Highway engineers and stakeholders can improve the quality of analysis and discourse through better understanding of what constitutes a “safe” design or decision. First, the subject of safety should always be addressed carefully. There is no such thing as perfectly “safe” highway; one should never promise this nor characterize safety in absolute terms. Second, properly understood, highway safety has been described as having two dimensions, nominal safety and substantive safety.

• Nominal safety refers to adherence to design practices, standards, warrants, etc.
• Substantive safety refers to actual (or expected) performance as defined by the frequency and severity of crashes.

Further Reading:

Nominal and Substantive Safety - CSS Quick Facts