Hilly Route is both a route and segment - largely, I think, because it was the only route in the game in the beta version. When Flat Route was added it was not made a separate segment in its own right. I’d enjoying racing others and tracking PRs for this whole, flat section of Watopia’s Roads.
Technically I guess you’d need to move the start finish arch(es) to the point at which Hilly and Flat divide - but this doesn’t seem too difficult to accomplish.