Research on Intersection Signal Switching Model under Emergency Situation

The frequent occurrence of the city emergency leads to rapid development of emergency traffic management which is an important part in the Emergency Rescue System. In the intersection with heavy traffic, the emergency rescue vehicles often with increased delay, reduced safety, and sometimes even with a collision, which restrict the efficiency of the rescue. This paper established the intersection signal control optimization model based on detail analysis of emergency rescue vehicles traffic characteristics and traffic signal control. The models, on one hand, were able to guarantee the emergency vehicles through the intersection quickly and without delay; on the other hand, could ensure the minimum impact to other vehicles in the process of the emergency vehicles through the intersection. Finally, the model’s practicality was verified by real cases.


Introduction
In modern life, the emergency events, natural disasters, and the non resistance events happen frequently.Moreover, these events are unpredictable and bring to several damages.Statistics show that every year our country has big loss because of the disasters such as the natural disasters, accidents, the social and public events , and the loss caused by rescue that does not come in time increased year by year 1 .Therefore, the accurate and reliable traffic control measures are an effective way to reduce the response time of the emergency vehicle, improve the efficiency of the emergency rescue, and reduce the loss of lives and property.But in the process of the emergency response, the emergency vehicle often with increased delay, reduced safety, and even with a collision in intersections.According to the results of the emergency vehicle accident investigation in Virginia, about 31% accidents of the emergency vehicles' happened in the intersection.Moreover, the consequences are even more serious than happened in other places 2 .Therefore, the study on signal optimization control under emergency situations has an important significance in emergency rescue, traffic safety, and traffic management.
For the research on the right-of-way of the emergency vehicle, the scholars in transportation area in different countries focused on different aspects.Bachelder and Foster proposed the emergency vehicles preferred system based on the induction coils 3 .Mussa and Selekwa raised one conversion process of the traffic signal timing based on quadratic optimization method 4 .Nelson and Bullock pointed out that the signal control should back to normal operation rapid and safely after the emergency vehicles through the intersection 5 , but the conversion mode of the signal control they proposed back to normal often used the ways of smooth, add only, or dwell, all of which are set according to the experience and are hard to adapt to the change of complex traffic condition.
In recent years, as the development of intelligent agent technology, many domestic scholars put forward the methods to solve the traffic problem based on this innovative, real-time and intelligent technology.Cao fulu Chang an University proposed a kind of urban traffic signal optimization control system for the optimization of signal control through the coordination of the intelligence agents between adjacent intersections.Cheng Xiangjun Beijing Jiaotong University presented one distributed traffic signal coordination control method based on the multiagent technology, this method makes the vehicles through the intersection without delay under the higher utilization of the each phase in crossing traffic time.
We can conclude from the above analysis that the current study is mainly concentrated in the hardware and system design of the signal priority.The purpose of emergency signal optimization control can be expressed as one objective function, and this function subjects to some restrictions.We are able to obtain the optimal control strategy by the mathematical programming model and various kinds of optimization methods.

Control Process
This paper chooses the four phase signals cross-intersection as the research object.The signal optimization control measures under emergency situations are built according to the traffic conditions when the emergency vehicle's detector detects the request vehicles.The control process as in Figure 1.

General Vehicle Detector Position Setting
As shown in Figure 2, the position of the general vehicle detector can be expressed as where, L VD is the setting distance of other vehicle's detector; L QD is the length of the other queuing vehicles under peak traffic condition; L DD is the length of the slowly moving vehicles under the peak traffic condition.
To ensure the emergency vehicle could pass through the intersection safely and without delay, we must make sure the other vehicles in front of the emergency vehicle dissipated in time.L QD can be determined by the average value of multiple times

Normal traffic
Driving direction

Emergency vehicle detector
Other vehicle detector measurements.L DD is difficult to detect, it can be replaced by the sum of the mean value and two times standard deviation of L QD 6 , as follows:

2.2
Then, x i is the detected length of the other queuing vehicles under peak traffic condition; X is the average queue length of the other vehicles; σ is the standard deviation of the other vehicle queue length.

