The routes are the result of a joint venture between the Federal Aviation Administration and the Department of Defense to provide for high-speed, low-altitude military activities.
Airspace above the United States from the surface to 10,000 feet above sea level is limited to 250 knots (indicated airspeed) per the FAA's Federal Aviation Regulations.
MTRs do not constitute an official airspace, and are all open to VFR or IFR civilian traffic; however only military aircraft are allowed to squawk 4000 and exceed 250 knots.
The FAA requires these bases to 'NOTAM' out the routes at least two hours prior to use to allow for civilian traffic to de-conflict if needed.
Military training routes are depicted on FAA VFR sectional aeronautical charts as narrow gray lines with IR or VR designator prefixes.