Large vehicles differ dramatically from passenger cars in many physical and operational characteristics. This paper aims to study how large vehicles in mixed traffic flow affects speed on freeway. Jingjintang freeway is selected to be the investigated section. The whole freeway was divided intosixty-four sub-sections according to their specific characteristics. Bivariate test was used to identify factors that relate to large vehicles. Speed trends for passenger cars, traffic flow and large vehicles are also examined. Results show that the proportion of large vehicles has an obvious hourly characteristic. The paper also identifies a relatively small but statistically significant correspondence between large vehicles percent and total crash rates. Speed limit as a safety strategy to avoid crashes shouldnot ignore large vehicles factor. Nowadays , new technology enable speed limit adapt to different traffic conditions. Linear regression techniques were used to find relationships between proportion of large vehicles and traffic flow speed .The results provide a method for speed limit to responses to the change of large vehicles.
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