Chapter 5 Safety Analysis Concepts
5.1 Introduction
5.1.1 Overview
Chapter 5
provides background information on the purpose of the safety chapter; the intended users and limitations of the safety analysis concepts; and overviews of TxDOT safety policies and programs, AASHTO HSM, and Crash Modification Factors (CMFs).5.1.2 Background and Purpose
TxDOT has adopted the aspirational target, referred to as Road to Zero of zero traffic fatalities on Texas roadways by 2050. A variety of approaches are being taken to facilitate the potential reduction of fatal crashes and improve the overall safety for all road users. The application of proven safety countermeasures is key to reaching the Road to Zero target. One of the objectives of this manual is to promote safe-by-design practices by integrating safety analysis during project development, analyzing existing safety conditions, comparing alternatives, and predicting safety performance for future conditions. This manual also promotes consistency and standardization in safety analysis across the State and advances general transportation safety knowledge. Each District prepares annual safety plans to improve safety within their District.
5.1.3 Intended Users
The safety analysis concepts, methodologies, and tools presented in this chapter and
Chapter 6
are intended for engineers, planners, modelers, and designers who perform safetyrelated analyses for TxDOT projects. A safety practitioner is defined as a TxDOT employee and/or consultant who is tasked with performing, reviewing, and/or approving safety analyses. The safety methodologies and tools in this chapter may also be appropriate for use by local agencies and MPOs.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.