Tang Zihan, Zhu Minqing, Li Junli, Zhao Mengxuan, Zhang Shaohua
Hebei University of Technology
Abstract: In order to reduce the delay and conflict caused by the large vehicles passing through the suburban highway signal intersection due to the spatial layout of the existing highway level signal intersection in Tianjin, which is generally not suitable for the driving of large vehicles, and improve the traffic capacity and safety of the highway level signal intersection in Tianjin. Through field observation, video surveillance and camera, the traffic investigation of five typical intersections in Tianjin was carried out, the driving characteristics of large vehicles and the problems existing when passing through the highway signal intersection were analyzed, and the intersection of Gaowang Road and Meishi Road, which had the highest proportion of large vehicles, was taken as an example, and the problems were found and the spatial layout was optimized through the investigation of the traffic volume of the intersection by vehicle type. Finally, the optimization scheme is evaluated by VISSIM simulation software, and the results show that the optimization scheme significantly reduces the delay of vehicles at the intersection with a high proportion of large vehicles, and improves the traffic capacity of the intersection.
Keywords: signal intersection; spatial layout; proportion of large vehicles;
About author:TANG Zihan (2001—), female, from Handan, Hebei Province, research direction is urban and rural planning.
0 Introduction
With the rapid development of the mainland's economy and science and technology in recent years, the construction of the mainland's transportation infrastructure is also in full swing, the transportation scale allocation has been continuously improved, the service level has been significantly improved, and the transportation supply capacity has been further increased. Due to the fact that the economic development of the mainland still presents certain regional differences, and the economic development and transportation level development between regions are still unbalanced, the transportation demand between regions is diversified[1].
The proportion of large vehicles in road traffic is typically 25% and can reach as much as 60% [3]. The model structure of large vehicles is huge, the driving space demand is high, and because of its own poor power performance, driving speed should not be too fast, and the operation flexibility is low, resulting in more consumption and waste of road time and space resources, and the potential safety hazards are greater.
If the road design does not fully consider the characteristics and driving needs of the traffic flow dominated by large vehicles, it will lead to the formation of a large number of moving bottlenecks in areas with a high proportion of large vehicles such as suburban roads, which will seriously hinder the driving of small and medium-sized vehicles, resulting in the reduction of their driving speed and traffic efficiency, and the vehicles with large differences in vehicle models will interfere with each other, resulting in the decline of road traffic capacity and the waste of road space resources. Taking Tianjin as an example, the traffic composition of suburban highways is mixed and the proportion of large vehicles is relatively high, but the characteristics of small and medium-sized vehicles are mainly used as the basis for determining the parameters of plane signal intersections, which lags behind the structural characteristics of freight vehicles dominated by large vehicles [4].
Many scholars at home and abroad have conducted various studies on the optimization of road level intersections. However, there are still few studies on the level signalized intersection of suburban highways, which are dominated by large vehicles. With the upgrading of the complexity of highway traffic composition and the increase of large vehicles, the spatial layout optimization of the plane signal intersection of suburban highway under the field of view of large vehicles has certain reference value for ensuring the safety of highway and improving the efficiency of road traffic operation in mainland China.
1 Research on the characteristics of plane signal intersections of suburban highways under the vision of large vehicles
1.1 Selection of research objects
In this study, five suburban highway signal intersections, including the intersection of Jiuyuan Line and Beiyang Line, Gaowang Road and Meishi Road, Jinghuan Line and Jinyong Line, Jinwei Line and Jiuyuan Line, Jiuyuan Line and Wujing Line, were selected to summarize the driving characteristics and driving problems of large vehicles through field observations and questionnaire interviews.
1.2 Driving characteristics of large vehicles
Large cars refer to: a total mass of 4.5t (inclusive), the number of passengers (except the driver) 20 (inclusive) or a vehicle length of 6m (inclusive) or more, such as ordinary buses, medium-sized trucks and large special vehicles are large vehicles. Due to the differences in vehicle models and power performance and other factors, large vehicles and small and medium-sized vehicles present great differences in the characteristics of highway driving, which are mainly reflected in three aspects: speed, acceleration and deceleration, safety distance and sight distance:
(1) Speed: Affected by factors such as automobile structure and dynamic performance, the speed of large vehicles is generally low, with an average speed distribution range of 50~80 km/h and a maximum speed distribution range of 70~100 km/h [5]. Both the average speed and the maximum speed are quite different from the speed of other models, so the resulting speed difference will obviously lead to the reduction of traffic flow stability and road smoothness.
