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TECHNICAL VALUES DETERMINED BY THE EXPERT
In addition to the initial data accepted on the basis of the investigator's decision and the case materials, the expert uses a number of technical quantities (parameters) that he determines in accordance with the established initial data. These include: the reaction time of the driver, the delay time of the brake actuator, the rise time of deceleration during emergency braking, the coefficient of adhesion of tires to the road, the coefficient of resistance to movement when the wheels are rolling or the body is sliding on the surface, etc. The accepted values of all quantities must be justified in detail in research part of the expert opinion.Since these values are determined, as a rule, in accordance with the established initial data on the circumstances of the incident, they cannot be attributed to the initial ones (i.e., accepted without justification or research), regardless of how the expert determines them (according to tables, calculated by or as a result of experimental studies). These values can be taken as initial data only if they are determined by investigative actions, as a rule, with the participation of a specialist and are indicated in the investigator's decision.
1. DECELERATION DURING EMERGENCY BRAKING OF VEHICLES
Deceleration J - one of the main quantities required in calculations to establish the mechanism of an accident and to resolve the issue of the technical possibility to prevent an accident by braking.The magnitude of the established maximum deceleration during emergency braking depends on many factors. With the greatest accuracy, it can be established as a result of an experiment at the scene. If this is not possible, this value is determined with some approximation from tables or by calculation.
When braking an unladen vehicle with serviceable brakes on a dry horizontal surface of asphalt pavement, the minimum allowable deceleration values during emergency braking are determined in accordance with the Traffic Rules (Article 124), and when braking a loaded vehicle, according to the following formula:
| Where: |  | - | the minimum allowable deceleration value of an unladen vehicle, m/s, |
 | - | braking efficiency coefficient of an unladen vehicle; |
|
 | - | braking efficiency coefficient of a laden vehicle. |
The deceleration values for emergency braking with all wheels are generally determined by the formula:
| Where | ? | - | friction coefficient in the braking area; |
 | - | vehicle braking efficiency coefficient; |
|
| | - | slope angle in the deceleration section (if ? 6-8°, Cos can be taken equal to 1). |
The sign (+) in the formula is taken when the vehicle is moving uphill, the sign (-) - when driving downhill.
2. TIRE GRIP COEFFICIENT
Adhesion coefficient ? is the ratio of the maximum possible value of the adhesion force between the tires of the vehicle and the road surface on a given section of the road R sc to the weight of this vehicle G a :
The need to determine the friction coefficient arises when calculating the deceleration during emergency braking of the vehicle, solving a number of issues related to maneuver and movement in areas with large angles of inclination. Its value depends mainly on the type and condition of the road surface, so the approximate value of the coefficient for a particular case can be determined from Table 1 3 .
Table 1
| Type of road surface | Coating condition | Adhesion coefficient ( ? ) |
| asphalt, concrete | dry | 0,7 - 0,8 |
| wet | 0,5 - 0,6 |
|
| filthy | 0,25 - 0,45 |
|
| Cobblestone, paving stone | dry | 0,6 - 0,7 |
| wet | 0,4 - 0,5 |
|
| Dirt road | dry | 0,5 - 0,6 |
| wet | 0,2 - 0,4 |
|
| dirty | 0,15 - 0,3 |
|
| Sand | wet | 0,4 - 0,5 |
| dry | 0,2 - 0,3 |
|
| asphalt, concrete | icy | 0,09 - 0,10 |
| packed snow | icy | 0,12 - 0,15 |
| packed snow | without ice crust | 0,22 - 0,25 |
| packed snow | icy, after sand scattering | 0,17 - 0,26 |
| packed snow | without ice crust, after sand scattering | 0,30 - 0,38 |
The speed of the vehicle, the condition of the tire tread, the pressure in the tires and a number of other factors that cannot be taken into account have a significant impact on the value of the coefficient of adhesion. Therefore, in order for the conclusions of the expert to remain fair even with other possible values in this case, when conducting expert examinations, one should take not the average, but the maximum possible values of the coefficient ? .
If you need to accurately determine the value of the coefficient ? should conduct an experiment at the scene.
The friction coefficient values that are closest to the actual, i.e., to the one that was at the time of the incident, can be established by towing a braked vehicle involved in the incident (with the appropriate technical condition of this vehicle), while measuring the adhesion force using a dynamometer.
Determination of the coefficient of friction using dynamometer carts is not practical, since the actual value of the coefficient of friction of a particular vehicle may differ significantly from the value of the coefficient of friction of the dynamometer trolley.
When solving issues related to braking efficiency, experimentally determine the coefficient? impractical, since it is much easier to establish the deceleration of the vehicle, which most fully characterizes the braking efficiency.
