I think you can ride on gravel roads in Missouri, I cant quote book and verse on it though.
In Arkansas, you are allowed to ride on roads if only going from trailhead to trailhead. But, I doubt many deputys know that.
Trouble is, the kids get out there and rut the roads up, and generally wreak havoc and give us all a bad reputation. A few years ago, I was riding on the gravel with my daughter on the back, just putting in 2nd or 3rd, barely off idle, and a sheriff deputy stopped me and said to get it off the road. Hell, I ride faster in my yard than I do on the gravel, at least I can get traction in the grass!