The main goal of this study is to propose a safety performance index assessment tool to improve the construction safety. Formulation of the safety performance index of construction sites is achieved upon a validated multidimensional safety performance model. The contribution of this study could be summarized as incorporation of fuzzy set theory into structural equation modeling to develop a safety performance index assessment software tool. Case studies were conducted at 11 international construction sites and the results of their site safety performance indices were benchmarked. A short (simple) safety performance model was developed as an alternative to the full model (proposed model) to assess safety performance of construction sites. Results showed that short model predicts the safety performance with an acceptable accuracy and requires less time to complete. Finally, a safety performance index assessment software tool for construction sites was proposed by developing a site safety performance (SSP) application for mobile devices based on the validated multidimensional safety performance model. The paper attempts to numerically validate the influencing factors of construction safety with the help of a mobile device application. The paper also develops a mobile application tool to measure safety performance at any construction site.