Runways also are labeled with vertical lines, to indicate their length. Each vertical line represents 1000 ft.

Taxiways—the connections between runways and the terminal building— are usually built at 30° angles to the runways. This allows planes to turn from one to the other without coming to a stop.

Taxiways are labeled with letters, to indicate the different paths from runways to terminals.

O’Hare International Airport in Chicago, Illinois, is one of the busiest airports in the world, with more than 900,000 take-offs and landings per year.

The United States has more than 18,000 airports. More than 8000 have paved runways, while many of the others have grass runways. Approximately 4900 have lighted runways.

Parallel runways are labeled R and L. For example, two north-south runways might be labeled R18-36L and L18-36R.

Most interstate highways in the United States are numbered according to the following system: east-west highways are even- numbered, with the numbers increasing from south to north; north-south highways are odd-numbered, increasing from west to east.