(2) Acceleration/deceleration: Large vehicles have low driving flexibility, so the performance of starting and braking is low, which leads to the weak ability of large vehicles to accelerate and decelerate [6]. When emergency braking or stop-and-go situations occur on highway roads, the performance of large vehicles is limited. Therefore, in most cases, large vehicles tend to maintain a low speed and maintain a long distance from the front and rear vehicles to ensure driving stability and safety.
(3) Safety distance and sight distance: large vehicles require more driving space due to their larger size, so the driving flexibility is far less than that of small and medium-sized vehicles, and the dynamic space required for driving is also larger. And large vehicles have heavy loads, low power factors, small driving power, and far sight distance.
1.3 Analysis of large vehicle driving problems
(1) Large vehicles have a long wheelbase and wide body, so the space occupied when driving is much larger than that of ordinary vehicles. Large vehicles can easily get stuck when turning and making a U-turn at highway level signal intersections, affecting the smooth flow of the road and even affecting traffic safety.
(2) When other conditions are equal, the impact force generated by the collision of the vehicle with larger mass is much greater than that of the ordinary small and medium-sized car impact, and because there is a large gap between the driving speed of the large vehicle and the small and medium-sized vehicle, the impact force is further aggravated. Therefore, once a large vehicle encounters an accident on the road, the resulting losses are usually relatively significant.
(3) Due to the difference of driving speed, the middle interval distance of large vehicles is usually large when following small and medium-sized vehicles, and the excessive gap in the traffic flow can make the efficiency of vehicle passage decrease, and cause great waste of time and space resources to highway plane signal intersections.
(4) Due to the large size of large vehicles, the sight of the surrounding drivers is seriously blocked, and it is easy to produce negative psychological emotions such as a sense of oppression and tension, so the driving behavior of the surrounding vehicles is unpredictable, and it is easy to change the driving behavior frequently, such as changing lanes, overtaking, acceleration and deceleration.
2. Optimization of the spatial layout of the intersection for large vehicles
2.1 Intersection location
The proportion of large vehicles on suburban roads in Tianjin is relatively high, so the intersection of Gaowang Road and Meishi Road in Wuqing District, Tianjin was selected for spatial layout optimization research.
Gaowang Road is the main road in the north-south direction of Tianjin, and Meishi Road is the main road in the east-west direction. Gaowang Road is a county-level highway connecting Wangqingtuo Town, Xianggu Port, Douzhangzhuang Township, Sicundian Town, Baigutun Township and Gaocun Town, also called 231 County Road. There is a lot of farmland around the road, which is affected by the Beijing-Tianjin-Hebei urban agglomeration, and the traffic flow is relatively large. There are no residential areas and commercial land around the road intersection, and the flow of people is very small, and the vicinity is mostly farmland and industrial parks, and there are many large vehicles such as trucks.
2.2 Intersection status
Through field observation, the traffic survey of the geometric conditions of the intersection was carried out, and the direction of Gaowang Road was two-way four-lane, and the direction of Meishi Road was two-way six-lane, and the current canalization map of the intersection was drawn according to the field investigation as shown in Figure 1.
At the intersection of Gaowang Road and Meishi Road, the method of drone real shooting and video counting is adopted, and the cycle is used as the counting point to count the flow of each vehicle type in each entry road every 9min, and the flow rate of each vehicle type at the intersection is obtained by summarizing the data of each entrance road. The survey results show that the proportion of large vehicles in the east and west import roads is 71% and 63% respectively, which is relatively high, and the proportion of large vehicles in the north and south import roads is 37% and 41% respectively.
Fig.1 Current canalization diagram Download the original image
2.3 Intersection problem analysis
Through the preliminary investigation, it is found that the intersection of the east and west entrance road and the north entrance road through the large car more than small car, the south entrance road through the large car slightly less than the small car, on the whole, the large car more than the small car, but the intersection design vehicle for the standard car, does not fully consider the intersection through the large car through the situation of the intersection. Therefore, this article mainly analyzes the problems of large vehicles when passing through this intersection:
(1) The weight and load of large vehicles are relatively large, so the time and distance required to slow down before passing through the intersection are long [1], and the location of the sign indicating the vehicle to slow down before the intersection is too lagging for large vehicles.
