In theory, it would often be possible for the rule simply to be a restatement of the background justification. A few years ago, for example, the state of Montana eliminated all fixed speed limits, requiring instead only that driving should be "reasonable and prudent."' But drivers have widely divergent ideas of what is reasonable and what is prudent, and so do police officers and judges. As a result, there developed wide variations in speed limit enforcement, the consequence being that drivers became highly uncertain about just how fast they could go without running afoul of the law. This much uncertainty was too much for the Montana Supreme Court, which struck down the "reasonable and prudent" rule as excessively vague.

Unknown, Frederick Schauer - Thinkin…, loc. 30