Research on Route Selection Model of Road Network Emergency Evacuation under Ice and Snow

The route selection of road network emergency evacuation is of great practical significance for possible emergencies in future. Based on analysis the principles of emergency path selection, the paper considers urban roads, which meets certain basic conditions as evacuation after the initial choice, take the traffic capacity, intersection density, vehicle delays in the intersection into consideration, and establish the route selection model of road network emergency evacuation under ice and snow.


The emergency lane and the shoulder setting.
When emergencies happen and emergency evacuation is needed, evacuation vehicles are mostly large vehicles, and it is possible for mechanical accidents and the possibility of misuse to take place under emergency conditions. Therefore, it is necessary for vehicles to park on the emergency lane so as not to affect others emergency vehicles, otherwise it will cause the traffic congestion and slow down the progress of emergency evacuation. The connecting roads quantity and intersection density.
"Connecting road" refers to the link road between the public roads and residential main road in the foreign definition, and the link road between the two adjacent signalized intersections in emergency evacuation, as well as other minor roads or the import and export of everyday life and commercial services. If the number of connection is too much, it is bound to affect the evacuation efficiency of vehicles on the main roads. The high density of intersections will also aggravate the vehicles intertwined exacerbate.

Traffic control methods and information level.
In the existing urban road traffic conditions, the information level will directly influence the evacuation efficiency and the enforcement of evacuation plan in the process of emergency evacuation. Intelligent transportation facilities could transmit information of all sections in road network to the control and command center, and transmit the corresponding guidance and decision-making information back to the road users, so that real-time monitoring and induction for the evacuation in site could be realized.

Regional climate conditions.
In the existed study on emergency routes selection, the premise is normal weather conditions. It is needed to take the extreme climatic conditions into consideration for establishing and selection emergency evacuation routes. According to the weather conditions, it will determine the number of evacuation routes, the performance of evacuation vehicle, and the equipment of vehicle ancillary facilities and so on.

Emergency Evacuation Network Optimization
After the initial selection of evacuation routes, the selected evacuation routes are generally irregular, and the actual evacuation capability needs to be verified. The capacity of evacuation networks is calculated as follows: Where, C p is possible capacity (pcu/l/h), N is the lanes number of One-way road, f w is the capacity correction factor by Lane width and lateral clear width, f s is the capacity correction factor by snow and ice conditions, f B is the capacity correction factor by bus, f D is the capacity correction factor by driver conditions in Emergency situation, f V is the capacity correction factor by Fault vehicle, f p is the capacity correction factor by interference of pedestrian and bicycle .
On the existing urban roads, buses occupy a certain proportion. When the bus is in and out of temporary stops, it will inevitably lead to the reduction of road capacity. The urban road network capacity will be reduced in different snow conditions, and with snow at a different level, the reduce extent of capacity is different.

Emergency evacuation route selection model
The basic selected steps of emergencies route in the snow and ice conditions is: firstly, determine urban roads which meet certain basic conditions as an evacuation road objects by initial selection, and then in accordance with the evacuation route selection model, evacuate from the Advanced Engineering Forum Vol. 5 145 shortest time and vehicle flow reasonable allocation point of view, and take the traffic capacity, intersection density and vehicle delays in the intersection into consideration to determine a reasonable evacuation route.

The influence of different evacuation routes on evacuation time.
In the actual evacuation, the evacuation route is divided into series route, parallel route or mixing route, and the series route is divided into series route with same lanes or not. Evacuation route is the series route of single-path: in order to analyze and calculate conveniently, the paper refers to the concept of evacuation time in unit road length, known as the "equivalent evacuation time". The formula is as follows: Where, t 0,ij is travel time per unit length of i kinds of road in j kinds of weather conditions, l d is length of sections , V f,ij is free speed of i kinds of road in j kinds of weather conditions, q is traffic of sections ； C n ij is capacity under the condition, α ij, β ij is lock coefficient of the road and snow conditions respectively, it is shown as follows in Fig.1. Therefore, when the evacuation route is connected by multi-path with different lanes, the total travel time for the evacuation section is that: Where, T is total evacuation time of series route, L k is the length of k-th route, t k,ij is unit travel time of k-th route in corresponding climate condition and road types. Evacuation route is the parallel multi-path: In parallel route, the simplest case is two parallel roads; it is shown as follows in Fig.2: The solution of this equation could be obtained by equation 5 and 3, in order to obtain the travel time of two evacuation slip. Similarly, when the evacuation route is multiple parallel routes, the calculation method is similar:

