With the continuous improvement of people's living standards and the continuous development of science and technology, Vehicle While constantly making breakthroughs and innovations in terms of quantity and quality appearance, the negative impact of traffic congestion is particularly obvious. The reason why people begin to pay attention to and effectively alleviate traffic congestion is that traffic congestion not only brings a lot of inconvenience to people's daily life and work, but also severely restricts the economic development of the city. [2]
The phenomenon that vehicles are blocked on the carriageway is called traffic jam. Traffic congestion has caused many problems. First of all, due to the blocked traffic, the vehicles drive slowly on the road, and the most direct impact is to increase the travel time and cost of residents. The increase in travel costs not only wastes time, but also reduces work efficiency.
Secondly, people's physical and mental health has also been hurt. When people are stuck in traffic, their mood will fluctuate greatly, and they are usually anxious and impatient. If you are in this mood for a long time, your physical and mental health will be affected.
Moreover, if you are in a bad mood, your work efficiency will be low, which will directly affect the working condition of the day. In addition, the urban environment has also been seriously threatened. Traffic jam vehicles can only move forward at a slow speed. As a result, parking and starting become more frequent, not only increasing exhaust emissions, but also causing noise pollution. [2]
Urban roads are composed of intricate road sections and intersections. The composition and flow of traffic flow are different at different times and places, and the causes, places, and impact areas of traffic congestion are also different, showing obvious characteristics of time and space distribution.
Characteristics of congested traffic flow
Traffic congestion is a direct reflection of the imbalance between supply and demand of roads in a certain period of time. When traffic congestion occurs, the traffic flow in the upstream and downstream of the road changes significantly. In the upstream section, the traffic capacity decreases, the traffic density increases, the road occupancy increases, the driving speed decreases, the traffic flow through the road section decreases, and the vehicle delay increases, thus increasing the travel time to the destination.
Classification of urban road traffic congestion
According to the causes of traffic congestion, it can be divided into frequent congestion and occasional congestion (normal congestion and abnormal congestion). Frequent congestion refers to the phenomenon that the traffic demand exceeds the traffic capacity of the road in a certain period of time, resulting in traffic congestion. The main reason for this kind of congestion is the increase of traffic flow, which exceeds the normal capacity of the road. It usually occurs at the bottleneck of the road, and usually occurs during the rush hour of commuting. This kind of congestion has a certain regularity. It usually occurs repeatedly in a fixed road section, intersection and fixed time. It belongs to periodic congestion. The duration of congestion fluctuates within a certain range, and its occurrence is predictable. Incidental congestion is mostly the sudden occurrence of traffic events, which leads to a sharp decline in the road capacity, thus causing traffic congestion. The traffic incidents that cause such traffic congestion mainly include sudden traffic accidents, vehicle breakdown, road construction and maintenance, and large-scale commercial activities. The occurrence time and location of traffic emergencies are random, irregular, and usually unpredictable, and belong to non periodic congestion. The duration of congestion is uncertain due to different causes of traffic incidents. According to the type of traffic congestion, it can be divided into primary congestion and secondary congestion. Primary congestion refers to the congestion caused by road bottlenecks or traffic events, which is caused by direct causes. Secondary congestion is usually caused by primary congestion, which makes the traffic flow can not be evacuated in a local area in time. The stranded traffic flows back and spreads to other branches, resulting in sudden increase in traffic demand of other relevant sections and traffic congestion. Secondary congestion is usually caused by accidental congestion. If it is not controlled and dredged in time, it will cause a large area of traffic paralysis. In order to avoid the deterioration of urban traffic conditions, secondary traffic congestion should be avoided as far as possible. Therefore, for the traffic control and management departments, it is very important to find the time and place of the original congestion in time, and accurately analyze and judge the nature and degree of congestion, which is the focus of traffic dispersion and control. According to the degree of traffic congestion, it can be divided into congestion, very congestion, serious congestion and deadlock. When a deadlock occurs, the vehicle can hardly run and the traffic is paralyzed.
