The Highway Safety Manual was developed by American Association of State Highway and Transportation Officials (AASHTO). It provides guidance and tools to conduct quantitative safety analyses alongside other transportation performance measures. Before the Highway Safety Manual engineers were limited in their ability to quantify safety impacts of project decisions due to a lack of an authoritative source and thus safety considerations were typically minimized in the decision making process. The Highway Safety Manual changed that and brought safety to the forefront of any operation decisions or transportation facility design. The manual is broken into four different sections:
- Part A – Introduction, Human Factors, and Fundamentals of Safety;
- Part B – Roadway Safety Management Process;
- Part C – Predictive Methods; and
- Part D – Crash Modification Factors.
Of particular interest to the RoadSafe GIS system are the elements of part B, the roadway safety management process. The first component of the roadway safety management process is known as network screening, which refers to the review of the transportation network to identify and rank sites according to their potential for crash frequency or crash severity reduction after implementation of a countermeasure.
Network screening consists of five steps:
- Establish focus
- Identify network and establish reference populations
- Select performance measures
- Select screening method
- Screen and evaluate results
For more details on each step you should refer to the Highway Safety Manual. For RoadSafe GIS, however, we want to highlight the performance measures available in step 3. The following table of performance measures and data inputs was adapted from the Highway Safety Manual:
| Performance Measure | Crash Data | Roadway Information for Categorization | Traffic Volume | Calibrated Safety Performance Function | Other |
|---|---|---|---|---|---|
| Average Crash Frequency | X | X | |||
| Crash Rate | X | X | X | ||
| Equivalent Property Damage Only (EPDO) | X | X | EPDO Weighting Factors | ||
| Relative Severity Index | X | X | Relative Severity Indices | ||
| Critical Rate | X | X | X | ||
| Excess Predicted Average Crash Frequency Using Method of Moments | X | X | X | ||
| Level of Service of Safety | X | X | X | X | |
| Excess Predicted Average Crash Frequency Using Safety Performance Functions (SPFs) | X | X | X | X | |
| Probability of Specific Crash Types Exceeding Threshold Proportion | X | X | |||
| Excess Proportion of Specific Crash Types | X | X | |||
| Expected Average Crash Frequency with EB Adjustment | X | X | X | X | |
| Equivalent Property Damage Only (EPDO) Average Crash Frequency with EB Adjustment | X | X | X | X | EPDO Weighting Factors |
| Excess Expected Average Crash Frequency with EB Adjustment | X | X | X | X |
A common scenario described in the Highway Safety Manual is using these performance measures to evaluate intersections in the transportation network. Outside of the AASHTO sponsored SafetyAnalyst software tool, however, there are not many options for calculating the performance measures for a reference population. RoadSafe GIS changes that since we can now incorporate many of these performance measures into our high collision intersections module. In our next blog post, we will go more in depth into the performance measures available in RoadSafe GIS, the data requirements and how our tools can aid network screening.