In the early days of United States activities at McMurdo, tracked vehicles were used for station transport of men and supplies. These functioned well, if slowly and expensively, over snow and ice but were impractical over land because of track wear. Wheeled vehicles were introduced but, while satisfactory over earth roads, very quickly bogged down in snow; this difficulty was to some extent solved by the substitution of high flotation, chevron-tread, or ribbon-tread (sand) tires, for conventional tires. By 1966, the Naval Civil Engineering Laboratory (NCEL) had developed high-strength, heavy-duty snow roads capable of supporting vehicles weighing up to 31 500 kg, and wheeled vehicles replaced tracked vehicles for station use.