Your sentence is the passive version of this sentence : 'Heavy traffic causes a lot of pollution.' In the active sentence above, 'Heavy traffic' is the subject and 'a lot of pollution ' is the object. In your passive version of the sentence, 'A lot of pollution' is the subject and 'heavy traffic' is the agent. The agent is the thing that CAUSES the action to happen in a passive sentence. The agent is always connected to the sentence with the preposition 'by'. Here are some more examples: Shakespeare wrote 'Hamlet' : active 'Hamlet' was written by Shakespeare : passive The Egyptians built the pyramids : active The pyramids were built by the Egyptians: passive

Because the traffic is the CAUSE OF the pollution. It's like saying that lung cancer is caused by smoking.

"By" is a word used to show the reason (or a maker) of a verb. For example: You are drawing a picture. A picture is drawn BY you.

Heavy traffic causes pollution. Therefore, pollution is caused [BY heavy traffic]. We use "by" to explain the cause of something. Why would you use "with" (=accompany)?