Per NEC 300-5 Minimum cover requirements. One row stipulates "in trench below 2" thick concrete or equivalent Non Metallic raceways require only 12" cover. Another row states under 4" thick concrete exterior slab with NO vehicular traffic 4" min cover. Yet the row stating under streets,highways, roads, alleys, driveways, and parking lots min cover shall be 24". Because the one row states NO vehicular traffic (4") the other for 12" cover makes no mention of vehicular use...can it be assumed then that 12" is adequate under 2" concrete regardless of traffic.