The Road Sweeper allows to trigger the scoring of any road. It is scored as if at the end of the game, taking into account the majority and all the possible bonuses affecting the road itself (inns, Mage/Witch...) or any meeples on it (fairy, ringmaster...), and after the scoring you remove all the meeples on it. So this is scoring a full scoring of one incomplete road (therefore no castle scorings triggered), not a partial scoring as the ones in Mini #2 - Messages. Finally, you can extend and occupy the just-scored road if possible and you are interested.
This way, the Road Sweeper can be used with one or several objectives in mind, for example:
* Sabotaging a road project worth many points for other players
* Taking over a road worth many points but controled by other players
* You name it...
Imagine a scenario with an incomplete road with an inn and a German cathedral but occupied by an opponent. Each tile would be worth 3 points if completed. The Road Sweeper would allow you to trigger the scoring of the road and the current owners would receive 0 points due to the inn, but you could take control of the road afterwads and complete it. Lovely, huh?