According to the influence scope of traffic congestion, it can be divided into local congestion and network (regional) congestion. Local congestion refers to the congestion at the bottleneck of a road or an intersection due to the traffic demand being greater than the traffic capacity, which can be restored to the normal operation state through the self regulating capacity of the road or the temporary dredging by the traffic management personnel. Network or regional congestion usually refers to the congestion of multiple roads and intersections in the region, resulting in too many vehicles gathered in the region, and the entire road network can not run smoothly. In this case, it is necessary to effectively control the traffic flow of some sections and reasonably dredge the traffic flow, otherwise the road traffic in the whole area will be in a closed loop locked state. [1]
In fact, from the analysis of traffic congestion, we can find that traffic congestion generally does not occur at every moment of the day, nor in every road section and every road in big cities. It happens at the time and road sections where citizens can easily gather. For example, the morning, noon and evening commutes are also the time for students to go to and from school in the morning, noon and evening. The sudden gathering and increase of people will inevitably increase the traffic congestion. Secondly, traffic jams are more likely to occur at the intersection of central roads and roads than at other sections of the city. However, the problem of traffic congestion cannot be simply attributed to the sudden gathering of people and the location of the road. There are also the following reasons for traffic congestion:
The plot ratio of the road is small
With the rapid increase of motor vehicles, the roads in big cities are still responding to the old capacity, and the speed of road development is far from keeping up with the increase of motor vehicles. In addition, people will gather in the center of the city during peak hours, and the flow of people will increase, which will lead to serious traffic jams on the central roads of many major cities in China, such as Beijing, Tianjin, Shanghai, Shenzhen, and so on. Especially in the peak hours of the concentrated sections of travel, it seems that these concentrated sections can be figuratively compared to "parking lots".
Road structure
The design of road structure also has an important impact on the accessibility of traffic. In our country, the roads in large cities are generally distributed in a radioactive way, extending around the city center in a divergent state. Although this structure facilitates the convenience of suburbs to the city center, it also causes congestion in the city center to a large extent. In addition, the number of crisscross road intersections will increase, which will cause serious back blocking.
Time distribution of total trips
In China, due to the uneven distribution of people's travel volume in time, it can be divided into peak and off peak periods. However, the short time of occurrence and the dense flow of people are the prominent characteristics of the peak period in China. In other words, at the peak time, the contradiction between the surge of people flow and the road structure is particularly prominent. It increases the pressure on the transportation system.
Urban building planning
The structural trend of the road is largely affected by the urban architectural planning. Generally, the downtown area of a large city is crowded with businesses, and some tertiary industries have become the mainstream of the downtown area. In this way, the people flow in the non downtown area will gather in the center, and this central layout will inevitably increase the road demand in the center, making the central road overloaded. On the other hand, the layout of urban traffic planning does not fully consider the layout of urban land, which is incompatible with the two. It may be that the layout of road network is ignored Urban land layout This also caused the pressure of overload operation in some sections. traffic control
On the one hand, due to the imperfection of traffic laws and regulations in China, the low publicity of traffic management departments on safety education, and the low awareness of people on safety awareness, there will be many violations of traffic laws and regulations.
Uncivilized phenomena such as lane change, congestion, overtaking and so on often occur on the originally congested roads, which not only aggravate the time of congestion, but also cause great potential safety hazards. On the other hand, China's Traffic management system The intelligent level of is far from enough. The traffic command center can not conduct comprehensive analysis and dredging of real-time road conditions, and can not deal with the situation in time. [2] As for the problem of traffic congestion, it can be seen that the factors that cause the problem of traffic congestion are not just one reason, but the result of the joint action of various factors. Therefore, we need to analyze and solve this problem from many aspects and angles.
With the improvement of people's living standards, the number of private cars has become a reality. In order to ease the traffic pressure and reduce the number of travel vehicles, we can implement the single and even number limit policy. This is not only fair, but also can reduce the number of travel vehicles to ease traffic congestion.
Increase the publicity effect of green travel
Green travel means taking buses, bicycles and even walking. This not only reduces the impact of vehicle exhaust on the environment, but also can achieve the purpose of exercise.
