traffic flow consists of separate cars with different dynamic characteristics and driven by drivers of different qualifications, i.e. it is not homogeneous.
In conditions of low-intensity traffic, when individual vehicles move along the road at long intervals, the driver's choice of driving mode is limited by the Rules of the road, the condition of the car and the road. In a dense traffic flow, the driver is not free to choose the speed of movement, he cannot always overtake, and his behavior is largely determined by the general rhythm of traffic on the road. Consequently, heavy traffic levels out differences in the performance of individual drivers and vehicles.
Observations have shown that the movement of a dense traffic flow along a street or road resembles the movement of water in a canal. If you quickly block the flow of water in the channel, it will instantly stop and a reverse wave will run across the surface. The same "waves" can be observed in traffic stopped by a red traffic light or entering a narrow section of the road. The effect of a reverse wave in relation to the traffic flow is expressed in a sharp decrease in speed along the column and a reduction in the intervals between cars.
It is well known that a channel of a certain cross section can pass a very definite amount of water per unit time. If we want to pass more water through the channel, we must increase its cross section. Something similar happens with a traffic flow moving along its own channel - a street or a road. A carriageway of a certain width can pass a very certain number of cars, and if we want to increase its capacity, we must widen the road.
This analogy gave specialists a reason to apply the laws of fluid motion to study the patterns of traffic flows. Such a model, however, with certain limitations, makes it possible to carry out important research and solve a number of practical issues related to traffic control.
The traffic flow can be characterized by three main parameters: intensity N (the number of cars passing through a certain section of the road per unit of time), average speed V (the average speed of all cars passing through this section in a certain period of time) and density D (the number of cars per unit road length, usually 1 km). These parameters are related by the basic traffic flow equation: N = DV.
Graphically, this equation is the main traffic flow diagram, the general view of which is shown in Fig. 3.
Using the equation and diagram, you can determine the characteristics of the traffic flow. So, the average speed is proportional to the tangent of the slope of the straight line connecting the origin of coordinates with a point whose coordinates characterize a certain intensity and density. The speed V, as follows from the above equation, is equal to the ratio of traffic intensity (N auto / h) to the density corresponding to it (D auto / km).
The maximum possible traffic intensity under given conditions is achieved at a certain density of the traffic flow (point A on the diagram) and is called the capacity of the lane or road as a whole. It is characteristic that at a flow density greater than at point A, the traffic intensity decreases. This is explained by the fact that with a high traffic density, congestion often occurs, the speed decreases, and this leads to a decrease in the number of cars passing through any section or section of the road per unit of time.
From the main diagram and the equation of the traffic flow, a very important conclusion for traffic control follows: in cases where there is a need to let the maximum possible number of cars pass on the road, it is necessary to set with the help of signs a certain speed regime that provides the greatest intensity. As observations show, under favorable traffic conditions, an ordinary two-lane road with a carriageway width of 7 - 7.5 m can pass no more than 2000 cars per hour. The maximum intensity is reached at a speed of approximately 50 - 60 km/h * .
* (Silyanov VV Theory of transport flows in the design of roads and organization of traffic. M., Transport, 1978.)
One of the characteristics of movement is the freedom to overtake in traffic. The need for overtaking appears due to the heterogeneity of the composition of the flow - cars and high-speed trucks tend to overtake slow moving vehicles to maintain the desired speed. As the traffic intensity increases, the need for overtaking increases, and the opportunities for their implementation decrease, since there are fewer and fewer intervals in the oncoming traffic that provide safe maneuvering conditions. Observations show that overtaking proceeds freely when, in the oncoming traffic, the interval between cars is such that it can be overcome in 20 seconds or more. If this interval is less than 7 s, then overtaking becomes practically impossible. Of course, individual experienced drivers driving a car with good dynamic qualities, they can overtake at shorter intervals, but this is associated with greater risk.
In table. 16 shows data characterizing the possibility of overtaking on ordinary road 7 - 7.5 m wide at different traffic intensity. As calculations show, at a traffic intensity of 100 vehicles per hour, 70% of all intervals in the traffic flow are longer than 20 s, and therefore overtaking can occur relatively freely. With an intensity of 900 vehicles per hour, only 4% of such intervals remain, and this greatly complicates the conditions for overtaking. Observations carried out by the Moscow Automobile and Road Institute show that overtaking is practically not carried out when the total intensity of traffic on the road in both directions reaches 1500 - 1800 avt / h. This happens due to a decrease in the traffic flow of safe intervals for overtaking.
From Wikipedia, the free encyclopedia
traffic flow- is the movement ordered by the transport network Vehicle.
The movement of passengers is called passenger traffic, movement of goods - cargo traffic , pedestrian traffic pedestrian flow .
The following main indicators are used to characterize traffic flows:
- traffic intensity,
- time interval,
- traffic density,
- speed.
