Modeling the Interactive Effects of Safety Countermeasures on Pennsylvania Highways
Open Access
- Author:
- Shao, Yen-wen
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 15, 2015
- Committee Members:
- Paul Peter Jovanis, Dissertation Advisor/Co-Advisor
Paul Peter Jovanis, Committee Chair/Co-Chair
Eric Todd Donnell, Committee Member
Martin T Pietrucha, Committee Member
Dennis Kon Jin Lin, Committee Member - Keywords:
- countermeasure
interaction
crash estimation
Crash Modification Factor
CMF
multiple treatments
safety effectiveness - Abstract:
- Crash Modification Factors (CMFs) are typically used to quantify the effects of road safety countermeasures by estimating the expected safety impact with the installation of certain treatments. When multiple treatments are implemented together, multiplying individual CMFs is a common way to estimate the overall reduction on crashes quantitatively, carrying with an assumption of statistical independence of the treatments. The validity of the assumption of treatment independence has not been studied extensively. When interaction among treatments exists, it is inappropriate to assume independence of the treatments. The inappropriate use of CMF multiplication is likely to cause errors in the estimation of the magnitude of the safety effects of the treatment (i.e. CMF). This research develops a methodology to identify and quantify the presence of interaction between countermeasures. Differences in safety effectiveness estimates are developed so as to better understand the implications of treatment interaction on safety. The cross-sectional modeling approach using negative binomial models was adopted to estimate the effect on CMFs of countermeasure interaction. This research analyzes the interaction using Generalized Linear Models (GLM) with an underlying negative binomial distribution. After careful selection and screening of predictor variables, a series of models with independent CMFs are compared to a comparable series without the assumption of treatment independence (i.e. with two-way interaction terms). This dissertation consists of three papers. For the first paper in Chapter 2, the methodology is applied to rural multi-lane highways by studying the potential interactions among lane width, right should width and median type. These roadway countermeasures interact with each other throughout the cross-section of rural multilane highways in terms of their association with safety performance. Chapter 3 studies a more homogeneous type of road environment, rural two-lane highways, with three different treatment combinations that were selected from the countermeasures throughout roadway, roadside, and alignment elements, including horizontal curve density and access density. Two types of interactions, categorical by categorical and categorical by continuous interactions, were developed and analyzed in the research. All treatment combinations have demonstrated the existence of interactions with the significance on interaction terms. For the last paper in Chapter 4, the interactive effects on limited access facilities (i.e. interstate freeways) were explored to see how the interaction of a same pair of treatments (e.g. lane width and shoulder width) vary across different road facilities (i.e. rural multilane and interstate freeways). The treatment combinations that are tested for independence include lane width, right shoulder width, and median type. The comparison of the GLM models demonstrates the existence of significant interactive effects. The findings of all papers have achieved the research objectives to develop a feasible method for examining treatment independence and to demonstrate the influences of interaction in CMFs by effectively estimating the combined safety effectiveness with interactions. The differences in CMF with and without interaction are shown to lead to the divergences of conclusions for predicting and assessing the combined treatment effects. It is necessary to be aware and examine the independence of multiple treatments when potential interactions exist; neglecting interactions should be avoided to prevent the combined treatment effects from being incorrectly estimated. The findings of this dissertation are expected to bring contributions to the field by enhancing the quality of safety estimation with the application of CMFs, and therefore provide innovative yet reliable tools and methods for roadway safety improvement.