Proactive planning is done to help prevent a problem, such as creating a traffic management system before it becomes a problem, or creating a sewage system that allows for easy expansion. Reactive planning would be how we are trying to help the environment now. The damage has already been done and now we are trying to minimize and reverse the damage.
I believe proactive planning is one of the major goals of regional and urban mapping. It is much easier to see a future problem using a map, and correcting it, than having the problem occur and trying to fix it then.
A new and interesting development in urban planning is a 3D modeling system that Placeways has developed called Scenario3D. This program is an extension to ArcGIS and allows a map-maker to create a street-level view of an environment, such as a neighborhood or shopping mall. This could be very helpful in the instances where an overhead view hides problems that are presented from the height of buildings or areas that could introduce crime that are not visible from an overhead view. Here is a sample picture from the program;
