I'm afraid it wont work, you will sink into it at that depth. It will also be horrible to walk on.
You should really use proper sub base which has a mix of fines in it to bind it when compacted, This forms a solid thick mat which helps spread the loading of a car. The ballast would not really spread the load but just get individually rammed into the earth below.
why the ballast? do you have access to some for free? If you want to use it i would have 6'' of proper sub base and then a coating as thin as possible probably 80mm or so ballast.
To get really technical you could use a geotextile on the sub grade then a fill of 6'' of ballast then 20mm stone as a blinding course then another geotextile, then maybe 4'' of compacted MOT sub base then a layer of ballast but really that would be the kind of thing you might use to prevent flooding etc and would depend on your ground conditions on site and all round it would be more work and expense than a normal 6-8'' sub base plus 50mm depth of gravel as per a standard driveway make up
