You know one thing I've noticed? I'm curious if it's universal, because I've never seen a dirt/gravel road I can drive 40 MPH on before. I've mostly seen gravel roads, and I'd stay down towards 20 or so, due to the surface, and the noise from rocks and stuff bouncing around inside the wheel wells. But here, I can get up to 40 or so without trouble. They're smooth--not as many potholes or big rocks and stuff.
I've lived in New Mexico and Missouri, and the roads I know you can't drive that fast.