Emergency Vehicle Detector Position Setting
The setting distance of the emergency vehicle detector should be able to guarantee that it has enough time to dissipate the vehicles before the emergency vehicle arrived at the intersection.So, there exists where t ED is the ideal travel time of emergency vehicle from the detected position to the intersection parking line; L is the distance between the emergency vehicle detector and intersection; v ED is the ideal average speed of the emergency vehicle.
In actual situation, the emergency vehicle could reach the intersection in any time.Assume the driving direction is already known, according to the different signal phase condition when the signal control machine receives the detected information from the emergency vehicle, the location of the emergency vehicle's detector can be set as follows.
1 The signal phase of the emergency vehicle's driving direction is green: Then, the disappearing time of the other vehicles is: where t CL is the disappearing time of the other vehicles before the emergency vehicle appears; v QD is the dissipation speed of the other vehicle's queue in front of the emergency vehicle.
There should be a certain safety time interval between the last dissipated vehicle and the emergency vehicle.The formula is as follows: where t SI is the time interval between the last dissipated vehicle and the emergency vehicle.

Emergency vehicle Other vehicles
Emergency vehicle detector Other vehicles detector So the minimum distance of the emergency vehicle's detector in this condition is 2 The signal phase of the emergency vehicle's driving direction is yellow.
According to the conduct process we proposed above, the minimum distance of the emergency vehicle's detector in this condition is where t SL is the loss of the start time see Figure 4 .
3 The signal phase of the emergency vehicle's driving direction is red.The minimum distance of the emergency vehicle's detector in this condition is in Figure 3 L where t AR is the all red light duration of this phase; t AR is the faded signal time of this red phase.
In practical situations, it is not easy to ensure t AR , which could be resolved by use the "real position" and the "virtual position".The "real position" is the actual position of the emergency vehicle detector.The formula is as follows:  The "virtual position" is the position set by L 3 .In this condition, we should deduct the setting position of emergency rescue vehicle's detector during the red signal phase.So there exists: L 3 ≥ L 3 see Figure 5 .
Therefore, the minimum distance of the emergency vehicle detector is:

Optimal Control
At first, divide the entire process into three periods: detection period, before, and after the intersection period.

Detection Period
After the distance of the emergency vehicle detector is determined, the information about emergency vehicle, such as the location, speed, and the direction can be determined.The formula as follows 7-9 : Once the control machine request received the request of the emergency vehicle, we could decide whether to carry out the signal priority control by the formulas we proposed above according to the signal phase and fixed signal timing cycle.

The Optimization before the Vehicles through the Intersection
The optimization purpose of this period is 1 to guarantee the emergency vehicle safety and without delay; 2 to minimize the influence to other vehicles.
According to the actual situation and with above optimization goals, we could implement optimization methods by different control measures, which can be specified as 10- 13 1 The signal phase in emergency vehicle's driving direction is green when the detector detected emergency vehicle to ensure the safety when the emergency vehicles passes through the intersection.We do not consider the emergency vehicles through the intersection in the yellow signal phase.
i Assume t GR > t ED > t CL .
The signal control plan is still running as normal.t GR is the remaining signal time of this green phase in emergency vehicle's driving direction when the detector detected emergency vehicle; t AG is the full signal time of this green phase in emergency vehicle's driving direction.
ii Assume t ED ≥ t GR > t CL .
The signal control plan is changed according to the difference between the surplus green time of this phase and the required time for emergency vehicles arrived.a If t ED − t GR − t SW − t SI ≥ T , the signal control will be switched to normal, and the next phase can get the time for green is t HG t ED − t GR − t SW − t SI .After t HG , convert the green signal phase to the emergency vehicle's driving direction.
b If t ED − t GR − t SW − t SI < T, extend the show time of the green light phase, there exist t DG t ED − t GR T EI .Where, T is the time of the static car needed to through the intersection as usual.t DG is the extended time of the green signal phase in the emergency vehicle's driving direction.T EI is the time of the emergency vehicle needed to through the intersection.
iii If t ED > t CL > t GR , the signal control plan is changed according to the difference between the surplus green time of this phase and the required time for emergency vehicles arrived also.
a Convert the pedestrian traffic light in emergency vehicle's driving direction to red until the emergency vehicle passed the intersection and make speakers or tips to notice to the crowd in the meantime.
The priority control process of the emergency vehicle.

The Optimization after the Vehicles Passes through the Intersection
The control process of the emergency vehicle under the emergency situation can be described in Figure 6.
According to the Figure 6, the following relationships can be calculated: 12 where t P is the time interval between the moment of the emergency vehicle detected and the beginning of next signal cycle; t D is the time interval between the beginning of the signal cycle and the moment of the emergency vehicle detected; t RE is the time length of the intersection signal optimization control after the emergency vehicles through the intersection; m, n is the signal cycle number.In this period, establish the optimal model with the minimum delay of other vehicles' as the optimization target.Such that 14-18 13

First Period
The selected experimental data and the average length of the vehicles' queue are shown in Tables 1 and 2.
The average speed of the dissipating vehicles is 3.3 m/s and the setting position of the detector are shown in Table 3.

