How do go about making "Frogger" traffic?

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?