What do you think are some of the obstacles for a large scale evacuation effort?

There could be a problem with transportation, since it's large scale they might not have enough vehicles. Traffic jams in general may be a problem as not everyone can get out as quickly as desired. It's hard to plan in advance a large scale evacuation as events that would require large scale evacuations only happen rarely, so if the worst was to happen not everyone would be prepared and have the correct equipment to evacuate safely.