The National Bridge Inventory (NBI) is a database, compiled by the Federal Highway Administration, with information on all bridges and tunnels in the United States that have roads passing above or below. This is similar to the grade crossing identifier number database compiled by the Federal Railroad Administration which identifies all railroad crossings. This bridge information includes the design of the bridge and the dimensions of the usable portion. The data is often used to analyze bridges and judge their conditions. The inventory is developed with the purpose of having a unified database for bridges to ensure the safety of the traveling public as required by the Federal-Aid Highway Act of 1968. It includes identification information, bridge types and specifications, operational conditions, bridge data including geometric data and functional description, and inspection data. Any bridge more than 20 feet (6 meters) long used for vehicular traffic is included.