My experience with driveways is that the gravel used in them raises hell with any detector I have used. Not to mention people used to clean out there coal stoves and dump clinkers in the driveway as well..... and clinkers and metal detectors dont mix.
The only thing I have found that works on old driveways is a FBS machine, but even with them... performance is greatly reduced.
The answer to your question is probably different in different parts of the country but your pretty close to CT, and places where coal was burned always give my machines fits.