What is the main idea of the news item? [br] [originaltext] The United State

The United States is introducing a new public alert system that will deliver warnings to mobile phones in the event of emergency. The system is first being rolled out in New York and Washington and will operate nationwide by April.
    Top mobile phone company executives joined federal officials and New York Mayor Michael Bloomberg at the World Trade Center site to announce the so-called Personal Localized Alerting Network, or PLAN. Bloomberg said the system will allow cell phones to receive free alerts about imminent safety threats.
    "If people need to take action, the system will transmit instructions accurately and in a timely manner, which is something that could save countless lives," he said.
    The warnings will be localized by using only local cell phone towers. The notifications could warn residents and visitors about an approaching tornado, hurricane or storm. If there is an urban terror threat, everyone in the area with a cell phone can be warned and advised what to do.
    All major U.S. cell phone companies will participate in the system. The system will give priority to emergency messages, overriding all regular telephone and text traffic.