Let's say four hours. If large numbers of residents outside of the evacuation zone decide to join the exodus the plans could become overwhelmed. Roads could become choked and law enforcement officers would be unable to manage the excess traffic. That is exactly what happened during Hurricane Floyd. Let's carry the above example a step further.
FIRMs are pre-identified flood prone areas.
This could include low-lying areas adjacent to rivers and lakes. So, as you listen to news reports of an evacuation order, listen with care. If so, evacuate. If not, you are considered safer where you are than at risk on the road. The key to evacuating is knowing if you live in an area which has been identified as an evacuation zone.