In Python, if the user's solution modifies traffic, test cases break. In the tester code, pass a COPY of the list to the solution with traffic[:] to fix this.
In the meantime, I managed to submit my solution by copying the traffic list like this: t = traffic[:] and using my copy to solve the problem.

I know it sounds kinda stupid, but the sensei should mention that multiple cars can pass in one go, and that the order of the cars is important
Otherwise, this kata is quite a fun one to solve

All random tests expect an answer of one, even when the correct result is clearly way above one.

The user can modify the input.