Rear-end Collision Risk Management Of Freeways In Heavy Fog

Heavy fog may easily cause traffic accidents,thus freeway closures are frequently taken in order to ensure traffic safety in China,which not only seriously affect the travel of people,but also bring great economic losses. This paper mainly studied how drivers’ behaviors and psychological characteristics are influenced by heavy fog on a one-way two-lane freeway,as well as traffic measures in heavy fog weather in order to ensure traffic safety.The driving behavior studiesadopt the method of obtaining specific indicators such as the driving speed and the space headway from a driving simulator experiment,with the combination of questionnairesurveys to obtain drivers’ psychological characteristics. The results show that:(1) the participants’ space headway is shortened in heavy fog when there are no cars in a neighboring lane;(2) the participants’ space headway is also shortened in heavy fog when there are carsdriving at a constant speed in a neighboring lane. Moreover,the increased speed of the neighboringcars leads to a corresponding increase of the average free-flow speed and shortened space headway;(3)most participants are not aware that they have shortened the space headway in heavy fog;(4) the corrective measure based on intelligent transportation systemsenables participants to reduce the standard deviation of free-flow speed in heavy fog,as well as eliminate the differences of space headway between clear weather and heavy fog weather.The traffic measurestudies mainly include single-lane and two-lane intermittent release measures on freeway. By proposing cellular automaton(CA) models of traffic flow that consider driving behaviors in heavy fog and by simulation, different intermittent release measures are studied in this paper. The simulation resultsof single-lane roadmodel indicate that the risk indicatorfain fog is much higher than normalweather when cellular occupancyρ<0.5. After taking intermittent release measures,the magnitude offawill drop from 10-4to 10-5 under the same fog condition,which greatly enhances the safety.In addition, this paper concludes the appropriatevehicle number released for each time and the time intervalht between the released fleets and the maximum number of vehicles Qmax which can be released per hour.The study resultsshow that two-lane intermittentreleasemeasures can also reduce the risk of rear-end crashes which means the measures are effective. We found that there existed different critical values(hmax) under different speed limit conditions. Intermittent release measures could effectively controlrear-end collisionrisk when ht>hmax. However,both fa and hmax would be increased by fog zone expansionand asynchronous release in the two lanes.The experiment results show that driving behaviors in heavy fog on freeway are different from those in clear weather. The drivers tend to keep the space headway much shorter than in normal weather conditions,due to the cognitive limitations. The reduction of space headway in heavy fog often causes rear-end collisions. After a suitable intermittent release measure is applied,the probability of traffic accidents becomes reasonable. Therefore, freeway management departments could effectively control the rear-end collision risk through adopting a suitable intermittent release measure. This study results can be used as a theoretical basis and reference for traffic management departments to develop intermittent release measures.