Traffic flow theories
In world literature, the very first and largest monograph on the theory traffic flows- the work of S. Drew and R. Donald "Theory of traffic flows and their management." It examines in detail the elements of the "driver - car - road" system and builds traffic flow models, describes the process of formation and further functioning of the traffic flow, its formalization and description based on mathematical models, considers methods for regulating traffic on complex road junctions and highways and designing high-performance transport systems with high throughput.
Considerable attention is paid to a systematic approach to transport problems, and the methods of probability theory, mathematical statistics, and queuing theory that are important for applications are described. Of great interest is the so-called deterministic approach to transport problems and the method of physical analogies. Part of the book is devoted to some practical problems associated with the design of roads and traffic control.
Deep research in the field of studying traffic flows was carried out by T. Metson, R. Smith, W. Leitzbach and other scientists of the University of Tokyo H. Inose and T. Hamada prepared a monograph that addresses the problem of collecting and processing information about the parameters of traffic flows, and also questions of their assessment and forecasting.
A sharp increase in motorization led to a change in the pattern of intensity fluctuations. Fluctuations in traffic intensity during the year are characterized by coefficient of annual unevenness: K G = W m / W G, Where W m And W G– monthly and annual volume of traffic, respectively.
Coefficient K G used in calculating the annual traffic volume: W G = N a D m / ( K G K With), Where N A– measured traffic intensity, avt./h; D m- the number of days in a month; K With– coefficient of daily non-uniformity of movement.
The distribution of traffic intensity by day of the week is characterized by its maximum value on Fridays, when the car is used by the largest number of individual owners. This intensity value should be taken as the calculated one.
During the day, as a rule, the highest traffic intensity is observed in morning hour the peak, followed by a slight decline, after which the traffic intensity gradually increases until the evening peak hour, which is much more extended in time than the morning one.
Transport is divided into three categories: transport common use, non-public transport and personal or individual transport.
The composition of the traffic flow is characterized by the ratio of vehicles of various types in it. The assessment of the composition of the traffic flow is carried out mainly by the percentage composition or share of vehicles various types. This indicator has a significant impact on all parameters traffic. However, the composition of the traffic flow largely reflects the overall composition of the car fleet in the region. The composition of the traffic flow affects road congestion, which is explained, first of all, by significant difference V overall dimensions cars. If the length of domestic cars 4-5 m, cargo 6-8, then the length of buses reaches 11, and road trains 24 m. The articulated bus has a length of 16.5 m.
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An excerpt characterizing the traffic flow
- Adieu, ma bonne, [Farewell, my dear,] - answered Prince Vasily, turning away from her.“Ah, he is in a terrible position,” said the mother to her son, as they got back into the carriage. He barely recognizes anyone.
- I don’t understand, mother, what is his relationship with Pierre? the son asked.
“The testament will say everything, my friend; our destiny depends on it...
“But why do you think he would leave anything for us?”
- Ah, my friend! He is so rich and we are so poor!
“Well, that’s not enough reason, mother.
- Oh my god! My God! How bad he is! mother exclaimed.
When Anna Mikhailovna went with her son to Count Kirill Vladimirovich Bezukhy, Countess Rostova sat alone for a long time, putting a handkerchief to her eyes. Finally, she called.
“What are you, dear,” she said angrily to the girl, who kept herself waiting for several minutes. You don't want to serve, do you? So I will find a place for you.
The countess was upset by the grief and humiliating poverty of her friend and therefore was not in a good mood, which was always expressed in her by the name of the maid "dear" and "you".
“Guilty with,” said the maid.
“Ask the Count for me.
The count, waddling, approached his wife with a somewhat guilty look, as always.
- Well, Countess! What a saute au madere [saute in Madeira] of grouse will be, ma chere! I tried; I gave a thousand rubles for Taraska not for nothing. Costs!
He sat down beside his wife, valiantly leaning his hands on his knees and ruffling his gray hair.
- What do you want, countess?
- Here's what, my friend - what do you have dirty here? she said, pointing to the vest. "That's sauté, right," she added, smiling. - Here's the thing, Count: I need money.
Her face became sad.
- Oh, Countess! ...
And the count began to fuss, taking out his wallet.
- I need a lot, count, I need five hundred rubles.
And she, taking out a cambric handkerchief, rubbed her husband's waistcoat with it.
- Now. Hey, who's there? he shouted in a voice that only people shout, confident that those whom they call will rush headlong to their call. - Send Mitenka to me!
Mitenka, that noble son, brought up by the count, who was now in charge of all his affairs, entered the room with quiet steps.
“That’s what, my dear,” said the count to the respectful man who entered. young man. “Bring me…” he thought. - Yes, 700 rubles, yes. Yes, look, don’t bring such torn and dirty ones as that time, but good ones, for the countess.
“Yes, Mitenka, please, clean ones,” said the countess, sighing sadly.
“Your Excellency, when would you like me to deliver it?” Mitenka said. “If you please, don’t worry, don’t worry,” he added, noticing that the count had already begun to breathe heavily and quickly, which was always a sign of anger. - I was and forgot ... Will you order to deliver this minute?
- Yes, yes, then bring it. Give it to the Countess.
