I'll admit, due to the lack of and orientation of signage, the many intersections on Prairie, and the fact it changes direction every 6 months it can get confusing.. Especially among the non-readers and more careless among us. 

Apparently a number of folks still riding it when the trail sign says closed, as well..

This morning I was there around 8:30, and the lady said a number of bikers had already exited. The sign said clearly, hiking only. 

I talked to the Ranger and he said the determination is made around 9 or 10 and the sign is changed at that time.. Normally this is a good time for me. But this time of year I'm riding earlier for obvious reasons..
I got back in my truck and will ride another day.

Here's the phone number for trail status - 803-547-1019

He said a person answers the phone and it's updated in real time.