1) People who would by interested in this statistic are transportation and environmental agencies, like the DMV or EPA. They are interested in it because using this information they can plan new road locations and city layouts, as well as protect the environment.
2) The Federal Highway Administration collects the data and with it estimates the national VMT and congestion on highways. They report a new VMT every month. The data is compiled more monitoring 4000 traffic counting locations nation wide, and by The National Household Travel Survey which is conducted every 5 years.
3) Total US passenger highway miles (in millions):