The need for experimental determination of the coefficient ? may arise in the study of issues related to maneuvering, overcoming steep ascents and descents, keeping vehicles on them in a braked state.
3. BRAKING PERFORMANCE FACTOR
The braking efficiency coefficient is the ratio of the calculated deceleration (determined taking into account the value of the friction coefficient in a given section) to the actual deceleration when the vehicle is moving in this section:
Therefore, the coefficient TO uh takes into account the degree of use of the grip qualities of tires with the road surface.
In the production of autotechnical examinations, it is necessary to know the braking efficiency coefficient to calculate the deceleration during emergency braking of vehicles.
The value of the braking efficiency coefficient primarily depends on the nature of braking, when braking a serviceable vehicle with wheel locks (when skid marks remain on the roadway) theoretically TO uh = 1.
However, with non-simultaneous blocking, the braking efficiency coefficient may exceed unity. In expert practice, in this case, the following maximum values of the braking efficiency coefficient are recommended:
| K e = 1.2 | at? ? 0.7 |
| K e = 1.1 | at? = 0.5-0.6 |
| K e = 1.0 | at? ? 0.4 |
If the braking of the vehicle was carried out without blocking the wheels, it is impossible to determine the braking efficiency of the vehicle without experimental studies, since it is possible that the braking force was limited by the design and technical condition of the brakes.
| Table 2 4 |
||||
| Vehicle type | K e in the case of braking of unladen and fully laden vehicles with the following friction coefficients |
|||
| 0,7 | 0,6 | 0,5 | 0,4 |
|
| Cars and others based on them |  |  |  |  |
| Freight - with a carrying capacity of up to 4.5 tons and buses up to 7.5 m long |  |  | | |
| Freight - with a carrying capacity of more than 4.5 tons and buses with a length of more than 7.5 m |  |  |  |  |
| Motorcycles and mopeds without a sidecar |  |  | | |
| Motorcycles and mopeds with a sidecar | | |  |  |
| Motorcycles and mopeds with an engine displacement of 49.8 cm 3 | 1.6 | 1.4 | 1.1 | 1.0 |
In this case, for a serviceable vehicle, only the minimum permissible braking efficiency (the maximum value of the efficiency coefficient; braking) can be determined.
The maximum permissible values of the braking efficiency coefficient of a serviceable vehicle mainly depend on the type of vehicle, its load and the friction coefficient in the braking section. With this information, it is possible to determine the braking efficiency coefficient (see Table 2).
The motorcycle braking efficiency values given in the table are valid for simultaneous braking with the foot and hand brakes.
If the vehicle is not fully loaded, the braking efficiency factor can be determined by interpolation.
4. DRIVING RESISTANCE COEFFICIENT
In the general case, the coefficient of resistance to the movement of a body along the supporting surface is the ratio of the forces that impede this movement to the weight of the body. Therefore, the coefficient of resistance to movement allows taking into account the energy loss when the body moves in this area.Depending on the nature of the acting forces in expert practice, different concepts of the coefficient of resistance to movement are used.
Rolling resistance coefficient - ѓ called the ratio of the force of resistance to movement during free rolling of the vehicle in a horizontal plane to its weight.
By the value of the coefficient ѓ , besides the type and condition of the road surface, is influenced by a number of other factors (for example, tire pressure, tread pattern, suspension design, speed, etc.), so a more accurate value of the coefficient ѓ can be determined in each case experimentally.
The energy loss when moving on the road surface of various objects thrown away during a collision (collision) is determined by the coefficient of resistance to movement ѓ g. Knowing the value of this coefficient and the distance that the body has moved along the road surface, you can set its initial speed, after which, in many cases.
Coefficient value ѓ can be approximately determined from Table 3 5 .
Table 3
| road surface | Coefficient, - |
| Cement and asphalt concrete in good condition | 0,014-0,018 |
| Cement and asphalt concrete in satisfactory condition | 0,018-0,022 |
| Crushed stone, gravel treated with binders, in good condition | 0,020-0,025 |
| Crushed stone, gravel without processing, with small potholes | 0,030-0,040 |
| paving stones | 0,020-0,025 |
| Cobblestone | 0,035-0,045 |
| The soil is dense, even, dry | 0,030-0,060 |
| The ground is uneven and muddy | 0,050-0,100 |
| The sand is wet | 0,080-0,100 |
| Sand dry | 0,150-0,300 |
| Ice | 0,018-0,020 |
| snowy road | 0,025-0,030 |
As a rule, when moving objects thrown away during a collision (collision), their movement is hampered by road irregularities, their sharp edges cut into the pavement surface, etc. It is not possible to take into account the influence of all these factors on the magnitude of the force of resistance to movement of a particular object, therefore the value of the coefficient of resistance to movement ѓ g can only be found experimentally.
