English splits 'less' and 'fewer' by countability. Since 'traffic' can't be counted one-by-one, which fits 'There is ___ traffic today'?

English divides quantity words by whether the noun can be counted individually — "traffic" describes a mass or flow rather than discrete countable units, so it takes "less," the modifier reserved for uncountable nouns, giving "less traffic today."

"Fewer" is reserved for countable nouns you could count one by one, like "fewer cars" or "fewer people," making it the wrong fit for an uncountable noun like traffic. "Many" also only pairs with countable nouns, so it fails here for the same reason.

This less/fewer distinction is one of the most commonly broken rules even among native speakers, especially in casual speech and supermarket signs like "10 items or less," which grammar purists often flag as technically incorrect.