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 the route is flown entirely below 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 entirely below 1,500 feet AGL; three-digit numbers may extend above.SOURCE → FAA AIM Chapter 3CHECKED APR 21ACS II.A.K3EASY