Using Observational Before-after Studies To Evaluate Road Safety Projects (2009)

The main objective of this technical brief is to provide practitioners with a quick reference on the key considerations and components of a valid before-and-after observational study. This document also serves as a tool to increase the level of understanding of before-and-after study techniques so that they can be more effectively conducted in the future.

By providing better assessment tools and resources to allow practitioners to explicitly consider safety impacts in their decision-making, there has been a positive shift in North American road safety. However, there are cases where local studies and research have employed inferior analysis techniques in before- and-after assessments due to lack of understanding of proper techniques, time or resource constraints and/or budget limitations. As transportation safety practitioners, shortcomings must be identified in before-and-after study methodologies so that good research results are identified and poorly conducted work is not propagated.

This document is not intended to be a prescriptive reference on the methodologies and formulae for completing different types of before-and-after studies. This has been left to more comprehensive documents such as the updated Highway Safety Manual and research work available in the public realm and provided in the final section of this brief.

Section 2 of this technical brief outlines the background fundamentals and definitions required to understand the primary components of a before-and-after study, the techniques adopted to conduct such a study and how each technique differs from the others. Section 3 provides an introduction and a brief description of each technique, its requirements, strengths and weaknesses.

The Transportation Safety Council would like to thank the following authors for their contributions in the preparation of this brief:

• Pedram Izadpanah, research fellow, Department of Civil Engineering, University of Waterloo
• Alireza Hadayeghi, vice president, Synectics Transportation Consultants
• Hossein Zarei, transportation safety analyst, Synectics Transportation Consultants

The council would also like to acknowledge the assistance of Russell Brownlee, transportation safety engineer at Giffin Koerth Forensic Engineering and Accounting, and Brian Malone, president of Synectics Transportation Consultants, for being co-editors of this document. In addition, the council’s gratitude goes to Elizabeth Wemple and Calvin Mollett for being reviewers of this document.