“What gold I have this Mitenka,” added the count, smiling, when the young man left. - There is no such thing as impossible. I can't stand it. Everything is possible.
“Ah, money, count, money, how much grief they cause in the world!” said the Countess. “I really need this money.
“You, countess, are a well-known winder,” said the count, and, kissing his wife’s hand, went back into the study.
When Anna Mikhailovna returned from Bezukhoy again, the countess already had money, all in brand new paper, under a handkerchief on the table, and Anna Mikhailovna noticed that the countess was somehow disturbed.
- Well, my friend? the countess asked.
Oh, what a terrible state he is in! You can't recognize him, he's so bad, so bad; I stayed for a minute and did not say two words ...
“Annette, for God’s sake, don’t refuse me,” the countess suddenly said, blushing, which was so strange with her middle-aged, thin and important face, taking out money from under her handkerchief.
Anna Mikhaylovna instantly understood what was the matter, and already bent down to deftly embrace the countess at the right time.
- Here's Boris from me, for sewing a uniform ...
Anna Mikhaylovna was already embracing her and crying. The Countess was crying too. They wept that they were friendly; and that they are kind; and that they, girlfriends of youth, are occupied with such a low subject - money; and that their youth had passed ... But the tears of both were pleasant ...
Countess Rostova was sitting with her daughters and already with a large number of guests in the drawing room. The count ushered the male guests into his study, offering them his hunter's collection of Turkish pipes. Occasionally he would come out and ask: has she come? They were waiting for Marya Dmitrievna Akhrosimova, nicknamed in society le terrible dragon, [a terrible dragon,] a lady famous not for wealth, not for honors, but for her directness of mind and frank simplicity of address. Marya Dmitrievna was known by the royal family, all of Moscow and all of St. Petersburg knew, and both cities, surprised at her, secretly laughed at her rudeness, told jokes about her; yet everyone, without exception, respected and feared her.
In an office full of smoke, there was a conversation about the war, which was declared by the manifesto, about recruitment. No one has yet read the Manifesto, but everyone knew about its appearance. The count was sitting on an ottoman between two smoking and talking neighbors. The count himself did not smoke or speak, but tilting his head, now to one side, then to the other, he looked with evident pleasure at the smokers and listened to the conversation of his two neighbors, whom he pitted against each other.
One of the speakers was a civilian, with a wrinkled, bilious, and shaven, thin face, a man already approaching old age, although he was dressed like the most fashionable young man; he sat with his feet on the ottoman with the air of a domestic man, and, sideways thrusting amber far into his mouth, impetuously drew in the smoke and screwed up his eyes. It was the old bachelor Shinshin, the cousin of the countess, an evil tongue, as they said about him in Moscow drawing rooms. He seemed to condescend to his interlocutor. Another, fresh, pink, officer of the Guards, impeccably washed, buttoned and combed, held amber near the middle of his mouth and with pink lips slightly pulled out the smoke, releasing it in ringlets from his beautiful mouth. It was that lieutenant Berg, an officer of the Semyonovsky regiment, with whom Boris went to the regiment together and with which Natasha teased Vera, the senior countess, calling Berg her fiancé. The Count sat between them and listened attentively. The most pleasant occupation for the count, with the exception of the game of boston, which he was very fond of, was the position of the listener, especially when he managed to play off two talkative interlocutors.
“Well, how about it, father, mon tres honorable [most respected] Alfons Karlych,” said Shinshin, chuckling and combining (which was the peculiarity of his speech) the most popular Russian expressions with exquisite French phrases. - Vous comptez vous faire des rentes sur l "etat, [Do you expect to have income from the treasury,] do you want to receive income from the company?
When generating information about the state of traffic, first of all, data characterizing the traffic flow is needed.
Many years of foreign and domestic experience in scientific research and practical observations of traffic flows made it possible to identify the most objective indicators. With the improvement of methods and equipment for the study of traffic flows, the nomenclature of indicators used in the organization of traffic continues to develop. The most commonly used are: the intensity of the traffic flow, its composition by types of vehicles, traffic density, traffic speed, traffic delays. Let us characterize these and other indicators of the traffic flow.
Traffic intensity (traffic intensity) N a is the number of vehicles passing through the section of the road per unit of time. A year, a month, a day, an hour and shorter periods of time (minutes, seconds) are taken as the estimated time period for determining the intensity of traffic, depending on the task of observation and measuring instruments.
On the road network, it is possible to single out separate sections and zones where traffic reaches maximum dimensions, while in other areas it is several times less. Such spatial unevenness primarily reflects the uneven placement of cargo and passenger points and places of their attraction. On fig. 2.1 shows an example of a cartogram characterizing the intensity of traffic flows (in cars per hour) on the main streets of the city.
Uneven traffic flows in time (during the year, month, day and even hour) is of paramount importance in the problem of organizing traffic (Fig. 2.2, 2.3). A typical curve for the distribution of traffic intensity during the day on a city highway is shown in fig. 2.2. Approximately the same picture is observed on the roads. The curves in fig. 2.2 make it possible to single out the so-called "peak hours", during which the most difficult tasks of organizing and regulating traffic arise.
