The USSR had an extensive and overburdened transportation system. Population and economic activity being highly dispersed, it had to haul five to ten times as much freight to produce a unit of GNP as the United States and Western Europe did. The backbone of the system was the railroads, which operated 147,000 km (91,000 mi) of public track, about a third of it electrified, in 1988; approximately half of the rail lines predated 1917. The absolute volume of freight traffic on the Soviet railroads that year was more than double the United States and Western Europe combined. Ninety-one percent of all ground-hauled freight was moved by rail, as compared to 48 percent in the United States and between 10 and 30 percent in Europe. The government consistently neglected highways and motor vehicle transport, evidently deterred by the cost of building a modern system in so enormous a country. There were 970,000 km (602,730 mi) of public highways in 1988, of which about 90 percent was classified as hard-surfaced. Only 3 percent of Soviet ground freight was hauled by truck in 1988. About 15 percent of all cargo traveled by sea or inland shipping. Aeroflot, the USSR’s one airline, carried a fraction of 1 percent of freight traffic.