Very interesting, but unfortunately the one example they pulled was incorrect. If you look at the street sign is says "Exit | 45 mph", which means this is the speed limit when taking the exit. The captured example looses this information.
And it's not even the speed limit, it's the recommended speed for that exit. IOW, go as fast as you like, right up to the posted speed limit, but if you're a tall semi hauling a load it is recommended that you slow to the speed posted on the yellow sign. But I digress...