It should be remembered that when a body falls from a height at the moment of impact, a part of the kinetic energy of translational motion is extinguished due to the pressing of the body to the road surface by the vertical component of inertia forces. Since the kinetic energy lost in this case cannot be taken into account, it is also impossible to determine the actual value of the body's velocity at the moment of fall, only its lower limit can be determined.
The ratio of the force of resistance to movement to the weight of the vehicle when it is free rolling on a section with a longitudinal slope of the road is called the coefficient of the total road resistance ? . Its value can be determined by the formula:
The sign (+) is taken when the vehicle is moving uphill, the sign (-) is taken when driving downhill.
When moving along an inclined section of the road of a braked vehicle, the coefficient of the total resistance to movement is expressed by a similar formula:
5. DRIVER RESPONSE TIME
In psychological practice, the driver's reaction time is understood as the period of time from the moment the driver receives a signal of danger to the start of the driver's influence on the vehicle controls (brake pedal, steering wheel).In expert practice, this term is commonly understood as a period of time t 1 , sufficient to ensure that any driver (whose psychophysical capabilities meet professional requirements), after an objective opportunity arises to detect danger, has time to influence the vehicle controls.
Obviously, there is a significant difference between these two concepts.
Firstly, the danger signal does not always coincide with the moment when an objective opportunity arises to detect an obstacle. At the moment an obstacle appears, the driver can perform other functions that distract him for some time from observing in the direction of the obstacle that has arisen (for example, monitoring the readings of control devices, passenger behavior, objects located away from the direction of travel, etc.) .
Consequently, the reaction time (in the sense that is put into this term in expert practice) includes the time elapsed from the moment when the driver had an objective opportunity to detect an obstacle until the moment when he actually discovered it, and the actual reaction time from the moment receiving a signal of danger to the driver.
Secondly, driver reaction time t 1 , which is accepted in the calculations of experts, for a given road situation, the value is constant, the same for all drivers. It can significantly exceed the actual reaction time of the driver in a specific case of a traffic accident, however, the actual reaction time of the driver should not exceed this value, since then his actions should be assessed as untimely. The actual reaction time of a driver within a short period of time can vary widely depending on a number of random circumstances.
Therefore, the reaction time of the driver t 1 , which is accepted in expert calculations, is essentially normative, as if establishing the necessary degree of attentiveness of the driver.
If the driver reacts to the signal more slowly than other drivers, therefore, he must be more careful when driving in order to meet this standard.
It would be more correct, in our opinion, to name the quantity t 1 not the reaction time of the driver, but the standard delay time of the driver's actions, such a name more accurately reflects the essence of this value. However, since the term "driver reaction time" is firmly rooted in expert and investigative practice, we retain it in this work.
Since the required degree of driver attention and the ability to detect obstacles in different traffic conditions are not the same, it is advisable to differentiate the standard reaction time. To do this, complex experiments are needed to determine the dependence of the reaction time of drivers on various circumstances.
In expert practice, it is currently recommended to take the standard driver reaction time t 1 equal to 0.8 sec. The following cases are an exception.
If the driver is warned of the possibility of a hazard and of the place of the expected occurrence of an obstacle (for example, when bypassing a bus from which passengers are getting off, or when passing a pedestrian with a short interval), he does not need additional time to detect the obstacle and make a decision, he should be prepared for immediate braking at the moment of the beginning of dangerous actions of a pedestrian. In such cases, the standard response time t 1 it is recommended to take 0.4-0.6 sec(greater value - in conditions of limited visibility).
When the driver detects a malfunction of the controls only at the moment a dangerous situation arises, the reaction time naturally increases, since additional time is needed for the driver to make a new decision, t 1 in this case is 2 sec.
The traffic rules prohibit a driver from driving a vehicle even in a state of the slightest alcoholic intoxication, as well as with such a degree of fatigue that may affect traffic safety. Therefore, the effect of alcohol intoxication on t 1 is not taken into account, and when assessing the degree of driver fatigue and its impact on traffic safety, the investigator (court) takes into account the circumstances that forced the driver to drive the vehicle in such a state.
We believe that the expert in the note to the conclusion can indicate an increase t 1 as a result of overwork (after 16 hour driving work by about 0.4 sec).
6. DELAY TIME FOR BRAKE ACTIVATION
The delay time of the brake actuator ( t 2 ) depends on the type and design of the brake system, their technical condition and, to a certain extent, on the nature of the driver pressing the brake pedal. In case of emergency braking of a serviceable vehicle, the time t 2 relatively small: 0.1 sec for hydraulic and mechanical drives and 0.3 sec - for pneumatic.If the hydraulically actuated brakes are applied from the second pedal application, the time ( t 2 ) does not exceed 0.6 sec, when triggered from the third pressing on the pedal t 2 = 1.0 sec (according to experimental studies conducted at TsNIISE).
