I would like to discuss the difference between the usage in these sentences below:
I was delayed by a traffic jam.
I was delayed by heavy traffic.
The second one is correct, based on a TOEFL test sample. I also wonder why the sentence below is incorrect in the test:
I was delayed by a heavy traffic
Both traffic jam and heavy traffic are objects. I know the preposition a must be put before traffic jam in this example, because the event is specific.