I was riding shotgun in the car when I noticed.... THESE WORK!!
(https://i.imgur.com/hwNcs7C.jpg)
(https://i.imgur.com/DEMtP9u.jpg)
(https://i.imgur.com/8Nl5N13.jpg)
(https://i.imgur.com/uBV8eW9.jpg)
(Sorry for crappy quality, it was snowing and crappy weather)
I was under the impression that these were never used, as I have driven by accidents in this area before, but they never worked!
On a side note, I want one of these for my traffic control collection :-/. What can ya do though.
I've seen the ones on US 36 used quite a bit. Most often those ones step down the speed as you approach congestion.