Interesting concept, I would love to see you expand on it some with a few of the other suggestions in the comments like having people slip around or to cause chaos. Maybe have a single car or NPC where your job is to just rain on them as much as possible, give a percent hit and a limited amount of water. You could make it level based with different cities or different routes with the game goal. I think the game has an interesting premise, it could be cool to just see you run with it. Best of luck!