Devastation has hit NYC in zones that were not forecast to deem mandatory evacuation. As it turns out, the hurricane had caused greater devastation than expected and many people are unexpectedly experiencing loss of water and power and some loss of life. So in response to your first statement, no, most people did not know their areas would be in the path of danger. They are traveling now because where they are now is not habitable.
In response to the concept of supply and demand, yes, that is generally how the economy works. If NYC is in a higher demand for private car service, then it will likely cost more to get more drivers on the road.
I don't think the question at this point is why people are looking for relief via traveling - it should be why are they seeking help and how can we help.