How to make traffic behave as it does in "Frogger?" (3D)

So, in my game, I have a level that starts out like Frogger, with lanes of traffic at the start, followed by a river you have to cross by jumping on row boats. My question is, how would I make the traffic and boats seamlessly scroll in one direction, endlessly, while being on the road/water from the very beginning (instead of driving out of the tunnel onto an empty road at the start of the level) and keeping each vehicle's colliders intact?

I'm very new to coding (but I'm still learning), and...

How to make traffic behave as it does in "Frogger?" (3D)