"Excessive speed" crashes and yellow advisory speed signs are related. If you're driving a 55-mph road at only 45-mph, but run off the road on a yellow-signed 25-mph curve-- CONGRATULATIONS! You've just had a speed-related crash! You violated ORS 811.100, the Basic Speed Rule by driving "a vehicle upon a highway at a speed greater than is reasonable and prudent" regardless of the legal maximum.

Oh, you thought speeding crashes were due to violating the speed limit? Hardly, most are due to adverse conditions like curves, rain, snow, and fog. 

In fact "almost 50 percent of speeding-related fatalities occur on lower speed collector and local roads, which carry only 28.1 percent of the total vehicle miles traveled in the United States." About the only place you'll have a speed-related crash on an interstate is a ramp.. with a yellow advisory limit!