5.1.4 Limitations

It is not possible to cover all the ever-changing safety analysis processes, procedures, methods, approaches, and tools in this chapter. Safety practitioners are aggressively working to develop new research to fill in existing gaps and limitations in the application of safety analysis methodologies. Some of these gaps include (but are not limited to):
  • Data unavailability, including lack of necessary data inputs for crash analysis;
  • Traditional crash rate analyses assumed a linear relationship between traffic volume and crash frequency;
  • Lack of consideration for variability of driver population from one location to another;
  • Lack of consideration for traffic variability throughout the day (current safety analysis procedures use ADT volumes);
  • Changes in existing tools and versions of those tools — some methods and tools do not consider variables that may be important to roadway design, creating the potential for statistical bias and uncertainty;
  • Randomness, natural variations in crash data, and/or changes in site conditions; and
  • Safety analysis approaches or predictive methodologies have not been developed for all facility types, alternative intersections, or roadway designs
Due to these limitations, professional judgement is necessary when performing safety analysis.