Responders to spill accidents need to have at their fingertips a method of rapid prediction of chemical cloud dispersion behavior and other relative information to protect the public from death and injury during evacuation of the affected area. A survey of 123 chemical spill accidents in the United Stares where evacuations occurred revealed that available mathematical models were not used to predict cloud dispersion behavior while the event was under way, although modeling was sometimes done after the fact The varied reasons for why modeling was not done include the fact that models require complex information on meteorology, the accident took place when modelers were not available, and sometimes, responders did not know what chemicals were released. The evacuation distances based on a gir;en downwind concentration (level of concern) vary considerably, depending upon the chemical released and upon atmospheric turbulence resulting from solar insolation and wind blowing across surface features. Examples are presented. The problem is complicated because available mathematical models and look-up tables often do not give the same answer under the same conditions, nor do authorities agree on the concentration levels representing a level of concern on which to base evacuation. The responders to chemical spills need to be aware of these limitations, recognizing that modeling is a tool and does not represent ultimate truth. Yet modeling is useful to responders and is required by law in the United Stales in developing possible accident scenarios under risk mangement plans as part of the Clean Air Act. Western Research Institute, in surveying the problems, has determined a need to communicate the benefits of modeling in a format designed for responders and others in the decision-making processes. A system for estimating evacuation distances has been developed for a hand-held computer for field use. The major features are that the user inputs the chemical spilled, spill location, time of day, cloud cover, and source spill information; evacuation distances based on a level of concern are calculated in seconds. The system is designed for planners and emergency response personnel without special modeling or computer skills. The results compare favorably with real field data and with dense gets and neutrally buoyant models in the public domain.