The term "rush hour" is conditional and is explained only by the fact that the hour is the basic unit of time. The duration of the highest traffic intensity can be more or less than an hour. Therefore, the most accurate concept will be the peak period, which means the time during which the intensity measured over small time intervals (for example, 15-minute observations) exceeds the average intensity of the period of the busiest traffic. The busiest period on most urban and extra-urban roads is usually a 16-hour period of time during the day (approximately from 6 am to 10 pm). In conditions of oversaturation of the UDS with traffic on a number of highways in Moscow and other large cities, during almost the entire active period of the day, a "peak" intensity is observed (line 3 in Fig. 2.2), accompanied by congestion phenomena.
Temporal irregularity of traffic flows can be characterized by the corresponding coefficient of unevenness TO n. This coefficient can be calculated for annual, daily and hourly traffic irregularities. Irregularity can be expressed as the proportion of traffic intensity attributable to a given period of time, or as the ratio of the observed intensity to the average for the same time intervals.
| Rice. 2.1. Cartogram of the average daily intensity of traffic flows in the city Fig. |
Coefficient of annual unevenness
,
where 12 is the number of months in a year; N am– traffic intensity for the compared month, avt/month; Nag– total traffic intensity for the year, avt/year.
Daily irregularity coefficient
,
where 24 is the number of hours in a day; N Ah– traffic intensity for the compared hour, avt/h; Nac– total traffic intensity per day, avt/day.
It should be noted that traffic publications use the concept of volume of traffic as opposed to intensity of traffic. The volume of traffic is understood as the actual number of cars that have passed on the road during the accepted unit of time, obtained by continuous observation for the designated period.
To characterize the spatial non-uniformity of the transport or pedestrian flow, the corresponding non-uniformity coefficients for individual streets and road sections can also be determined similarly to the temporal non-uniformity.
Most often, the intensity of the movement of vehicles and pedestrians in the practice of organizing traffic is characterized by their hourly values. Moreover, this indicator is most important during peak periods. However, it must be borne in mind that the intensity of traffic during "peak hours" on different days of the week may have different values.
On roads with more high level the intensity of traffic of vehicles is less irregular in traffic and the intensity is more stable during peak periods.
For two-lane roads with oncoming traffic, the total intensity is usually characterized by the total value of oncoming flows, since the traffic conditions and, in particular, the possibility of overtaking are determined by the loading of both lanes. If the road has a dividing strip and oncoming flows are isolated from each other, then the total intensity of the opposite directions does not determine the traffic conditions, but characterizes only the total work of the road as a structure. For such roads, the intensity of traffic in each direction has an independent value.
In many cases, especially when solving traffic control issues in urban environments, not only the total traffic intensity in a given direction is important, but also the intensity per lane, or the so-called specific traffic intensity M A. If a specific distribution of traffic intensity across lanes is known and it is significantly uneven, then as calculated intensity M and you can take the intensity of traffic in the busiest lane.
Time interval t i between vehicles following one after another in the same lane is the reciprocal of traffic intensity. Expected value E(t i) determined by dependency E(t i) = 3600/M A. If the interval t i between cars following each other along the lane for more than 10 seconds, then their mutual influence is relatively weak and the driving conditions are characterized as "free". The stochastic process of distribution of cars in the traffic flow and the time intervals between them is considered in more detail in subsection 2.4.
The composition of the traffic flow is characterized by the ratio of vehicles of various types in it. This indicator has a significant impact on all traffic parameters. However, the composition of the traffic flow largely reflects the overall composition of the car fleet in the region. Thus, on the roads of the United States and many Western countries, cars predominate, which make up 80-90% of the total fleet. As motorization grows and the share of passenger cars in the fleet of our country increases, it will also increase in the traffic flow. In many cases, this proportion already reaches 70 - 90%.
The composition of the traffic stream affects road congestion (traffic constraint), which is primarily due to a significant difference in the overall dimensions of cars. If the length of passenger cars is 4–5 m, trucks 6–8 m, then the length of buses reaches 11 m, and road trains 24 m. An articulated bus (trolleybus) has a length of 16.5 m. However, the difference in overall dimensions is not the only reason for the need for a special taking into account the composition of the flow in the analysis of traffic intensity.
When driving in a traffic flow, the difference is important not only in static, but also in dynamic dimension of the car, which depends mainly on the reaction time of the driver and the braking qualities of the vehicles. Under the dynamic envelope L d (Fig. 2.4) means a section of the road that is minimally necessary for safe movement in a traffic flow with set speed car, the length of which includes the length of the car l and the distance d called safety distance.
There are three fundamentally different approaches to the calculation definition L e proposed by various authors (see subsection 2.4).
Table 2.1
The braking qualities of cars of various types in operation differ significantly. This difference is confirmed by the requirements for braking efficiency (Table 2.1) established by GOST 25478–91 “Motor vehicles. Requirements to technical condition in terms of traffic safety. Verification Methods.
