"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)?"
If "Gab es viel Verkehr" is correct, then why is "Hat es viel Verkehr gegeben" incorrect (Duo proposes "Hat es da viel Verkehr gegeben" instead)?
Also, why is "Hat es viel Verkehr da gegeben" incorrect? Why "da" should follow the verb in this case?