Good morning, drivers! There are too many cars on the highway today, which means we have a lot of stop-and-go traffic heading into the city. Grab a coffee and be patient!

Cars is a countable noun, so we use many and add an "-s" for the plural form.

Traffic, however, is an uncountable noun. Even though it represents many vehicles, the word itself is uncountable, so it never takes an "-s" or "a/an".