Standard Asphalt Driveway
According to nationally syndicated newspaper columnist Tim Carter in his article "A Strong Asphalt Driveway," a standard asphalt driveway needs a sub-layer of crushed gravel between 8 and 12 inches deep.
Full Depth Asphalt Driveway
A full-depth asphalt driveway needs a sub-layer of gravel between 6 and 8 inches deep, according to the National Asphalt Pavement Association. A full depth asphalt driveway requires that at least 3 inches of asphalt be laid over the gravel.
According to the National Asphalt Pavement Association, a full depth asphalt driveway is the best option. "We recommend full-depth asphalt for driveways," says Dave Newcomb, NAPA Vice President of Research and Technology. The NAPA also states that "Full-depth asphalt provides a better balance of strength and flexibility---plus durability."