Empirical Study Of Operational Performance And Safety Assessment Of Underground Diverging Area Using Vissim

With the growing economic development and urbanization in China for the past 10 years,many highways especially underground highway tunnels are being constructed throughout the country.The underground road system in China has transcended the USA,Europe and other developed countries and the number of highway tunnels in China exceeded 1200 in 2013.Recently,many underground tunnel interchanges are also being constructed.Driving in the underground highway tunnel is different than the driving on the open highway.Although,less crashes happen in the underground highway tunnel but the severity of the crashes is higher than the open highways.Moreover,Interchanges are critical segments of the highway where merging and diverging of traffic take place.Underground tunnel interchanges are different than the open highway interchanges which offer a limited section of tunnel highway for lane change and diverging of vehicles.Therefore,this study focuses on the operational performance and safety assessment of the underground highway tunnel along with the underground tunnel diverging area.For this purpose,the microscopic simulation model,VISSIM,was used to analyze the operational performance.The operational performance of the underground highway tunnel was analyzed during low,medium and high traffic conditions with different lengths of the lane changing section 100 m,200m and 300 m before the entrance of the exit ramp of underground tunnel interchange.For safety assessment,Surrogate Safety Assessment Model was used to perform the traffic conflict analysis and results of each scenario were compared.For the operational performance of the underground highway tunnel,vehicles’ delay,speed and lane changing speed measurement were made.The results show that during low and medium traffic conditions,travel time delay of the traffic do not vary significantly.But during high traffic conditions,mean travel time delay was reduced by 94% when the length of the lane changing section was increased from 100 m to 300 m.Similar results were obtained by analyzing the lane changing speed of the vehicles.Results show that during high traffic conditions,mean lane changing speed was increased from 23.2 km/h to 51.5km/h when the length of the lane changing section was increased from 100 m to 300 m.Analysis of Variance was performed to find the statistically difference in mean of operational performance indicators due to traffic volume factor.For safety assessment,traffic conflict techniques were used using SSAM.SSAM filtered the conflicts using the TTC threshold values based on the vehicles’ trajectories.Results of SSAM gave very few conflicts during low and medium traffic conditions.But during high traffic condition when the length of the lane changing section was 100 m,SSAM reported 202 total number of conflicts and these conflicts were reduced by 74% when the length of the section was increased to 300 m.ANOVA test was also performed to find the difference in mean of safety performance indicators due to traffic volume.