No doubt the internet is useful for general alarm systems for all sorts of natural and criminal disasters and dangers and warnings about dangers from bad foods and drugs but it is also very subject to hacking and could in itself create false warnings that could cause disaster.

There would need to be some way for users to register their geographical location so that the authorities upstream would know who to alert. But if enough people shared their location, it would allow local-area residents to assist one another in emergencies, without having to wait for official help. This could be very valuable, especially if there were tailored forum-type software residents could access to set up threads on topics as they arose. Otherwise exchanges of information and questions would get into a muddle.

There also ought to be local-area phone books, arranged by address, so neighbors within a few blocks of each other could offer each other assistance. (But that wouldn't be as helpful as an Internet forum.)

General geographical location is somewhat easy, and good enough for most.It could be a hierarchal code, which you could go to the street, town, county, state, or region, or other specific data, like school your children attend.

Once that is determined, an RSS type feed could be used to alert users with that ID, or a certain level of hierarchy.