Safety Evaluation of Intersection Conflict Warning Systems

Chapter 2. Objective

This research examined the safety impacts of the application of ICWSs in Minnesota, Missouri, and North Carolina. The objective was to estimate the safety effectiveness of this strategy as measured by crash frequency. Target crash types included the following:

• Total crashes (all types and severities combined).
  
  
• Injury crashes (K (fatal injury), A (incapacitating injury), B (non-incapacitating injury), and C (possible injury) on KABCO scale).
  
  
• Right-angle crashes (all severities combined).
  
  
• Rear-end crashes (all severities combined).
  
  
• Nighttime crashes (all severities combined).

While the ICWS strategy specifically targets right-angle crashes, other crash types were considered to determine whether there were supplemental benefits or drawbacks. Rear-end crashes were the only additional crash type determined to occur commonly enough to be reasonably considered independently in the analysis. The research team surmised that there was potential for other drivers to be alerted in addition to the conflicting vehicles for which the systems were designed. This could lead to reductions in rear-end crashes on both the major and minor routes, owing to increased awareness. This would differentiate the outcome from the effects of traffic signals, which typically produce an increase in rear-end crashes while reducing right-angle crashes.

A further objective was to address questions of interest, such as whether effects varied depending on the following characteristics:

• Type of installation (i.e., specific type or combination of ICWS).
  
  
• Location of installation (i.e., post mounted, overhead, or in advance or at the intersection).
  
  
• Intensity (i.e., number of approaches).
  
  
• Level of traffic volume.
  
  
• Posted speed limit on the major route or minor routes.
  
  
• Presence of turn lanes.
  
  
• Presence of intersection lighting.
  
  
• Before period expected crash frequency.

The evaluation of overall effectiveness also included the consideration of the installation costs and crash savings in terms of the B/C ratio.

Meeting these objectives placed some special requirements on the data collection and analysis tasks, including the need to do the following:

• Select a large enough sample size to detect, with statistical significance, what may be small changes in safety for some crash types.
  
  
• Identify appropriate reference sites without ICWS installation.
  
  
• Properly account for changes in safety due to changes in traffic volume and other factors unrelated to ICWS installation.
  
  
• Pool data from multiple jurisdictions to improve reliability of the results and facilitate broader applicability of the products of the research.