1 Introduction
2 Related work
3 Method
3.1 Free-velocity analysis
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Within a certain range in the front, the number of vehicles which are moving in the same direction is not sufficient to affect the driver to make a reaction on velocity. For instance, the threshold value of the number can be determined as Nln − 1, where Nln is the number of lanes in one direction. It means that the vehicle still has a free lane to move at its desired velocity without the influence of the slow vehicles in front of it.
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The free-velocity collection cannot be executed in the vicinity of intersections in consideration of the forced decelerating, waiting, accelerating processes of vehicles due to the traffic lights and security considerations.
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The free-velocity collection can be executed only when the condition mentioned above has been active for a certain time. It ensures that there is enough time for the driver to convert to his desired velocity from the previous state.
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According to different conditions such as the number of lanes, lane width, and the value of speed limit, we classify the streets into several classes in advance. Thus, the individual vehicle needs to gather and calculate its free velocity for each class respectively. Such is helpful to the accuracy of information collection.
3.2 Recording at the intersection
3.3 Connectivity calculation and delay estimation for street selection
3.3.1 Connectivity probability in light traffic
3.3.2 Queues and individuals
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Connectivity in the queue. The influence of queue in the urban street is very likely to be negligible when the length exceeds 2 mi (3.21 km) [42]. However, in an urban environment, the length of the street between adjacent intersections is generally less than such 2 mi. In other words, the queue generated at last traffic lights will not be dispersed in the current street. And in view of the transmission range of about 250 m, we consider that the connection in the queue is linked from the head vehicle to the last one in the whole street which they entered.
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Head vehicle and tail vehicle. In MMIR, we refer to the head vehicle in the queue as the headmost vehicle at the time of executing the connectivity calculation rather than the time when the queue formed. On the contrary, the tail vehicle is also the meaning. Common sense says that with fewer disturbances from other vehicles, the one with the fastest free velocity in the front of the queue accelerates and more likely runs at its free velocity without loss. On the other side, from starting to move to the last communication for intersection records, the rear vehicles in the queue have more time and practicable distance (is about queue length plus transmission distance) to accelerate than others. Furthermore, the one with the slowest free velocity in the rear can get its free velocity more quickly and then run without disturbance (the vehicles behind have overtaken it almost). Therefore, from respective recording time in the intersection records, we consider both the processes of head vehicle and tail vehicle as acceleration (it is not needed if the vehicle has reached its free velocity) and then running at the free velocity without loss until reaching the range of next intersection or catching the queue ahead.
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Integration and overlap. Once a queue is formed and enters the new street, there are three occurrences we need to notice: the queue catches up with an individual (Fig. 2), an individual catches up with the queue (Fig. 3), and the queue catches up with another queue (Fig. 4). In the first case, the head vehicle overtakes the individual vehicle which means the individual is integrated into the queue, and then we no longer consider it independently. The second case is similar to the first, after the individual vehicle overtakes the tail vehicle in the queue, and then we no longer consider it. Note that the individual vehicle can hardly overtake or be overtaken by all the vehicles in the queue within the distance of usual urban street length, and moreover, there is little probability that its velocity is faster or slower than all the vehicles. In the last case, two queues overlap with each other and are integrated into a new queue. Then, we consider the head vehicle in the queue in front as the new head vehicle and the tail vehicle in the queue behind as new tail vehicle.
3.3.3 Delivery delay
3.3.4 Improvement and adjustment for connectivity calculation
4 Parameter setting
Parameters | Value |
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Street length | 1 km |
Transmission range | 250 m |
Vehicle free-velocity | v ~ N (70, 10.5) KPH |
Sigma (driver imperfection) | 0.5 |
Number of vehicles | 12/30/50 |
Average time interval | 0.5/1/1.5/2/2.5/3 s |
5 Results and discussion
5.1 Accuracy of connectivity probability
5.2 Analysis of estimated delay
Parameters | Value |
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Streets length | 1 km |
Transmission range | 250 m |
Vehicle free-velocity | v ~ N (70, 10.5) KPH |
Sigma (driver imperfection) | 0.5 |
Traffic flow | 50/150/200/250/300 vehicles/lane/h |
Simulation time | 3000 s |
Beacon interval | 1 s |
Test packet sending rate | 10 s |
Test packet’s TTL | 100 s |
Traffic lights’ period | 160 s |