5 (Refer to FAA-CT-8080-2G, Figure 59, area 2.) The chart shows a gray line with "VR1667, VR1617, VR1638, and VR1668." Could this area present a hazard to the operations of a small UA?
B. Yes, this is a Military Training Route from 1,500 feet AGL.
This is correct, and not. But it's probably the answer the FAA is expecting.
Why is the MTR width in nautical miles?
In general, ground-based measurements are in Statute miles and airborne measurements are in nautical miles. For example, visibility is a ground-based observation and will be stated in statute miles. Almost all aeronautical navigation and instrumentation is based on the nautical mile.

Why is this important to Drone Pilots?
MTR's are a safety concern for all pilots because contact with anything flying more than 250 knots would be bad. Fortunately, when outside of the Military Operating Areas (MOA), military aircraft blend into the national airspace quite well. If you will be flying a drone near an MTR, you can call Flight Service (1-800-WX-BRIEF)

Which FSS you're routed to is determined by the number you're calling from. This works well if you're on a landline or calling from your cell phone's local calling area, but what if you're away from your home area code? Luckily, the FAA has published the numbers for all the FSSs, all are toll-free if you've got one of those ancient plans without free long distance.
These gray lines on the VFR charts indicate Military Training Routes (MTR).

§91.117 'Aircraft speed.'
(a) Unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet
MSL at an indicated airspeed of more than 250 knots (288 m.p.h.).

MTR, or Military Training routes are low-altitude training routes generally below 10,000' Mean Sea Level
(MSL) for speeds in excess of 250 knots.
MTRs are depicted as thin, light gray lines on the sectional. Each has its own identification, and the identifier has two parts. "VR" means that pilots flying the routes will be flying under visual flight rules. "IR" means the pilots will be flying under instrument flight rules. The second part of the identifier is either a three or a four-digit number. Four digits means the route will be flown at or below 1500 feet AGL. A three-digit number means the route will be flown both below and above 1500 feet AGL. Thus, on the chart section below, VR-1267 means a training route flown under VFR at a relatively low level. IR-218 would be a route flown under IFR conditions at any level. A jet fighter traveling toward you at over 300 miles per hour can be very hard to see, so it's a great idea to keep any local MTRs fixed in the back of your mind, and to be especially alert when operating near an MTR.
Arrows are shown to indicate the direction of flight along the route. MTRs can vary in width from 4 to 16 miles. The width of the route determines the width of the line that is plotted on the chart.  Route segments with a width of 5 NM or less, both sides of the centerline, are shown by a .02” line.  Route segments with a width greater than 5 NM, either or both sides of the centerline, are shown by a .035” line.

It is very subtle and without a measuring stick (engineering scale) you would be challenged to identify one by itself. Here are two MTR's on the Arizona Sectional Chart, VR1267-1268 and IR218 that show the difference. IR218 is the thinner line of the two shown here.
Most MTR minimum low altitude operations are governed by FAA and military rules and are mostly above 500 ft., making this the mostly correct answer. 14 CFR§91.119 Minimum safe altitudes: General, and AFR 60-16 : General Flight Rules (Airforce Flight Rules) must be complied with by military aircraft, but there are exceptions. For example, military aircraft operating within the boundaries of an MOA (Military Operating Area) can be from the surface up to the MOA boundary.

This question is mostly correct because the MTR's in the image accompanying the question are not inside an MOA, but MTRs below 1500 ft are not charted,. MTRs with no segment above 1,500 feet AGL are identified by four number characters; e.g., IR1206, VR1207. Typically aircraft on the MTRs in this image will be between 500 ft (because of §91.119) and 1500 ft AGL (because of the 4-digit MTR identifier).
If in doubt, call Flight Service (1-800-WX-BRIEF) to ask if the MTR is active.
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