The data related to registered vehicles is called Vehicles in Operation (VIO) and it can be accessed with the help of the Federal Highway Administration (FHWA), the Bureau of Transportation Statistics and from the commercial companies.

The two different types of sources which are used for collecting the data of registered vehicles are also called Vehicles in Operation (VIO). One is the source of government agencies like the Federal Highway Administration (FHWA) and the other is some private companies such as IHS and Hedges.

This cumulative data is mainly collected by the Federal Highway Administration (FHWA) and the number of vehicles which is slowly going higher since last 50 years had slowed down during the economic crisis of 1990-91. But the number of vehicles has now significantly increased and is estimated to be more than 3.75 million. The largest annual growth was seen during 1998 to 2000 when there was a stiff increase in the vehicles all over the country. The total number of vehicles increased during the year 1998 –2001 which was about 8.5 million and remains to be the largest increase in the recent years. But in the last few years, the number of vehicles has increased to 11 million.

The commercial data related to VIO differs from the data given by the government agencies. While registering the vehicle, the FHWA-561 form is used which is essential for the fiscal year. The government rules regarding the classification of vehicles change according to the time. Also, the government cannot count the vehicles which are not in use and sometimes the vehicles are counted twice during their transportation. The government agencies keep registering particular vehicles as per the month and year.