The notification system is a type of cyber siren to guide people on what to do during an emergency - which could trigger a very different response depending on the incident.

An explosion at a chemical plant, for instance, might demand that people remain inside, while a hurricane moving up the Delaware River could warrant a large-scale evacuation.

"My biggest fear is people not sheltering in place when they should, and they go out into something dangerous," said MaryAnn Marrocolo, Philadelphia's deputy managing director of emergency management.

Marrocolo and other local officials said at a news conference yesterday at the National Constitution Center that the emergency text-messaging system would be an efficient way to get directions out quickly and broadly.

Marrocolo said text messages have proven to be a more reliable way to communicate during an emergency than cell-phone calls. Text messages are more compact packets of information that use less bandwidth than cell-phone calls.

Marrocolo said that during the 9/11 attacks, when she was working for New York City's emergency management office, she was able to send and receive text messages but not phone calls.

Anyone can sign up for the emergency-text service, even residents from outside the city and four-county area.

There are two methods: Either online at www.ReadyNotifyPA.org, or on a cell phone by texting your county code (BUCKS, CHESCO, DELCO, MONTCO or PHILA) and dialing 411911.

In addition to information on weather-related emergencies or accidents, people can sign up to receive alerts on other things such as municipal or crime matters.

Thomas Sullivan, director of public safety for Montgomery County, said the text service will enable city and county officials to get information out "in the biggest, most comprehensive way."

He said that knowing whether to shelter in place or evacuate is a critical decision in the early moments of a disaster. In the past, he said, a disaster siren would warn people of danger, "but doesn't tell you what to do. We now can tell people exactly what they need to know."

Philadelphia and surrounding Pennsylvania counties used $900,000 in federal homeland security funding to pay for the service, called ReadyNotifyPA. The service can send an e-mail or text message to a cell phone, BlackBerry or pager.

In New Jersey, Gloucester County has a similar emergency text service, while Camden and Burlington Counties are working together to start one, said Don Elmer, director of emergency management for Camden County.

In a separate news conference that followed the news of the emergency text service, Mayor Nutter also announced evacuation routes that will cover every section of the city.

Breaking the city into 25 sections, the grid outlines how traffic and people would flow in the event of an evacuation.

The lack of an evacuation plan was cited as one of eight major shortcoming in Philadelphia's emergency preparedness, according to James Lee Witt Associates, an outside consultant hired in 2006 by Mayor John F. Street to assess the city's readiness.

Philadelphia Police Commissioner Charles H. Ramsey said that emergency managers in Washington, where he worked prior to being hired by Nutter, had a citywide evacuation plan and that Philadelphia needed one too.

"You have to have predetermined routes," Ramsey said.

He said the odds are low that Philadelphia will ever experience a mass evacuation, but a localized incident could force parts of the city to clear out.

Nutter added, "If conditions outside are too dangerous, officials will tell you to shelter in place. If we determine you must evacuate, we've made evacuation routes available to you today."

In stressing the need for citizens to be prepared for any scenario, Philadelphia's emergency managers are advising residents to put together a "go bag" - things to have along during an evacuation.

They say it should be a backpack or small suitcase and include copies of important documents; extra car and house keys; credit and ATM cards; cash in small bills; medications; and provisions such as bottled water and nonperishable food.

Asked if he had a go bag, Nutter said: not yet.

"I will be developing one over the course of the weekend," he said.

"We do want people to take this seriously," he said. But he admitted, "I didn't know what a go bag was two weeks ago."