I think this is the conflation of two different types of roads with different purposes that simply share the same name. The most common use for "collector road" refers to the bulk of the streets tagged as tertiary in the Las Vegas Valley. The other use of "collector road" applicable to these sort of ramps would be a more informal term for collector-distributor lanes, which do not serve as busier streets that traffic uses to get from local residential streets to the larger arterial roads, but rather are to separate traffic from the mainline highway between close-by interchanges, collecting all exiting/entering freeway traffic into a single consolidated ramp and reducing merging with thru traffic. These sort of roadways may occasionally have names/signed numbers, especially the longer multi-lane ones that are just as wide as the mainline freeway (see those along I 95 through Fredericksburg and 495 in Alexandria, VA), but generally they are both treated by traffic and designated by DOTs as just highway ramps, albeit longer and more complex than your typical slip lane or exit ramp. Since both the I 15 and US 95 ramps that were tagged as tertiary are both signed simply as exits rather than local lanes part of the main freeway, the best tag for them would be motorway links.