The country can increase the publicity of green travel during some public service advertisements, and can also hold some public service activities to make citizens feel the importance of green travel. In addition, citizens can also choose "carpooling", which greatly reduces the use of private cars.
road construction
Reduce the use of forked roads, including elevated roads, underground railways, underground carriageways, etc. The public transportation system can effectively alleviate the traffic jam.
Perfect transportation system and intelligent public management system
Improving the intelligence of traffic management in China can greatly reduce the inconvenience caused by traffic congestion. [2]
Traffic operation status refers to the overall performance of traffic operation status of a road or a region in the city. Drivers and passengers often use qualitative language such as "crowded" and "very crowded" to describe the actual feeling, which is related to the size of the city, road class, traffic mode and travel purpose of the traffic participants. The same traffic volume, different operating environments and people have different feelings, so different countries have different criteria for quantitative measurement and discrimination of traffic congestion.
The early traffic flow status mainly depends on manual investigation. With the development of electronic information technology and the continuous improvement of detection technology, it is possible to automatically collect traffic parameters. Since the 1960s, foreign scholars have studied Automatic Traffic Congestion Identification (ACI) and proposed many automatic identification methods, mainly including parameters threshold Law numerical analysis And pattern recognition methods. Parameter threshold method
Based on the analysis of traffic characteristics, this method uses one or more of the collected flow, speed, occupancy and other parameters as the discriminant indicators. When the parameter exceeds the set threshold, it is considered that a traffic jam event has occurred. The most representative algorithms are California algorithm, Mc Master algorithm and Monica algorithm. The California algorithm was used in the Los Angeles Highway Management and Control Center in 1965. The algorithm obtains the occupancy data of the upstream and downstream double sections of the road section through fixed detectors, compares the difference of the average occupancy with the threshold value of the corresponding location, and judges the road congestion events.
Numerical analysis method
This method usually obtains the predicted value through mathematical statistical analysis of the historical data, and uses this value to distinguish it after comparing with the actual measured data, mainly including the standard deviation method (SDN), double exponential smoothing (DES) algorithm Time series method Etc. Texas Transportation Association developed the standard deviation method during 1970-1975. This method takes the arithmetic mean of n traffic parameters collected before time t as the prediction value of the current time, and then processes the actual collected data with standard normal deviation. When the normal deviation exceeds the preset threshold, it is determined that accidental traffic congestion has occurred. Method of pattern matching
Willsky (1980), et al. proposed a dynamic model algorithm for macro detection. This method considers that the distribution relationship between traffic flow parameters follows a certain established pattern. By collecting historical data, the distribution models of flow density and speed density relationships are established, and several predictable patterns are set. Then the collected real-time traffic flow data are used with the General Likelihood Method (GLR) And multiple model method (MM) to check whether the relationship between parameters matches the given traffic mode, and then judge the traffic operation state.
Domestic experts and scholars have also carried out relevant research on the automatic identification of traffic congestion, but it started late. The early research mainly refers to the experience and methods of foreign countries. With the emergence of traffic congestion in large and medium-sized cities in China, many scholars began to study the identification methods of traffic congestion that are suitable for the characteristics of traffic flow in China.
With the expansion of the scope of urban traffic congestion and the intensification of the degree of congestion, the study of the traffic conditions of a single road or independent intersection can no longer fully reflect the requirements of regional road control and management. The identification and analysis of traffic conditions oriented to the network level has gradually attracted the attention and research of scholars at home and abroad.
