A Military Training Route (MTR) depicted on a sectional chart as "VR1667" indicates:

Why →"VR" prefix indicates a Visual Route flown at speeds above 250 knots, often below 1,500 feet AGL. A four-digit route number (like VR1667) means no segment of the route is above 1,500 feet AGL. Military aircraft can transit these routes at 400+ knots with no ATC communication to alert other airspace users.

The trap →"IR" prefixed routes are Instrument Routes flown under IFR. VR routes are Visual Routes. High speed, low altitude. Four-digit route numbers indicate the route stays at or below 1,500 feet AGL; three-digit numbers have at least one segment above.

Field note →For drone pilots, a VR route overhead is a genuine hazard. A fighter at 400 knots and 500 feet has nearly zero reaction time. Before flying in rural areas near VR or IR routes, check their scheduled usage times in the Chart Supplement.