This took me to Feb this year when the ARRL announced that they were dropping support of the Spring Sprints, a few weeks before it was to start, leaving people to scramble to organise it themselves. I contacted the organisers for each band and they were quite happy to run a script at their end and forward the cleaned logs to me to draw 'activated grid' maps.
Here's the maps for 50,144 and 432 MHz. Wayne N0POH who cleaned the logs for me never received the logs for 222, 903 and 1296. The logs only have the grid activated and not the grids at both ends of the contact, so I can't draw qso paths or see if any interesting openings occured. However the maps do show the locations of VHF activity.
The maps can be compared to the maps of ham population derived from the zip codes in the FCC database. Here's the US calls by 6char gridsquare and US calls by 4char gridsquare. The maps look similar to those of the US population and it would seem that hams are anywhere that people are. The maps indicate that anywhere you drove in a straight line for 500 miles east of the Mississippi you should get lots of contacts and that varying your path by 100 miles in one direction or another wouldn't make much difference, if the VHF people were evenly distributed amongst the ham population.
Here are the activated grid maps for 6m, 2m and 432 for the 1999 Spring Sprints. The contact count for 6m is low, because this was conducted the same weekend as the Dayton Ham fest.
These are the first maps I've seen of VHF activity by location during a contest.
VHF activity appears to be strongly linked with big cities or areas of high population density and does not correlate well with general ham population. There is a void of VHF activity below the Chicago-NY line - a rover trip below this line would not get many contacts. (The 2m activity in FM05, where I now live, is less than on 432 where I used to live in FM19 - you don't really want to contest in FM05).
Notice that Denver, St. Louis, Salt Lake City and Minneapolis/St. Paul have the same ham population, with Chicago having a much higher (>4x) ham population. However Chicago, Salt Lake City and St.Louis hardly register for VHF contests. One can suppose that the Salt Lakers are hemmed in by moutains but it's not clear why Chicago and St. Louis are so poorly represented.
The Minneapolis/St. Paul area has a disproportionately large amount of VHF activity for their population and this is due to an active club.
VHF activity is dependant on high population density or active clubs.
Next contest I would like to get information about the gridsquares at both ends of the contacts allowing me to draw QSO paths and look for openings.