The Federal Emergency Management Agency defines evacuations as the organized, phased and supervised movement of people from dangerous or potentially dangerous areas. Some evacuations occur without the direction of government officials. This happens when residents either perceive of or are aware of an actual emergency and evacuate an area on their own. Voluntary and Mandatory/Directed Evacuations involve government direction.
The definitions for these follow:
One factor to consider when deciding whether or not to evacuate is the type of disaster or emergency. Some emergencies allow you time to prepare (such as hurricanes or winter storms), others can come upon you quickly (such as flooding, wildfires or tornadoes).
If you live in a flood-prone area, it's best to evacuate during the voluntary phase so you will have enough time to move. Flood waters can come upon you suddenly and trap you, making it difficult or impossible to leave at a later time.
If you feel you will need assistance in moving the individual you care for, it's best to plan early and move early. If you wait too late to evacuate, you may not be able to find friends or neighbors to help you.
If you choose not to evacuate during the voluntary phase, it is imperative that you monitor the emergency situation closely.
Whatever you decide, it is extremely important to have an evacuation plan. FEMA provides some guidelines to help you prepare: http://www.fema.gov/plan/prepare/evacuation.shtm.
Source: SLG 101: Guide for All-Hazard Emergency Operations Planning, FEMA, 1996.