Safety Analysis of Florida Urban Freeways with Special Focus on the Influence of Truck Lane Restriction Policy
Urban freeways and tollways in the United States are characterized by a significant number of truck traffic hauling freight between various origins and destinations. The truck traffic varies in size from single-unit trucks to multi-unit tractor trailers. Trucks also have different operational characteristics dissimilar to those of passenger cars such as low deceleration capabilities resulting in longer stopping distances. Statistics show over 200 percent increase in truck vehicle miles of travel in the United States since 1970. The increase in truck vehicle miles of travel on US urban highways has posed significant challenges to policymakers given that truck traffic contributes to the reduction of efficiency and safety of the highway systems especially on limited access roadways. To address challenges and concerns posed by the growth of truck traffic on highways, policymakers have implemented a number of strategies including changing highway design standards to accommodate trucks, introduction of intelligent transportation systems, and the implementation of operational control policies aimed at heavy vehicles. One of the most prevalent operational control policies is the truck lane restriction. The implementation of truck lane restriction is mainly predicted on the principle that the restriction would increase the level of service and operational safety on limited access highways especially those located in urban areas. Like many other states, Florida has implemented truck lane restriction on approximately 430 miles of the state highway system. However, the effectiveness of this strategy in improving safety and efficiency of these highways has not been properly studied or documented. The objective of this study was therefore to analyze the effect of truck lane restriction on safety of the urban limited access highways in Florida. The analysis of safety was achieved through performing a before-after study that compared crash characteristics of highway sections before truck lane restriction was imposed to crash characteristics after the imposition of truck lane restriction. In addition, a negative binomial regression model was used to determine the influence of truck lane restriction as a modeling variable together with other variables derived from geometrics, traffic, and socio-economic characteristics related urban limited access highway sections in different metropolitan areas in the state of Florida. Data on geometric, traffic, and crash attributes were collected from various databases maintained by the Florida Department of Transportation including the Roadway Characteristics Inventory (RCI) database, the Crash Analysis Reprot (CAR) database and various other data provided by FDOT district offices and through field data collection. The before-and-after crash analysis data were from 200x to 200x while the geometric, traffic, and crash data used in the negative binomial regression modeling were from year 2005. All the data were checked for accuracy to eliminate sections that were under construction to avoid skewing the results. The results of the crash prediction model that was based on negative binomial regression model showed that the coefficient of the truck lane restriction variable in the model was negative but insignificant (p ≤ 0.808). The negativity of the variable indicates that highway sections with truck lane restriction had insignificantly less crashes than sections without truck lane restriction. A statistical marginal effect analysis was used to determine the percentage decrease in crashes that can be expected if truck lane restriction was implemented on a highway section. The results showed that in year 2005 there was a 4 percent decrease in crashes on sections with truck lane restriction compared to sections that did not have truck lane restriction. In addition, the results showed that when truck percentage variable was changed from minimum of 2 percent to a maximum of 15 percent in the model, there was a decrease of crashes by 22 percent. These results coupled with the results of the before-and-after analysis suggests that there is no clear cut safety benefits associated with the imposition of truck lane restriction. These results are in line with literature findings which showed mixed results arising from a number of studies conducted on different highways in different states in the United States. However, it should be noted that truck lane restriction has documented operational benefits such as decreasing lane changing by passenger cars and is generally popular with the traveling public.
July 3, 2007.
A Thesis submitted to the Department of Civil and Environmental Engineering in partial fulfillment of the requirements for the degree of Master of Science.
Includes bibliographical references.
Ren Moses, Professor Directing Thesis; Lisa Spainhour, Committee Member; John Sobanjo, Committee Member.
Florida State University
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