Document Type


Date of Award

Spring 5-31-2013

Degree Name

Doctor of Philosophy in Transportation - (Ph.D.)


Civil and Environmental Engineering

First Advisor

Janice Rhoda Daniel

Second Advisor

Athanassios K. Bladikas

Third Advisor

I-Jy Steven Chien

Fourth Advisor

RongFang Liu

Fifth Advisor

Jian Yang


Studies have shown that, roadway safety has become an intensively investigated topic with the objective of improved understanding of the factors that cause crashes to occur. However, it has been shown that as traffic volumes continue to increase across the United States, 52% of drivers feel less safe on the roads today more than they did five years ago and that the American public feels that traffic safety is a serious problem that needs both the government and media to pay more attention to this issue.

In response to these public and driver grievances, State and National transportation agencies have been and continue to pursue and understand the causes and solutions that would significantly reduce roadway crash frequencies. At national level, through various and rigorous studies, the American Association of State Highway and Transportation Officials, AASHTO has published the Highway Safety Manual to quantify safety using predictive models and CMFs. Various efforts have been attempted at state level too, for example, Texas DOT has developed an Interim Roadway Safety Design Workbook that describes the relationship between various roadway elements and each element influences roadway safety.

In an effort to contribute towards understanding and resolving the factors that influence crash frequencies on roadways, through a thorough literature search. This study realizes that although there has been vast research in this area, no study has explicitly explained why there is variation in crash frequencies on roadways segments with similar physical/geometric features and annual average daily traffic (AADT). Studies suggest that these variations are due to volume changes throughout the day, an effect literature shows that can only be addressed by hourly volumes and not AADT.

Driven by these literature conclusions, this dissertation develops crash modification factors (CMFs) for urban freeways by considering level of service (LOS) deterioration due to change in hourly traffic volumes. Here, this study investigates LOS when it deteriorated from A to B, B to C, C to D, D to E and E to F using hourly volume and hourly crash data collected on urban freeway segments, specifically routes US 1, NJ 3 and NJ 21 in the State of New Jersey. Data were collected on 14 miles of urban freeway segments and 1344 hours of traffic volume count and crash data were analyzed for a period of four years, 2008, 2009, 2010, and 2011.

Results from this investigation, shows that operational elements have some influence on urban freeway safety. This dissertation shows that as LOS deteriorated from A to B, B to C, C to D, D to E and E to F, the estimated CMFs were 0.673, 1.110, 0.865, 1.452, and 0.370 respectively. These findings concur with those referred to in this dissertation’s literature review findings, which showed that by adding capacity, that is, by reducing congestion initially results in safety improvement that diminishes as congestion increases.



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