Trying to avoid congestion, on the other hand, usually involves expanding roads, something which increases driving, and makes other forms of transportation less useful/comfortable, thus increasing gas usage overall.