Safety Evaluation of Multiple Strategies at Stop-Controlled Intersections

Chapter 3. Objective

This research examined the safety impacts of multiple strategies implemented at stop-controlled intersections throughout South 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 (fatal injury, incapacitating injury, nonincapacitating injury, and possible injury).
  
  
• Rear-end crashes (all severities combined).
  
  
• Right-angle crashes (all severities combined).
  
  
• Nighttime crashes (all severities combined).

A further objective was to address the following questions:

• Do effects vary by area type (i.e., urban versus rural)?
  
  
• Do effects vary by approach configuration of intersection (i.e., three-legged versus four-legged)?
  
  
• Do effects vary by lane configuration of intersection (e.g., four mainline lanes and two cross-street lanes versus two mainline lanes and two cross-street lanes)?
  
  
• Do effects vary by traffic volume?
  
  
• Do effects vary by expected crashes?

The evaluation of overall effectiveness included the consideration of the installation costs and crash savings in terms of the benefit–cost (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 untreated reference sites.
  
  
• Properly account for changes in safety because of changes in traffic volume and other factors unrelated to the strategy.