Traffic Data
Data and information on traffic volume, vehicle classification, and truck weight are basic to many highway and transportation functions.  A high level of nationwide and state data can be found on FHWA’s Highway Performance Monitoring System (HPMS) site.  More detailed information can be found at other states' Department of Transportation sites.
 
References
FHWA's Office of Highway Policy Information - Travel Monitoring
Data and information on traffic volume, vehicle classification, and truck weight are basic to many highway and transportation functions. The FHWA's Office of Highway Policy Information has traditionally maintained national programs to track traffic trends, vehicle distributions, and weight to meet data needs specified in highway legislation. Activities include development of guidelines, regulations, direct data collection (from State DOT sources), data processing, research, analysis, and professional conferences.
Travel Monitoring

FHWA's Highway Performance Monitoring System (HPMS)
The HPMS is a national level highway information system that includes data on the extent, condition, performance, use, and operating characteristics of the Nation's highways. In general, the HPMS contains administrative and extent of system information on all public roads, while information on other characteristics is represented in HPMS as a mix of universe and sample data for arterial and collector functional systems. Limited information on travel and paved miles is included in summary form for the lowest functional systems.​
Highway Performance Monitoring System