Waze is embarking on an unusual initiative: Using their app as a way for police departments to notify the public of road closures. I just listened to Chip Googe of the Mount Pleasant Police Department in South Carolina explain how his PD feeds road closure information into Waze in order to keep citizens updated on how the large Cooper River Bridge Run effects the town.
Other efforts of this sort include Texas' Harris County (Houston, essentially) and the New York Police Department sending information on road closings to Waze.
For the police departments, using Waze to reach citizens seems like a similar step to notifying television or radio stations. It remains to be seen how it would work, however, at scale.