You'll usually see it on a ticket, in a police report, or hear it from a lawyer or judge in a sentence like, "The posted speed was a prima facie limit." That means the speed limit creates a legal presumption about what is reasonable. If a driver goes over it, that is taken as initial evidence the speed was unsafe or unlawful, unless other facts change the picture. "Prima facie" basically means "enough on its face to make the case unless rebutted."

In everyday driving, this matters because a speed number is not always the whole story. A driver can sometimes argue that the surrounding conditions or roadway facts affect whether the speed was actually unreasonable. On the other hand, going under the limit does not automatically make a driver safe. Under Michigan's basic speed law, Michigan Vehicle Code MCL 257.627 (1949), a driver must travel at a careful and prudent speed for conditions. That becomes a real issue in late-fall freezing rain, when black ice can form before road crews are out.

For an injury claim, a prima facie speed limit can be evidence of negligence, but it usually is not the only evidence that matters. A speeding ticket, citation, or police finding may support an argument that the driver breached a duty of care. Still, insurers and courts will also look at road conditions, visibility, braking distance, and whether the driver could have avoided the crash.