It is quite countable. There may be two or more streets or areas where the traffic cannot move. In such a case, then, there would be multiple traffic jams. Of course, where a traffic jam begins and ends is not always exact, but once you're in the middle of one and the car is going less then 5 kpm, you know it.

It is countable. A countable noun has a plural form. ex:I was in a traffic jam yesterday afternoon. - You can tell where the traffic jam was. -->Traffic on the otherhand, in uncountable. ex: There isn't much traffic. - Uncountable nouns are always singular. They have no plural form. hope this helps!