From the Bureau of Transportation Statistics:
The NTAD is a compilation of datasets representing the nation’s transportation infrastructure, including more than 500,000 miles of roadway, over 600,000 bridges, and in excess of 19,000 airports.
This year’s NTAD consists of over 60 individual data layers, most of which have been updated within the last year. The NTAD also includes several new features: the U.S. portion of the North American Rail Network, truck parking, and road and airport noise layers. The 2016 NTAD marks the end of the traditional annual update.
BTS will now begin using a dynamic publication cycle, making updated data available throughout the year, allowing for more timely delivery to users. Also new this year, NTAD will be available exclusively online.
To offer larger and more complex data sets and enhanced functionality, DVDs have been eliminated. The enhanced online NTAD datasets allow for customizable downloads and APIs. They are available as GeoJSON and GeoServices (REST) services and are also available for download as shapefiles, kml and csv files.