Scholars at home and abroad have carried out relevant research on automatic recognition of traffic status, and have achieved some beneficial results, some of which have achieved good results in practical applications. In general, the early automatic identification of traffic status mainly focused on occasional traffic congestion, and the research objects were mainly highways and urban expressways with continuous traffic flow. Traffic congestion was treated as an abnormal event, but there were relatively few automatic identification methods for frequent traffic congestion on urban roads. The traffic flow characteristics of urban trunk roads and expressways are quite different. There are many factors affecting the operation of urban road traffic. In addition to the occasional congestion caused by road construction and traffic accidents, the frequent congestion is more prominent. because Urban road network The complexity of the composition of, and the complexity of the unique mixed traffic flow of motor vehicles and non motor vehicles in our country, has increased the difficulty to distinguish the urban road traffic state. [1] Research Status of Traffic Congestion Control Strategies and Methods
From the control range, traffic congestion control strategies can be divided into local control strategies and System control Policy. The local control strategy takes a part of the road network as the research object (such as a single road section or intersection), and further improves the traffic performance of the study area by optimizing the local traffic resources. From the perspective of control time, traffic congestion control can be divided into three types: static prevention, real-time control and dynamic prevention control. Static preventive control strategy can better reduce the propagation rate of traffic congestion, but the method is less flexible; The dynamic control strategy mainly adopts the method of manual intervention to implement temporary control on regional traffic. Common methods include turning prohibition measures and traffic guidance methods. For example, Middlesex University The congestion dissipation control of emergency single lane is studied by adopting the measure of vehicle prohibition. Scholars at home and abroad have made many research achievements on the mechanism of traffic congestion and congestion response strategies. The main means to solve traffic congestion in the early stage is to strengthen the construction of traffic facilities and increase the traffic capacity of the entire road network to meet the traffic demand. Since the 1980s, the focus has gradually shifted to how to improve the utilization efficiency of existing roads and make full use of the existing road network. With the increasing urban traffic congestion and the emergence of intelligent transportation (ITS), traffic engineering experts and scholars have gradually realized the importance of unbalanced traffic supply and demand. It is difficult to fundamentally solve the problem of urban traffic congestion only by increasing traffic supply, so people have turned to solving traffic congestion measures ITS To consciously change the spatial and temporal distribution of traffic flow in the road network and adjust the balance of traffic travel through the guidance and intelligent management of traffic flow. Many large cities at home and abroad have begun to pay attention to traffic network congestion management, such as GIS The urban traffic congestion management system is developed by technology, and the travel time and speed data are collected by GPS positioning technology. The travel time is taken as the main parameter to measure the performance of the road network. However, due to the lack of scientific understanding and grasp of the mechanism and spatiotemporal evolution characteristics of urban road traffic congestion, the location, time, scope and duration of occasional traffic congestion caused by frequent traffic congestion and sudden traffic events (accidents) cannot be correctly forewarned. Therefore, The existing traffic congestion mitigation control strategies are also often stuck at the level of principle, lacking systematic and operable measures. The essential cause of traffic congestion is the imbalance of traffic supply and demand. Since the cause of the imbalance of traffic supply and demand has not been fully analyzed, the diversion of traffic volume has been overemphasized in the formulation of traffic congestion mitigation strategies and control methods, and the strategies such as road expansion, purchase demand restriction, travel demand restriction and road restriction have been blindly implemented, While ignoring the formulation of traffic flow dredging strategies and control methods based on the road network structure, some even diverted the traffic congestion flow at the accident point to other places through simple traffic control measures, resulting in traffic congestion moving and spreading in the road network. In the aspect of traffic signal control, some scholars have conducted in-depth research on single intersection adaptive control and multi intersection signal coordination control. Although the traffic and transportation efficiency in local areas has been improved, the spatio-temporal characteristics, congestion properties and congestion levels of traffic congestion in the region have not been analyzed during the implementation process, and the effect of regional control has not been improved much. In addition, the existing research also does not consider the specific implementation of traffic congestion mitigation control methods enough, and there is little research on how the traffic information collection, traffic monitoring, route guidance and other application systems established within the framework of the intelligent transportation system can solve urban traffic congestion. The existing research is basically independent of the support that the intelligent transportation system may provide, It is carried out under the hypothetical and idealized road network environment, which affects the application of theoretical research results in practical projects. Therefore, in the context of intelligent transportation, it is necessary to further analyze and study the coping strategies and control methods to alleviate the increasingly serious traffic congestion in modern cities. [1]
Facing the problem of traffic jam, we should analyze and solve it from many aspects. We should not only understand the contradiction between population and roads, but also consider the road structure, urban planning and urban layout. Finally, we should not only solve the problem through some management policies and science and technology, but also improve the impact of human activities on traffic congestion. [2]
