Components of highway safety programs in other nations are often recommended for adoption in the United States and those here are sometimes tried elsewhere, the assumption in both cases being that program elements have universal relevance. This assumption overlooks the natural history of crash experience in which rates per unit vehicle and unit of travel usually decrease over time as more vehicles are introduced while that per unit population and absolute number of deaths increase, for reasons often unrelated to program elements. Several cultural, environmental and medical or related issues also affect safety programming and this must be considered in attempting to apply programs from one nation to another or even from one population to another within a single nation.