Exploring endogeneity of macroscopic speed parameters: empirical study during low volume conditions in construction work zones

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.