Michael Zeiler has been "playing around" with the GIS tools from ESRI (where he works, so it really isn't playing around per se). He has put together some awesome eclipse maps from the databases they have available for population, traffic, and so forth to predict how many people could show up for the eclipse at various locations.
Click here for The Great American Eclipse analysis.
These tools can be used for city and state planners to help anticipate the best and worst case scenarios in terms of visitor counts. Emergency response could be impaired in some cases and it is not a bad idea to pre stage responders in some places.
For eclipse chasers these tools show that the highways could become full of cars going to or leaving after the eclipse. The chances are greater that traffic jams will happen after the eclipse as everyone decides to head home about the same time.
I suppose it is handy to have a GPS or at least some good maps of the back roads just in case!
Fred Espenak has a road atlas that could prove quite useful if the local Internet resources are stressed.