The U.S. infrastructure and transportation system allows for a high level of mobility and freight activity for the nearly 270 million residents and 7 million business establishments. In 1997, over 230 million motor vehicles, ships, airplanes, and railroad cars were used on 6.4 million km (4 million mi) of highways, railroads, airports, and waterways. The transportation infrastructure also includes over 800,000 km (approximately 500,000 mi) of oil and gas transmission pipelines, and 18,000 public and private airports. The annual direct cost of corrosion in the infrastructure category is estimated at $22.6 billion, which is 16.4 percent of the total cost of the sector categories examined in the study.