"The street had a lot of traffic yesterday."

Translation: Ο δρόμος είχε πολλή κίνηση χθες.

[martin.mk]

Θα έλεγα ότι η σωστή μετάφραση είναι πολλή κίνηση, όχι πολύ κίνηση.

[Theo_Matrakas]

Σωστά, το διόρθωσα :)

[MiltiadisA1]

I believe that "a lot" means "πολυ" and the respond of Duolingo is incorrect

[mizinamo]

"a lot" means πολύ (mind the accent!) when it's an adverb, e.g. "I drink a lot", πίνω πολύ.

But here, you have "a lot of", which acts like an adjective, and then you need the appropriate ending depending on the gender of the noun, e.g. πολλή κίνηση because κίνηση is feminine.