In table. 2.2 is given complete classification vehicles established by UNECE ITC.
The actual dynamic dimension of the car also depends on the visibility, ease of control, maneuverability of the car, which affect the distance chosen by the driver. In this case, attention should be paid to the following circumstance. With a convoy of cars, each driver, thanks to the large glazing surface, as well as the small dimensions of the cars in front, can see quite well and predict the situation in front of several cars. At the same time, if a truck or bus is moving in front of a passenger car, the driver of a passenger car is deprived of the ability to assess and predict the situation ahead, and his driving actions become less confident. In this case, due to the impossibility of sufficient forecasting of the situation ahead, the danger sharply increases when overtaking, as well as in the event of an emergency stop of cars moving in a dense column.
When surveying traffic flows of high intensity, a certain difficulty is the task exact definition load capacity of each truck. Therefore, it is possible to resort to a simplified method of accounting for this category of vehicles and take a generalized coefficient 2 for all trucks with a carrying capacity of 2–8 tons.
When describing the characteristics of the traffic flow, both in writing and in the form of graphs, attention should be paid to the need to indicate the appropriate dimension in physical units (aut/h) or reduced (un/h).
Table 2.2
| Vehicle category | Vehicle type | Permitted maximum weight, t | Note |
| M 1 | Engine-powered vehicles intended for the carriage of passengers and having no more than 8 seats (except for the driver's seat) | Not standardized | Cars |
| M 2 | The same, having more than 8 seats (except the driver's seat) | Up to 5.0 | Buses |
| M 3 | Same | Over 5.0 | Buses, including articulated |
| N 1 | Vehicles with an engine intended for the carriage of goods | Up to 3.5 | Trucks, special vehicles |
| N 2 | Same | Over 3.5 to 12.0 | Trucks, tractors, special vehicles |
| N 3 | " | Over 12.0 | Same |
| About 1 | Vehicle without engine | Up to 0.75 | Single axle trailers |
| About 2 | Same | Over 0.75 to 3.5 | Trailers and semi-trailers, except for category O 1 |
| About 3 | " | "3.5 to 10.0 | Same |
| About 4 | " | " 10,0 | " |
To solve practical problems of ODD, recommendations on the choice of values can be used TO pr contained in domestic normative documents:
Using the reduction coefficients, you can get an indicator of traffic intensity in conventional reduced units, units / h
,
Where N i- the intensity of the movement of cars of this type; Knpi- the corresponding reduction factors for this group of vehicles; n is the number of vehicle types into which the observational data are divided.
Studies show that the reduction factors used are approximate and for modern models cars overpriced. Research experience K pr shows that with a more detailed approach to the role of the reduction coefficient, its values must also be differentiated depending on the level of the speed limit and the road profile.
Traffic flow density q a is a spatial characteristic that determines the degree of constriction of traffic on the road lane. It is measured by the number of vehicles per 1 km of the length of the road. The limiting density is achieved when the column of cars is stationary, located close to each other on the lane. For the flow of modern passenger cars, theoretically such a limit value qmax is about 200 vehicles/km. Practical research at the Department of Organization and Traffic Safety MADI showed that this figure ranges from 170-185 vehicles / km. This is due to the fact that drivers do not drive up close to the front car during a traffic jam. Naturally, at the limiting density, movement is impossible even with a centralized automatic control vehicles, as there is no safety distance. Density qmax at the same time, it is important as an indicator characterizing the structure (composition) of the traffic flow. Observations show that during the convoy movement of cars with low speed the flow density can reach 100 vehicles/km. When using the flux density index, it is necessary to take into account the reduction factor for different types of vehicles, otherwise the comparison q a for streams of different composition can lead to incomparable results. So, if we assume that a column of buses with a density of 100 vehicles / km (possible for cars) is moving on the road, then the actual length of such a column, instead of 1 km, will practically be 2.0–2.5 km. Considering the recommended value TO pr for buses, equal to 2.5, then the maximum traffic density of a column of buses in physical units can be 40 buses per 1 km, which is real.
The lower the flow density, the freer drivers feel, the higher the speed they choose. On the contrary, as q a, i.e. constrained traffic, drivers are required to increase their attentiveness and accuracy of actions. In addition, their mental tension increases. Accordingly, the probability of an accident increases due to a mistake made by one of the drivers, or a car failure.
Depending on the density of the flow, the movement according to the degree of constraint is divided into free, partially bound, saturated, columnar.
Numerical values q a in physical units (vehicles) corresponding to these flow states, very significantly depend on the parameters of the road and, first of all, on its plan and profile, the friction coefficient φ, as well as the composition of the flow by type of vehicle, which, in turn, affects the speed chosen by drivers.
Travel speed v a is the most important indicator, since it represents the objective function of traffic. The most objective characteristic of the process of vehicle movement along the road can be the graph of its speed change throughout the entire route of movement. However, obtaining such spatial characteristics for a plurality of moving vehicles is difficult, since it requires continuous automatic recording of the speed on each of them. In the practice of organizing traffic, it is customary to evaluate the speed of vehicles by its instantaneous values v a fixed in separate typical sections (points) of the road.
