It appears to be a sing that can be folded down to cover the message, but here it is August and it says:
Chains - Four Wheel Drive
REQUIRED

I first saw it last Saturday 8/22 but didn't have my camera handy, figuring I'd get it 'next' time. Unfortunately next time was today but I'd totally forgotten about it and drove by AGAIN. On our return trip today we'd didn't feel like making a u-turn to go back and take a photo.
I probably won't be up that way until a week after Labor Day weekend, so if anyone manages to take a photo sooner, feel free to post it in the HAZ Cartoon thread.