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Advances in Intelligent Transportation System and Technology When the candidate routes are the combination of several parallel roads, evacuation the route with the gross shortest vehicle traveling time is optimal.When the drawn route is the combination of series and parallel ones, which is shown as equation 7: Where, m is the number of parallel routes, T(L n ) is the road blocking function value of n-th parallel sections, the remaining parameters are the same meaning as above.
If the evacuation route is the combination of series and parallels, it needs to calculate the evacuation time of the connections. Respectively, the total evacuation travel time of entire routes could be obtained by the sum of the the evacuation time of sub-sections. The optimal route is the route with shortest evacuation time.
The influence of evacuation intersection on the evacuation time: under different weather conditions, the main reason for the vehicles delay in signal intersection is the vehicles interleaving of the different direction. According to the above analysis and existing reference, it is known that N junctions in an evacuation path, its total delay is: (9) Where, d k is average delays per vehicle of k-th intersection, C k is the cycle length of k-th intersection (S), X k is the imported Road saturation of k-th intersection = qC/sG, λ k is G/C, Q K is capacity (pcu/hr), y is λ k X.
In the reference [7], the ITS setting is considered as an impact factor on emergency evacuation of urban road network. According to the ITS setting of the evacuation routes, the impact factor is divided into three levels and take use of direct correction factor to standardize the emergency evacuation route selection model. This study considers the impact factor that ITS setting impact on emergency evacuation as 1 during the process of model standardizing, and that is no impact on emergency evacuation selection.

Selection Model of emergency evacuation route.
The emergency evacuation route selection process in the paper is divided into sections and intersection. The sections part should establish travel time impedance function in different snow and road grade conditions; while in the intersection, vehicles delays model in different snow conditions should be established. Through the combination and calculation of these two parts, the optimal evacuation route selection model based on a single OD points could be drawn. The formula is as follows: Where, j T is the evacuation time of the j-th route, Tj T is the evacuation time of the j-th route in the consideration of capacity, Dj T is the total intersection delays of the j-th evacuation route.
In summary, the optimal evacuation route selection model based on single OD points in different snow conditions can be obtained. Assuming that in an OD points, there are n routes, each route with m sections and l signalized intersections. The optimal evacuation routes selection model in different weather conditions is calculated as follows: (13) Where, m is the number of evacuation routes, n is the number of candidate routes, l d is the length of single section, V f,ij is the free speed of i kinds of road in j kinds of weather conditions, q is the volume of section, α ij 、β ij is the travel time parameters in section in the corresponding road and snow conditions, c n ij is the section capacity i kinds of road in j kinds of weather conditions; a j ,b j is the parameters in different snow and ice condition, k Q is capacity (pcu/hr), Ck is the cycle length of k-th intersection (S), y is λ k X k , X k is the imported Road saturation of k-th intersection =qC/sG; λ k is G/C.
For emergency evacuation, the evacuation purposes is to transfer evacuate personnel to a safe region quickly [8]. In general, the selected routes are many or more. The sort of evacuation routes by the evacuation route selection model could not effectively determine to meet the evacuation requirements or not, and the number of vehicles needed to allocate on road is not sure. So according to the characteristics of emergency evacuation, based on traffic emergency evacuation traffic allocation, multiple routes could be obtained by the emergency evacuation route selection model. It determines the number of emergency evacuation route selection and the flow on each route for minimizing the total evacuation time.

Conclusion
This paper determines the steps of the emergency evacuation route selection by analyzing the emergency evacuation route selection principle. Through the initial selection of the emergency evacuation route, it analyzes the influence of different evacuation routes on the evacuation time, and network optimization. Ultimately it establishes the emergency evacuation route selection model.