In areas subject to vehicle traffic, what is the minimum required thickness of compacted backfill when combined with asphaltic concrete?

The correct choice indicates a minimum required thickness of compacted backfill alongside asphaltic concrete of 30 inches and 6 inches, respectively. This requirement is designed to ensure adequate support and stability for structures exposed to vehicle traffic, which exerts significant loads that can lead to settling or failure if not properly supported.

The combination of 30 inches of compacted backfill ensures that there is enough material to absorb and distribute the weight from vehicles, thereby reducing the risk of deformation or damage to the asphalt surface. The 6 inches of asphaltic concrete serves as a protective layer that bears the vehicle loads while providing a durable and smooth driving surface.

This thickness combination is essential for effective load distribution, soil stabilization, and protection against frost heave or erosion, which can occur in transportation areas. Such specifications are typically guided by engineering standards and local regulations to ensure safety and longevity in pavement design where vehicle traffic is a factor.