Validation of Time-Safety Influence Curve Using Empirical Safety and Injury Data—Poisson Regression

Previous studies on designing for construction worker safety were founded in part on theory, derived primarily from the time-safety influence curve introduced by Szymberski in 1997, that the ability to mitigate construction safety risks is greater during early stages of facility development, as opposed to after the start of construction operations. It is concerning that the time-safety influence curve has not been validated using empirical evidence, and the relationship between time and safety was primarily established based on anecdotal evidence, including subjective judgment using personal experience. The present study attempts to address this concern by examining the validity of the time-safety influence curve using empirical data and statistical analysis. Injury and safety data were collected from 64 construction projects in the United States (U.S.) using the sustainable construction safety and health (SCSH) rating system, a tool designed to measure and evaluate the extent to which workplace safety and health is given priority during different phases of facility development. The result from the statistical analysis revealed strong evidence of correlation between timing of safety input and project safety outcomes, such as rate of incidents. This finding validates the theory behind the time-safety influence curve and provides statistical as well as empirical evidence that timing of safety input is associated with different incident rates. The present study is expected to help motivate construction stakeholders to address construction safety earlier in a project’s lifecycle to effectively achieve desirable safety outcomes