Study on the dynamic relationships between weather conditions and free-flow characteristics on freeways in Jilin

Weather conditions have considerable impact on freeway free-flow characteristics, several empirical studies have stated that precipitation, snow, and visibility loss may cause reductions in speed and capacity. However, relative few have considered the potential time-lagged effects of weather conditions on free-flow, while more attention should also now be paid to the trend, cycle, and irregular fluctuations inherent in temporally aggregated observed data. Therefore, a detailed investigation in this paper was carried out to examine the linkages between meteorological factors and key traffic stream parameters. The study was based on recent archived data from sensor devices, such as inductive loop detectors and weather sensors, located on provincial freeways of Jilin Province in China. The trend and cyclical components were firstly separated from weather and free-flow parameter series by using filtering technique. Then a multiple-equation system known as a vector autoregression (VAR) was proposed for characterizing the temporal dynamics inherent in these components, while Granger causality theory was adopted to identify the existence of a systemic causal relationship. Furthermore, the recently developed method of impulse response function provided insight into the cross-effects of these traffic parameters and their responses to weather conditions. Various causal relations can be found considering separately summer and winter periods. Besides, some interesting results were also concluded from our study, including descriptions of the dynamic interplay among variables, as well as the possible variations in hourly freeway traffic activities with respect of weather trends. It is hoped that this study will shed light on a fully understanding of how weather factors affect freeway traffic conditions.