Four years after the terrorist attacks of Sept. 11, 2001, we still do not have a simple way of notifying people in the Washington area of an emergency. We need a signal system.

When I was an engineer at a research facility that handled toxic chemicals, we had a system of whistle blows in case of an accident. The first set of whistles gave the general area of the emergency, the second set pinpointed the emergency site and the third set told people what to do -- shelter, evacuate the area, etc. After a pause, the signal sets repeated. Using indoor alarms, outdoor whistles and reverse 911 -- in which the government contacts residents via telephone during emergencies -- is a system that would work well for the District.

The general areas for an emergency system in the District could be the four quadrants of the city -- Northwest, Northeast, Southwest and Southeast. The quadrants then could be broken down into quarters for the second, more specific locating signal. Particularly vulnerable areas such as Capitol Hill, the White House or the Metro system also could be general areas that are broken into sectors.

Signs could be put up throughout Washington to let people know where they stood -- NE 1, for example. The designations could be painted at frequent intervals along the city's sidewalks, and Metro could post the information on its trains and in its stations.

The third set of whistle blows -- to tell people what to do -- could be similarly simple. For example, one whistle could mean "seek shelter if in affected region; no other regions affected." Two whistles could mean "seek shelter in affected region; everyone else evacuate." Three whistles could mean "everyone seek shelter." Four whistles could mean "everyone evacuate." So, for example, if a radiological "dirty bomb" hit SE 3, the whistle would blow four times for SE, pause, blow three times for the sector, pause, and blow two times for the "seek shelter/evacuate" order. After a long pause, the sequence would repeat. One long whistle would mean "all clear."

At the chemical facility where I worked, everyone was given wallet-size cards that decoded the whistle blows. These cards could be consulted quickly in an emergency. Such cards would work for residents and workers in the District, too. The information even could be put on the back of D.C. driver's licenses or other government-issued identification.

Hotels could include the same information in the booklets they put in their rooms or on the jackets for their room card keys. For people with hearing disabilities, lights on the whistles could communicate the same information as the audio signals.

A whistle system is simple, and it has worked well in the chemical industry. We need such an emergency warning system for the general public in Washington, which is widely seen as a likely target for terrorist attacks.

But will the authorities give us one?

-- Victoria K. Hall