US Ends.com suggested, when posting a photo of a similar sign on eastbound I-70, that there is one speed limit for regular travelers and a second, lower speed limit for dust storms.
I took this photo in 2006 on I-70 in Utah, while MOH and I were traveling in an easterly direction. The sign was located near Cisco (location way below), and it's apparently no longer there, or anyhoo it isn't shown on the current rendition of Google Street View. The Colorado River is less than 10 miles to the south of the interstate, and one happens to be driving through an upland largely underlain by Mancos Shale, which often makes paved roads lose their originally smooth surface, leaving drivers and passengers alike to feel about the same as they would while driving over bad frost heaves on the Alaskan Highway between Destruction Bay, Yukon, and Tok, Alaska.
Anyway, this is my first post in an off-and-on series of Thursday road sign posts. Many of the signs I've photographed over the years are standard mileage signs, exit signs, warning signs (curves, low flying aircraft, what-have-you), regulatory signs (stop, yield, do-this, do-that), mileposts (often photographed so I'll know where a certain roadcut or outcrop was located along the road), and other signs, some of which have nothing to do with the road (Iditarod signs, pub signs, signs on and inside buildings): anywhere signs, in other words.