Safety Evaluation of Centerline Plus Shoulder Rumble Strips

Chapter 2. Objective

This research examined the safety impacts of the combined application of CLRS and SRS in Kentucky, Missouri, and Pennsylvania. The objective was to estimate the safety effectiveness of this strategy as measured by crash frequency. Intersection-related and animal crashes were excluded. Excluding these crash types, target crash types included the following:

• Total crashes (all types and severities combined).
• Injury crashes (K (fatal), A (incapacitating), B (non-incapacitating), and C (possible) injuries on KABCO scale).
• Run-off-road crashes (all severities combined).
• Head-on crashes (all severities combined).
• Sideswipe-opposite-direction crashes (all severities combined).

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

• Do effects vary by level of traffic volumes?
• Do effects vary by the frequency of crashes before treatment?
• Do effects vary by vehicle speeds?
• Do effects vary by lane width and shoulder width?
• What is the difference between the combined effects of CLRS and SRS and effects of either in isolation?

The evaluation of overall effectiveness included the consideration of the installation costs and crash savings in the form of 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 due to changes in traffic volume and other non-treatment factors.
• Pool data from multiple jurisdictions to improve reliability of the results and facilitate broader applicability of the products of the research.