It depends on the context of the phrase "a bumpy road" eg I have a teenage daughter and she is starting to argue with me about everything, there's a bumpy road ahead until she gets through this stage. I have a son who never argues at all, so I am hoping the teenage years will be plain sailing with him.

A flat road

A smooth path

Are you talking literally or figuratively? If you are speaking figuratively, you might say it is "smooth sailing". Otherwise, the other answers work. Smooth and flat.

Smooth