With the rain and type of soil in our area,(clay sandy loam), we used 3" shot or crushed rock as the first layer, then 5/8 minus shot/crushed as the last layer. 6" for the first layer, then 4 inches the last. We've live here since 1998 and this year looking at putting a couple inches of 5/8. Our driveway is low use and mostly regular vehicles.
I've seen in a few places around the area that have used a honeycomb matrix that gravel is poured into. It seems to keep the gravel in place and spread the pressure from vehicles out pretty well. It could be economical to use depending on how long one would need. Another thing i've seen is recycled pavement. Results are variable, probably depending quality of the recycled material, on sublayer and knowledge of the person laying it down. It seems one would want to do it during hot weather and a roller/compactor would make a better product. It does get tracked around when freshly applied.