Message rate v c is a measure of the speed of delivery of passengers and goods and is defined as the ratio of the distance between the points of communication to the time spent by the vehicle on the way (time of communication). The same indicator is used to characterize the speed of vehicles on certain sections of roads.
Movement pace is the reciprocal of the speed of the message, and is measured by the time in seconds it takes to overcome a unit length of the path in kilometers. This meter is very convenient for calculating the time of delivery of passengers and goods over various distances. The instantaneous speed of the vehicle and, accordingly, the speed of the message depend on many factors and are subject to significant fluctuations.
The speed of a single moving car within its traction capabilities is determined by the driver, who is the control link in the VADS system. The driver is constantly striving to choose the most appropriate speed mode based on two main criteria - the minimum possible time consumption and traffic safety. In each case, the choice of speed by the driver is influenced by his qualifications, psycho-physiological state, the purpose of the movement, the conditions for its organization. Thus, studies conducted in the same road conditions on one type of car showed that the average speed of a car for different highly qualified drivers can vary within ± 10% of the average value. For inexperienced drivers, this difference is greater.
Consider the influence of the parameters of vehicles and the road on the speed of movement. The upper speed limit of a vehicle is determined by its maximum design speed. vmax, which depends mainly on the specific power of the engine. Max speed vmax, km/h, modern cars varies widely depending on their type and is approximately:
Passenger cars of large and medium classes ........ 200
Same small class 160
Medium Duty Trucks...................... 100
Same heavy duty and road trains .............. 90
Experience shows that the driver drives the car with maximum speed only in exceptional cases and for a short time, as this is associated with an excessively intense mode of operation of the vehicle's units; in addition, even slight slopes on the road require a reserve of power to maintain a stable speed. Therefore, even under favorable road conditions, the driver drives the car at the maximum speed of a long movement or cruising speed. Cruising speed for most cars is (0.75÷0.85) vmax.
However, real road conditions make significant adjustments to the actual range of observed speeds. Slopes, curves and uneven road surfaces cause speed reduction due to the limited dynamic properties of vehicles, and mainly due to the need to ensure their stability on the road. These objective factors especially affect the speed of the fastest cars. As observations show, the actual range of instantaneous speeds of free movement of cars on the horizontal sections of some main streets and roads in our country is 50 - 120 km / h, despite established by the Rules restrictions. These figures do not apply to unpaved or broken roads where speeds may drop to 10 to 15 km/h.
A significant impact on the speed of movement is exerted by those elements of road conditions that are associated with the peculiarities of the psycho-physiological perception of the driver and the confidence of control. Here again, it is necessary to emphasize the continuity of the elements of the WADS system and the decisive influence of drivers on traffic performance.
The most important factors, influencing the driving modes through the perception of the driver, are the distance (range) of visibility S In on the road and the lane width IN e, i.e., a "corridor" allocated for the movement of cars in one row. Visibility distance refers to the length of the section of road in front of the vehicle over which the driver is able to distinguish the road surface. Distance S B determines the ability for the driver to assess traffic conditions in advance and predict the situation. A prerequisite traffic safety is to exceed the distance S B above stopping distance value S o this vehicle in any specific road conditions: S B > S o .
With a short visibility range, the driver loses the ability to predict the situation, experiences uncertainty and reduces the speed of the car. Approximate values for speed reduction Δ v compared to the speed that is provided with a visibility range of 700 m or more, the following:
The width of the lane intended for the movement of cars in one row and usually highlighted by longitudinal markings determines the requirements for the trajectory of the car. The smaller the lane width, the more stringent requirements are placed on the driver and the greater his mental stress in ensuring the exact position of the car on the road. With a small lane width, as well as with oncoming traffic on narrow road the driver under the influence of visual perception reduces speed.
Based on research on the roads, Professor D.P. Velikanov obtained a relationship that approximately characterizes the relationship between speed and the required width of the road lane,
| , | (2.1) |
Where b a– vehicle width, m; 0.3 - additional clearance, m.
By analogy with the concept of "dynamic dimension" of a car, the indicator IN d can be called the "dynamic width" of the vehicle ("dynamic corridor"), since for confident movement at a speed v a the driver should have approximately such a free "corridor" of movement. In this dependence, one can once again trace the connections of the components of the VADS complex in road traffic. In formula (2.1) IN d is a road element (D), b a- characteristics of the car (element A), the coefficient 0.015 reflects the psychophysiological properties of the driver and the running properties of the car (subsystem VA).
According to the given dependence, the speed at which an average-skilled driver can drive a car for a long time and confidently is approximately: when driving a car and a lane width of 3 m, about 65 km/h, and with a lane width of 3.5 m, about 90 km/h; when driving a car with an overall width of 2.5 m and a lane width of 3.5 m, about 50 km / h.
However, this does not exclude the possibility that some drivers cannot accurately and timely assess the change in visibility distance or lane width and choose the right speed. Therefore, under the conditions limited visibility and narrow lanes are more likely to cause crashes.
