66 Airway/Route segments designated by the FAA as unusable will be depicted as shown below. Pilots should not file a flight plan for or accept a clearance that includes navigation on any route or route segment depict - ed as unusable. Pilots using RNAV may request ATC clearance to fly point-to-point between valid waypoints or fixes, even  those on routes depicted as unusable (refer to AC 90-108 for RNAV eligibility). When two airways/routes are coincident, but only one airway/route is designated as unusable, the following note indicat- ing which airway the unusable symbology applies to will be placed in close proximity to the airway/route identifiers. The Off Route Obstruction Clearance Altitude (OROCA) is depicted on IFR Enroute Low Altitude and Pacific charts and  is represented in thousands and hundreds of feet above MSL. OROCAs are shown in every 30 x 30 minute quadrant on  Area Charts, every one degree by one degree quadrant for IFR Enroute Low Altitude Charts - U.S. and every two de- gree by two degree quadrant on IFR Enroute Low Altitude Charts - Alaska. The OROCA represents the highest possible  obstruction elevation including both terrain and other vertical obstruction data (towers, trees, etc.) bounded by the ticked  lines of latitude/longitude including data 4 NM outside the quadrant. In this example the OROCA represents 12,500 feet. OROCA is computed just as the Maximum Elevation Figure (MEF) found on Visual Flight Rule (VFR) Charts except that  it provides an additional vertical buffer of 1,000 feet in designated non-mountainous areas and a 2,000 foot vertical buffer  in designated mountainous areas within the United States. For areas in Mexico and the Caribbean, located outside the  U.S. Air Defense Identification Zone (ADIZ), the OROCA provides obstruction clearance with a 3,000 foot vertical buffer.  Evaluating the area around the quadrant provides the chart user the same lateral clearance an airway provides should  the line of intended flight follow a ticked line of latitude or longitude. OROCA does not provide for NAVAID signal cover - age, communication coverage and would not be consistent with altitudes assigned by Air Traffic Control. OROCAs can be  found over all land masses and open water areas containing man-made obstructions (such as oil rigs). Military Training Routes (MTRs) are routes established for the conduct of low-altitude, high-speed military flight training  (generally below 10,000 feet MSL at airspeeds in excess of 250 knots Indicated Air Speed). These routes are depicted in  brown on IFR Enroute Low Altitude Charts, and are not shown on inset charts or on IFR Enroute High Altitude Charts. IFR  Enroute Low Altitude Charts depict all IFR Military Training Routes (IRs) and VFR Military Training Routes (VRs), except  those VRs that are entirely at or below 1,500 feet AGL. MTRs are identified by designators (IR-107, VR-134) which are shown in brown on the route centerline. Arrows are shown  to indicate the direction of flight along the route. The width of the route determines the width of the line that is plotted on  the chart: MILITARY TRAINING ROUTES (MTRs) FAA Chart Users’ Guide - IFR Enroute Terms OFF ROUTE OBSTRUCTION CLEARANCE ALTITUDE (OROCA) Coincident Airways/Routes with Unusable Segment UNUSABLE AIRWAY/ROUTE SEGMENTS Unusable Victor Route