"Was there a lot of traffic?"
Translation:Gab es viel Verkehr?
Most commonly, you will see the translations viel = "much/a lot of" for uncountable things and viele = "many" for countable things.
What I mean by this is that you can't count traffic (Verkehr): "How many traffics were there?" is nonsense. So here, viel fits.
If you rephrase to use a countable noun, like "traffic jams" (Staus), then you would use viele.
Gab es viel (Verkehr)? = "Was there much (traffic)?"
Gab es viele (Staus)? = "Were there many (traffic jams)?"