Impacts of helmet law changes on motorcyclists' fatalities in the United States

A motorcycle helmet has been considered a very important safety equipment for motorcyclists as it prevents or minimizes head and traumatic brain injuries which are often fatal. Hence, most of the states in the United States have enacted a law requiring all motorcyclists to wear helmets in the 1960s and 1970s, which is also known as the Universal Helmet Law (UHL). Nevertheless, some states began to repeal or restrict the law since then. After several years, one state has re-enacted the UHL as it has realized that the law was quite effective in reducing motorcycle fatalities. There have been three states with changes in their helmet laws in the 2000s. Florida and Pennsylvania repealed the UHL in 2000 and 2003, respectively, and they only require underage motorcycle riders to wear helmets. In contrast, Louisiana re-enacted the UHL in 2004. The prior studies have consistently discovered that the UHL repeal put motorcyclists in danger whereas the re-enactment significantly increases helmet-use and decreases motorcycle fatalities. Nevertheless, the prior studies have only evaluated short-term effects of the law change mostly using the traditional before-after approach (it is called naïve before-and-after study). The naïve approach assumes that there is no change from the ‘before’ to ‘the after’ period that affected the safety of the entity under scrutiny; hence, the naïve approach is not able to account for the passage of time and its effect on other factors such as exposure, maturation, trend, and regression-to-the-mean bias. In order to account for the influence of a variety of external causal factors that change over time, the before-and-after study with comparison group and the before-and-after study with Empirical Bayes (EB) approaches are adopted in this research. A series of SPFs (safety performance functions) were estimated for motorcycle crashes using county-based data. The expected numbers of crashes were calculated using the SPFs to estimate more dependable CMFs (crash modification factors). The authors have attempted to explore the long-term impacts of repeal and reinstatement of the universal helmet law by using 13 to 16 years of data. A range of safety performance functions were developed based on counties and calculated the expected motorcycle fatal crashes