My reasoning for going from motorway to primary rather than going through a sort of classification gradient would be that motorway classification is based off physical characteristics while tertiary, secondary, primary, and trunk are based off importance as a road. This means that a motorway could theoretically have the importance of a tertiary or primary road but is tagged as a motorway as it has freeway characteristics (a high speed limit, interchanges/overpasses, spurs off of another motorway, etc). When the freeway ends and becomes any sort of surface road, be it a neighborhood street, suburban thoroughfare, or an expressway, the classification should be tagged based on how traffic is using it, what destinations it links to, how it serves relative to other roadways, etc., and other tags can be used to describe the physical construction, such as expressway=* and access_control=*.

While this little expressway-like stub of Lake Mead Parkway may see more traffic than the rest of the road to the east, I would have to assume this number is only trivial as the traffic would be going to and from Eastgate Road, which is a relatively minor road itself and not worth in its own merits being linked by a trunk route.