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Abstract
This paper uses a simultaneous equation modeling approach to explore the relationships between mean speed, standard deviation of speed and work zone design characteristics. Data for model estimation were collected in 17 work zones on four-lane, divided freeways in Pennsylvania and Texas. The three-stage least squares estimator was used, as it is both consistent and efficient in the presence of endogeneity and error covariance. The fixed effects and random effects estimators with instrumental variables were also used to address possible unobserved heterogeneity. Results pointed towards a truly simultaneous structure, where mean speed is determined by exogenous design and traffic control features and the standard deviation of speed and the standard deviation of speed is dependent on design and traffic control features as well as mean speed. A number of work zone design and traffic control features directly influenced both speed parameters. Findings of this research indicate the relationship between speed magnitude, speed dispersion and work zone design and traffic control features is more complex than documented generalizations made in current work zone design and traffic control decision processes.
The data used for this paper was collected as part of National Cooperative Highway Research Program Project 3–69. The authors acknowledge and appreciate the cooperation of the Pennsylvania Department of Transportation and Texas Department of Transportation in enabling the research team to collect work zone speed data on active construction projects. The authors are also grateful to the reviewers of the first version of the manuscript. Their comments, questions and suggestions resulted in significant improvements to the paper.