If you're going to entertain yourself on a trip across western South Dakota by finding the alphabet (in order, and no cheating by skipping the Q) on billboards, I'd suggest starting well east of Kadoka.
That's assuming you're traveling from east to west. Going west you're heading toward the Black Hills, with its tourist attractions eager to catch the attention of I-90 travelers. Going east, don't bother with the game, because the billboards are so sparse that between one and the next you'll forget which letter you're looking for.
The challenge of the billboard game, of course, is finding the rarer letters: X, Z, and the infamous Q. The X (as in "exit") isn't a problem along the Interstate. The Z is rarer but not impossible, thanks to the CraZy Horse carving and occasional other amaZing attractions. Q can be more of a problem; thank goodness for Quick stops, antiQues, and Quiet campgrounds.
The hardest letter to find here, surprisingly, is J. This is why it's important to start east of Kadoka, where there is a sign advertising the Flying J truck stop near Rapid City. (Back when it was a Conoco, J's were really scarce.) For the discerning, there is also an inconspicuous J near the bottom of a billboard at the Kadoka off ramp. If you miss either of these, you might as well start hoping someone passes you in a Jeep.
During a recent trip across the western half of the state, I noticed quite a few new or freshly painted billboards for Black Hills tourist attractions. Based on this as an informal indicator of economic health, South Dakota is doing well.
I do have a few suggestions, though, for tourism businesses. As long as they're refurbishing billboards, how about making a few additions? Wall Drug could advertise its Zany cowboy Quartet and Quirky back alley and let us know the roaring T-Rex will make us Quiver in our flip-flops. The 1880 Town could add a Quick-draw contest. Reptile Gardens could promote its Jumping cockroaches and Jungle flowers—or maybe they could add a Jaguar or a Zebra.
You may think by now that I am a fan of billboards. Not so much. I do think they have their place—which probably includes the long stretch of Interstate across western South Dakota.
Still, creative travelers don't need billboards to entertain themselves. My daughter used to keep herself occupied by counting road kill, which she wrote down in a notebook under various categories: pheasants, deer, rabbits, raccoons, skunks, and UFO's (Unidentified Flat Objects).
Even someone who likes billboards might have to admit that there are way too many of them along the last few miles east of Exit 61 as you approach Rapid City. The road is littered with billboard after bigger billboard after enormous billboard, flashing lighted ads, and such an ugly clutter of signage that you can hardly find the exit. It isn't exactly the best way to welcome travelers to the beautiful Black Hills.
It reminds me of a parody by my favorite poet, Ogden Nash:
I think that I will never see
A billboard lovely as a tree.
Perhaps, unless the billboards fall,
I'll never see a tree at all.
Maybe some of the Exit 61 signs could be removed and spread out along I-90 eastbound. Only, of course, if they have plenty of Q's, Z's, and J's.