Here's how I would approach it... Based on a given detector geometry you can imagine it has a detection efficiency as a function of distance from the detector. Now you can imagine the source traveling in a straight line such that it passes some minimum some distance from the detector and at some velocity. From that path there is an expected number of counts that the detector would receive on average from that source as a function of minimum distance. Using that function and the background count rate one can estimate how close the source path needs to be from the detector to reliably detect it. From this you can optimize the altitude and velocity of the detector traveling along the road. I hope this brief explanation makes some sense. The hardest part would be to decide what signal to noise ratio is acceptable. For vehicle mounted it would simpler because you can probably just fix the elevation and only focus on the velocity as a variable.