Based on the research of the NIiPI of the General Plan of the City of Moscow, recommendations were developed for the desirable values of the lane width on straight sections of city roads (Table 2.3)
The actual speed of vehicles is also influenced by other reasons, and especially significant ones - meteorological conditions, and in dark time days - road lighting. Thus, the speed of free movement is a random variable and for the flow of cars of the same type in a given section of the road it is usually characterized by a normal distribution law or close to it (Fig. 2.5).
The better the road and meteorological conditions, the greater the amplitude of the speed fluctuations of cars of various types, which is due to their speed and braking qualities, as well as the characteristics of drivers.
Table 2.3
The influence of the considered factors on the speed of movement is manifested in the conditions of free movement of vehicles, i.e., when the intensity and density of movement are relatively small and there is no mutual constraint on movement. With an increase in the density of the traffic flow, traffic is constrained, and the speed drops. The influence of the intensity of the traffic flow on the speed of cars was studied by many foreign and domestic scientists. Various correlation equations of this dependence are derived, which have a general form:
 ,
|
Where v ac- the speed of the free movement of the car on this section of the road, km / h; k is the correlation coefficient for reducing the speed of movement depending on the intensity of the traffic flow.
The relationship between the main parameters of movement is considered in more detail in subsection 2.3.
Motion delays are indicators to which Special attention when assessing the state of traffic. Delays should include the loss of time for all forced stops of vehicles, not only before intersections, railway crossings, during traffic jams on stages, but also due to a decrease in the speed of the traffic flow compared to the prevailing average speed of free movement on this section of the road.
 ,
|
Where v f And vp- respectively, the actual and accepted design (or optimal) speeds, m/s; dl- an elementary section of the road, m.
As the design speed for a city highway, you can take the permitted by the Rules of the Road Russian Federation speed limit (e.g. 60 km/h). The starting point for determining the delay may be the standard speed of the message or the standard rate of movement for a given type of road, if any. So if on the road vp\u003d 60 km / h, which corresponds to the rate of movement without delay 60 s / km, and established by experimental verification v f\u003d 30 km / h (the pace of movement is 120 s / km), then the loss of time by each car in the stream is 60 s / km. If the length l of the considered section of the highway is equal, for example, to 5 km, the conditional delay of each car will be 5 minutes.
Total losses traffic flow time
| , |
Where tΔ– average total delay of one vehicle, s; T– duration of observation, h.
Vehicle delays at individual nodes or sections of the road network can also be estimated by the delay coefficient TO 3 characterizing the degree of increase in the actual travel time t f compared to calculated t r. Delay factor K 3 \u003d t f / t p. Traffic delays in real conditions can be divided into two main groups: at road sections and at intersections. Lane delays can be caused by maneuvering or slow moving vehicles, foot traffic, obstruction from stationary vehicles, including during loading and unloading operations, as well as traffic congestion.
Delays at intersections are due to the need to pass vehicles and pedestrians in crossing directions at unregulated intersections, downtime at prohibitory traffic lights.
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Characteristics of traffic flows
The most necessary and frequently used characteristics of the traffic flow are the intensity of the traffic flow, its composition by types of vehicles, traffic density, traffic speed, traffic delays. The intensity of the traffic flow is defined as the number of vehicles passing through the section of the road per unit of time. A year, a month, a day, an hour and shorter periods of time are taken as the estimated time period for determining the intensity of traffic, depending on the task of monitoring and measuring instruments.
On the road network, separate sections and zones can be distinguished where traffic reaches its maximum size, while in other sections it is several times less. Such spatial unevenness reflects, first of all, the uneven distribution of cargo-generating and passenger-generating points and places of their attraction. Irregularity can be expressed as the proportion of traffic intensity attributable to a given period of time, or as the ratio of the observed intensity to the average for the same time intervals.
It should be noted that in the literature on traffic, due to the unevenness of traffic flows over time, the concept of volume of traffic is often used in contrast to the intensity of traffic. The volume of traffic is understood as the actual number of cars that have passed on the road during the accepted unit of time, obtained by continuous observation for the designated period. The unevenness of traffic flows is manifested not only in time, but also in space, that is, along the length of the road and in directions. To characterize the spatial non-uniformity of a transport or pedestrian flow, the corresponding coefficients of non-uniformity for individual streets and road sections can also be determined. Most often, the intensity of the movement of vehicles and pedestrians in the practice of organizing traffic is characterized by their hourly values.
When researching and designing the organization of traffic, one has to resort to the description of traffic flows by mathematical methods. The primary tasks that served the development of traffic flow modeling were the study and justification bandwidth highways and their intersections. The behavior of the traffic flow is very variable and depends on the action of many factors and their combinations. Along with such technical factors as vehicles and the road itself, the behavior of drivers and pedestrians, as well as the state of traffic environments, have a decisive influence on it.
