"Fewer," my husband said to my comment, "There are less cars on the road." We were on the tail end of rush hour and on the way to the hospital for my fourth chemo treatment. More than once we were halted.
I once envied Marge Percy being married to another writer, but I don't now because I am able to list being a writer/journalist as just two more of Rick's wonderful attributes.
"Cars is a count noun. You use fewer with count nouns."
However, traffic is now a count noun so there is less traffic. One does not say, "I saw one traffic, two traffics." At least I don't any more because I'm married to a grammar-happy writer.
This is not a complaint, but a pleasure.