This road mainly exists to make an easy way for Seattlities to go to Salt Lake City
Well, I must say that I actually recognized the "Zzyzx" as the exit on I-15. The last time I went to Las Vegas I decided that I wanted to go to Los Angeles and thought that was a little odd when I saw it.
Before you start reading: bear with me, I have a point...
Now first of all, being a Yakima resident I must inform you that I am quite offended by your implication that Yakima is not a major city. ;> I'm assuming that, as you live in Washington, you have probably at least driven through it. You are suffering from classic "I Saw the 'Welcome to Yakima: The Palm Springs of Washington' Ultra-Cheesy Moronically-Placed Amazingly-Embarassing-to-a-City Sign Symdrome." This affects most new visitors and passersby who happen to notice the billboard, strategically positioned downtown along I-82 between the Lincoln Ave/Fair Ave and the Yakima Ave/Terrace Heights Way interchanges. Rarely has a guest in my house failed to notice it. I recently had visitors from both France and England, and while the kid from England didn't know where Palm Springs was and consequently didn't get it, both thought it was equally dumb. Radio call-in shows here make fun of it on a regular basis, and publically humiliated the one single person who phoned in and said they liked it.
Apparently it's the guy who owns Trail Wagons that owns the space, and likes the sign. It is also apparently illegal to tar and feather someone these days. These are both unfortunate situations.
It is also possible that you have no clue as to what I am ranting on about. Well, next time you on your way to Salt Lake City and see the traffic pick up a couple of miles past the Fred G. Redmon Memorial Bridge (the largest single-span concrete bridge in North America, if you want to add that to your facts list: 1336 feet long, 330 feet high, and on a sadder note, 2 suicides in the past year) spanning the Selah Gorge, keep a lookout on the right. It's sandwiched between the freeway and the offramp for Exit 33A (Lincoln/Fair).
I-82, while it is important, as are all interstates, in long-haul truck shipping and road-based vacationers to more interesting places, is essentially the lifeline of the Yakima Valley. The north-western part of the Lower Yakima Valley is also served by US 97, but commuter traffic between Ellensburg and Yakima (like myself), Yakima and the Lower Valley Cities and the Tri-Cities accounts for the vast majority of the actual traffic it carries.
Anyway, if you're still reading when you get to this point, it is nevertheless true that Yakima is equal to or larger than many other cities you list on other interstates, but those places don't get the "I know it's a stretch" line. According to the US Census Bureau the Yakima MSA contains about 200,000 people.
You might also want to include Richland on your list of major cities. The Tri-Cities area contains nearly as many people as Yakima, so it probably qualifies.
[Comments about I-238 deleted]
On I-82, exits 88 and 93 are the only interstate exits in the state of Washington still lit with Mercury Vapor streetlights. The closest possible exception is on I-5, at the 320th St. S. exit in Federal Way, where at the intersection of the Northbound offramp and 320th St. there is one lone mercury vapor light. It's not actually on the freeway, though, only on the offramp.
Mercury Vapor lights cast a cold, blue light. Most of the Washington interstate system has High Pressure Sodium lighting, the annoying yellow that you see everywhere. There are a few areas that have Metal Halide (clean, white light) as well, but those are growing in number while use of mercury is shrinking. Anyhow, if you were curious, here are some paces you can go in the Seattle area to see the differences in the lights. I-5 north of the Ship Canal Bridge has Metal Halide. SR-520 just east of 405 has some mercury lighting left over, as does the Valley Freeway (167) north of the SuperMall. Basically every where else, the lighting is sodium.
On I-90, exit 93, Elk Heights Rd. is the only unlit interchange. There are NO unlit interchanges on 5, 405, 705 or 182. 82 has at lest one unlit exit, the "Military Training Area" just south of Ellensburg.