Second Period
Select eight groups of data to simulate, as in Tables 4 and 5.
It is clearly shown in Tables 4 and 5 that all the emergency vehicles can pass through the intersection without delay and just cause little influence to other vehicles in most cases.
Mathematical Problems in Engineering 13

Third Period
Apply the first group data in Table 5 to calculate in MATLAB program, the results are shown in Table 6.
Apply the data in Table 6 to the simulation environment, the length of the other vehicles' queue are shown in Table 7.
Calculate the eight groups data, the average delay of emergency vehicle and other vehicles' are shown in Table 8.

Conclusion
This paper aims to establish the optimization model of the signal switching under emergency situations.On one hand, the models could guarantee the emergency vehicles are able to through the intersection safe and without delay; on the other hand, pass could ensure the minimum impact to other vehicles in the intersection during the processing of emergency vehicles pass through.Finally, real cases are employed to verify the practicality of the models.The models and algorithms can be also provided to some relevant management departments as the decision-making reference.

Figure 1 :
Figure 1: Intersection signal conversion control process under emergency conditions.

Figure 2 :
Figure 2: Classification of the traffic vehicle.

Figure 3 :
Figure 3: The location of emergency vehicle detector green .

Figure 5 :
Figure 5: The location of emergency vehicle detector red .
9The location of emergency vehicle detector yellow .
HG is the green time that other phases obtained; t SW is the loss time of conversion; t CL is the detected time of the dissipated vehicles in front of the emergency vehicle; t ED is the time from the position that emergency vehicle is detected to the stop line; v ED is the detected travel speed of the emergency vehicle; L QD is the actual queue length of other vehicles in emergency vehicle driving direction; L DD is the detected length of the slowly moving vehicles in emergency vehicle driving direction.
CL , 2.11 where t If t ED − t CL − t SW − t SI ≥ T , the signal control will be switched as normal, and the next phase can get the time for green is t HG t ED − t CL − t SW − t SI .After t HG , convert the green signal phase to the emergency vehicle's driving direction.b If t ED −t CL −t SW −t SI < T, extend the time of the green signal phase and the extended time is t DG t ED − t GR T EI . 2 The signal phase in emergency vehicle's driving direction is yellow when the detector detected emergency vehicle.i Assume t ED − t CL − t SW − t SI t YR ≥ T .The signal control machine will be switched as normal, and the next phase can get the time for green is t HG t ED − t CL − t SW − t SI − t YR .Where, t YR is the remaining signal time of this yellow phase in emergency vehicle's driving direction when the detector detected emergency vehicle; t AY is the full signal time of this yellow phase in emergency vehicle's driving direction, there exists 0 ≤ t YR ≤ t AY .After t HG , convert the green signal phase to the emergency vehicle's driving direction until the emergency vehicle passed the intersection.ii Assume t ED − t CL − t SW − t SI t YR < T. In this case, convert the green signal phase to the emergency vehicle's driving direction until the emergency vehicle passed the intersection.3 The signal phase in emergency vehicle's driving direction is red when the detector detected emergency vehicle.i Assume t CL < t ED < t RR .The other phases can get the time for green is t HG t ED −t CL −t SI .After that, convert the green signal phase to the emergency vehicle's driving direction and the break time of the red light in the emergency vehicle's driving direction is t DT t RR − t ED − t CL − t SI t SW .Where, t RR is the remaining signal time of this red phase in emergency vehicle's driving direction when the detector detected emergency vehicle; t AR is the full signal time of this red phase in emergency vehicle's driving direction.ii Assume t ED ≥ t RR .a If t RR ≤ t ED − t CL − t CL ≤ t RR t AG , the signal control machine will be switched as normal.b If t RR t AG < t ED − t CL − t CL , extend the time of the green signal phase and the extended time is t DG t ED − t RR − t AG T EI .c If t ED − t CL − t SI < t RR , convert the green signal phase to the emergency vehicle's driving direction and the break time of the red light in the emergency vehicle's driving direction is t DT t RR − t ED − t CL − t SI t SW .4 Pedestrians scheme under the emergency situation.

Table 1 :
Basic data of the experimental analysis.

Table 2 :
The average length of the vehicles' queue.

Table 3 :
The setting distance of the detectors.

Table 4 :
The second simulation of the optimization control low traffic .

Table 5 :
The second simulation of the optimization control high traffic .

Table 6 :
The optimization of the green light duration.

Table 7 :
The length of the other vehicles' queue.

Table 8 :
The average delay of the other vehicles.