The foundations of mathematical modeling of traffic patterns were laid in 1912 by the Russian scientist Professor GD Dubelir. The first attempt to generalize the mathematical studies of traffic flows and present them as an independent section applied mathematics was made by F. Hayt. Well-known and found practical application in the organization of traffic mathematical models can be divided into two groups depending on the approach. These are deterministic and probabilistic, that is, stochastic.
Deterministic models include models based on a functional relationship between individual indicators, for example, speed and distance between cars in a stream. It is assumed that all cars are at the same distance from each other. Stochastic models are more objective. In them, the traffic flow is considered as a probabilistic, random process. For example, the distribution of time intervals between cars in a stream can be taken not strictly defined, but random.
To clarify the relative spatial position of moving vehicles, such a concept as the dynamic dimension of the vehicle has been introduced. This parameter is defined as the sum of the length of the vehicle, the safety distance and the clearance to the vehicle stopped in front. For passenger cars, this gap ranges from 1-3 meters. There are at least three approaches to determining the dynamic dimension.
When calculating the minimum theoretical distance, they proceed from absolutely equal braking properties pairs of cars and take into account only the reaction time of the driven driver. Then the dynamic dimension consists of the sum of the length of the vehicle, the clearance, and the product of the speed and reaction time of the driver. In this case, the possible intensity of the traffic flow has no limit as the speed increases. However, this does not correspond to the real characteristics of the drivers and leads to an overestimation of the possible intensity of the flow. Here the main role is played by the practical significant increase in the reaction time at high speeds.
When calculating for complete safety, it is assumed that the safety distance should be equal to full stopping way rear car. This approach is more in line with the requirements for ensuring traffic safety at speeds exceeding 90 kilometers per hour. The most realistic approach is based on the premise that when calculating the safety distance, one must take into account the difference in the braking distances of cars, as well as the fact that the leader also moves a distance equal to his own during the braking process. stopping distance. As a result of the study of high-density traffic flows and special experiments conducted by American specialists, a theory of following the leader was proposed, the mathematical expression of which is a microscopic model of the traffic flow.
It is called microscopic because it considers a flow element, a pair of vehicles following each other. A feature of this model is that it reflects the patterns of the "driver-car-road-environment" complex, in particular, the psychological aspect of driving. It lies in the fact that when driving in a dense traffic flow, the actions of the driver are due to changes in the speed of the leading car and the distance to it.
Sergey ZOLOTOV
The concept of traffic flow
Definition 1
traffic flow- this is the number of units of vehicles of one type of transport that have followed a certain section of the road during a specified period of time.
The amount of traffic flow depends on the track capacity and processing capacity technical stations. The value of the traffic flow is directly proportional to the value of the cargo flow.
The traffic flow differs from the material and cargo in the following positions.
- First, the traffic flow does not necessarily imply the transportation of inventory items. The traffic flow can be cargo or passenger, laden or empty, as well as combined in various combinations.
- Secondly, the traffic flow in supply chains is considered separately for each mode of transport.
- Thirdly, the movement of the traffic flow is carried out not from the seller's warehouse to the buyer's warehouse (as a material flow), but from the point of departure of a particular type of transport to the destination of the same type of transport. In this case, the movement of the flow is provided by an appropriate transport infrastructure and technical means designed to perform loading, unloading and other operations with the rolling stock of a certain type of transport.
In some cases, the transport flow completely coincides with the material flows at the points of origin and redemption. In this case, we speak of a continuous traffic flow, which is understood as the transportation of goods by only one mode of transport on a door-to-door basis. This delivery technology is feasible for road transport, as well as in the case of rail transportation by sending routes. More common in logistics systems is the option when the material flow is moved by several modes of transport, that is, there is a discontinuous traffic flow.
Transport stream parameters
The transport stream is characterized by the following parameters:
- traffic intensity (the number of vehicles passing through a certain section of the road in a certain direction during a set period of time;
- coefficient of non-uniformity of the flow (measures fluctuations in the intensity of the flow during a given period of time - day, week, month, year);
- empty run coefficient (the ratio of empty run to the total mileage of the vehicle, the indicator tends to a minimum, shows the efficiency of the use of rolling stock);
- load capacity utilization ratio (the ratio of the mass of cargo to the carrying capacity of the vehicle, the indicator tends to a maximum)
Classification of traffic flows
Transport streams can be classified according to the following criteria.
Vehicle condition:
- laden flow, due to the movement of vehicles with cargo, is the productive mileage of transport;
- empty flow, due to the movement of vehicles without cargo, is the unproductive mileage of vehicles.
- one-way flow, due to the movement of vehicles in one direction;
- two-way traffic due to the movement of vehicles in the forward and reverse directions.
According to the object of transportation:
- freight, due to the transportation of goods by a specific mode of transport;
- passenger, due to the carriage of passengers;
- combined, due to the transportation of goods and passengers in one vehicle.
By type of transport:
- railway traffic, including wagon traffic and container traffic in rail transport;
- car flow (car flow);
- air flow (formed by the movement air transport- planes, helicopters);
- water (formed by the movement water transport, sea or river).
