Q: Why did the girl spread peanut butter on the road?
A: To go with the traffic jam!

Explanation:  Peanut butter and jelly is a common type of sandwich in the United States.  Jam and jelly are synonyms (they are not identical but very much the same).  A traffic jam is when there are so many cars in one place that the cars are stopped (or moving very slowly).  This joke plays with the word jam- the kind that goes with peanut butter and the kind that stops traffic.  Ha ha ha!