(2) Large vehicles have wider and taller bodies, so when waiting for a signal light in front of an intersection, large vehicles may obscure the view of drivers of small cars queuing behind [8]. The signal lights at this intersection are located on both sides of the sidewalk, and the vehicles behind cannot accurately judge the changes of the signal lights, resulting in problems such as the vehicles behind not being able to pass the intersection in time and running the red light
(3) Because there is no fast and slow lane at this intersection, all kinds of vehicles are mixed, resulting in other vehicles behind large vehicles starting but unable to drive when the signal light is on, which not only causes congestion at the intersection, but also causes more exhaust emissions.
(4) Large vehicles due to their longer body, so when making a U-turn at the intersection, the speed is slower and the requirements for the U-turn radius are greater, according to the relevant specifications for the minimum turning radius of large vehicles, when the vehicle nuclear load weight is 4~8t, the minimum turning radius is 9m [1]. According to the current situation of the intersection, the U-turn turning radius at each entrance road is 5.25m, which obviously does not meet the U-turn needs of large vehicles.
2.4 Spatial layout optimization scheme
According to the analysis above, the spatial layout optimization scheme for the intersection of Gaowang Road and Meishi Road is shown in Figure 2:
(1) The proportion of large vehicles in the north-south direction is relatively low, and the north-south direction is two-way two-lane, and in view of the delay and congestion caused by large vehicles turning around here, a no-U-turn sign is set up in front of the stop line.
(2) The proportion of large vehicles in the east-west direction is higher than that of small cars, and the contradiction between large vehicles is more prominent when passing through the intersection, so the spatial layout of the east-west direction is optimized.
Figure 2 Spatial layout optimization scheme Download the original image
(3) A central separation zone is added in the east-west direction, and the lanes are divided into two-way three-lane lanes. Among them, in order to solve the problem of delays caused by the slow speed of large vehicles, the outermost east-west lane is set as a special lane for large vehicles, so as to achieve the purpose of separating fast and slow.
(4) In order to solve the problem of small turning radius for large vehicles, the outermost east-west lane is set as a U-turn lane for large vehicles, which meets the standard of turning radius.
(5) In order to reduce the impact of large vehicles turning left on the traffic capacity of intersections, large vehicles turn left and adopt the remote U-turn method [7]. Left turns are prohibited in the lane for large vehicles, and after the vehicle is guided straight to the opposite lane of the intersection through the marking, make a U-turn through the center divider to the outermost lane of the divider, and then continue straight to the intersection and turn right.
3. Simulation evaluation of the optimization scheme
The optimized scheme plan is imported into VISSIM software, the road form of the intersection is edited and connected, the peak hour traffic flow is entered at each entrance, and the route decision of each entrance is set. And need to input the proportion of left-turn, straight-going, right-turn traffic flow of each import peak hour by model, then set up new signal light group, and finally set up travel time detector in each lane direction, evaluate the delay time of vehicle travel time in each lane direction, and the delay evaluation result is shown in Figure 3.
According to the simulation evaluation of VISSIM software, it can be found that the delay time of the left-turn lane in the east-west entrance lane with a high proportion of large vehicles at the intersection of Gaowang Road and Meishi Road after spatial optimization is greatly reduced, and the delay time of other entrance roads is reduced to varying degrees.
4 Conclusion
This paper investigates and analyzes the traffic characteristics of highway intersections in Tianjin, finds that the proportion of large vehicles is relatively high, analyzes and summarizes the problems caused by large vehicles driving at highway intersections, and takes the intersection of Gaowang Road and Meishi Road as an example, optimizes its spatial layout under the field of view of large vehicles, and evaluates the optimization scheme through VISSIM simulation software. The results show that the optimization scheme significantly reduces the delay time of vehicle traffic at the intersection. This optimization scheme is universally applicable to increasing the traffic capacity of intersections with a large proportion of large vehicles. Due to the limitation of the scope of data acquisition, this paper only selects five intersections in Tianjin as the research object, and with the further improvement of the data in the future, it can be extended to more highway intersections, so as to propose a more accurate optimization strategy for the spatial layout of highway plane signal intersections under the field of view of large vehicles.
Figure 3 Delay before and after optimization Download the original image
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