Safety Evaluation of Red-Light Indicator Lights (RLILs) At Intersections

Chapter 2. Objective

The research described in this report examined the safety impacts of the application of RLILs at signalized intersections. The objective was to estimate the safety effectiveness of this strategy as measured by crash frequency. The research team considered only intersection-related crashes, and further sub-target crash types included the following:

• Total crashes (all types and severities combined).
• Injury crashes (K, A, B, and C injuries on KABCO scale).^[1]
• Right-angle crashes (all severities combined).
• Left-turn crashes (all severities combined).
• Rear-end crashes (all severities combined).
• Nighttime crashes (all severities combined).
• Crashes in which driver(s) disobeyed traffic signal (all severities combined).

A further objective was to address questions of interest, such as the following:

• Do effects vary by traffic volume?
• Do effects vary by intensity of treatment (e.g., number of enforcement lights per intersection)?
• Do effects vary depending on the exact type of indicator light (e.g., blue LED versus white bulb)?
• Do effects vary by posted speed limit on the major route?
• Are the effects short-lived?
• Are spillover effects evident?

The evaluation of overall effectiveness 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 untreated reference sites. This included reference sites both adjacent and not adjacent to treated sites in order to consider potential spillover and migration effects. The term spillover is used to describe the effect of the treatment at nearby locations because motorists may be unsure of the exact locations of the treatment and may change their behavior at the nearby locations as well. Crash migration may occur if motorists change their behavior at the treatment sites and then compensate at other locations, or if motorists choose alternate routes to avoid the treatment.
• Properly account for changes in safety due to changes in traffic volume and other nontreatment factors.

^[1] The KABCO scale is used to represent injury severity in crash reporting (K is fatal injury, A is incapacitating injury, B is non-incapacitating injury, C is possible injury, and O is property damage only).