Experimental determination of the actual values of the delay time of operation of the brake drive of vehicles with serviceable brakes is in most cases unnecessary, since possible deviations from the average values cannot significantly affect the results of calculations and expert conclusions.
EXAMPLE #1.
Set the deceleration and vehicle speed before braking on dry asphalt concrete pavement, if the length of the braking tracks of all wheels is 10 m, the deceleration rise time is 0.35 s, the steady-state deceleration is 6.8 m/s 2 , the vehicle base is 2.5 m, the coefficient of adhesion - 0.7.
SOLUTION:
In the current traffic situation, in accordance with the recorded track, the speed of the car before braking was approximately 40.7 km/h:
j \u003d g * φ \u003d 9.81 * 0.70 \u003d 6.8 m / s 2
The formula indicates:
t 3 \u003d 0.35 s - the rise time of the deceleration.
j \u003d 6.8 m / s 2 - steady-state deceleration.
Sy = 10 m - the length of the recorded braking track.
L = 2.5 m -- car base.
EXAMPLE #2.
Set the stopping distance of the VAZ-2115 car on a dry asphalt concrete pavement if: the driver's reaction time is 0.8 s; delay time of operation of the brake actuator 0.1 s; deceleration rise time 0.35 s; steady-state deceleration 6.8 m/s 2 ; the speed of the VAZ-2115 car is 60 km / h, the adhesion coefficient is 0.7.
SOLUTION:
In the current traffic situation, the stopping distance of the VAZ-2115 is approximately 38 m:
The formula indicates:
t 1 \u003d 0.8 s -- driver reaction time;
t 3 \u003d 0.35 s - the rise time of the deceleration;
j \u003d 6.8 m / s 2 - steady-state deceleration;
V \u003d 60 km / h - the speed of the VAZ-2115 car.
EXAMPLE #3.
Determine the stopping time of a VAZ-2114 car on a wet asphalt concrete pavement if: the driver's reaction time is 1.2 s; delay time of operation of the brake actuator 0.1 s; deceleration rise time 0.25 s; steady-state deceleration 4.9 m/s 2 ; the speed of the car VAZ-2114 is 50 km/h.
SOLUTION:
In the current traffic situation, the stopping time of the VAZ-2115 is 4.26 s:
The formula indicates:
t 1 \u003d 1.2 s - driver reaction time.
t 3 \u003d 0.25 s - the rise time of the deceleration.
V \u003d 50 km / h - the speed of the VAZ-2114 car.
j \u003d 4.9 m / s 2 - deceleration of the VAZ-2114 car.
EXAMPLE #4.
Determine the safe distance between a VAZ-2106 car moving ahead at a speed and a KAMAZ car moving at the same speed. For the calculation, accept the following conditions: turning on the brake light from the brake pedal; driver reaction time when choosing a safe distance - 1.2 s; the delay time of the operation of the brake drive of the KamAZ vehicle is 0.2 s; the rise time of the deceleration of the KamAZ vehicle is 0.6 s; deceleration of the KamAZ car - 6.2 m / s 2; deceleration of the VAZ car - 6.8 m / s 2; the delay time of the operation of the brake drive of a VAZ car is 0.1 s; the rise time of the deceleration of the VAZ car is 0.35 s.
SOLUTION:
In the current traffic situation, the safe distance between cars is 26 m:
The formula indicates:
t 1 \u003d 1.2 s - driver reaction time when choosing a safe distance.
t 22 \u003d 0.2 s - the delay time for the operation of the brake drive of the KamAZ vehicle.
t 32 \u003d 0.6 s - the rise time of the deceleration of the KamAZ vehicle.
V \u003d 60 km / h - the speed of vehicles.
j 2 \u003d 6.2 m / s 2 - deceleration of the KamAZ car.
j 1 \u003d 6.8 m / s 2 - deceleration of the VAZ car.
t 21 \u003d 0.1 s - the delay time of the brake drive of the VAZ car.
t 31 \u003d 0.35 s - the rise time of the deceleration of the VAZ car.
EXAMPLE #5.
Determine the safe interval between the VAZ-2115 and KamAZ vehicles moving in the same direction. The speed of a VAZ-2115 car is 60 km/h, the speed of a KamAZ car is 90 km/h.
SOLUTION:
In the current traffic situation with passing vehicles, the safe lateral interval is 1.5 m:
The formula indicates:
V 1 \u003d 60 km / h - the speed of the VAZ-2115 car.
V 2 \u003d 90 km / h - the speed of the KamAZ vehicle.
EXAMPLE #6.
Determine the safe speed of the VAZ-2110 car according to the conditions of visibility, if the visibility in the direction of travel is 30 meters, the reaction time of the driver when orienting in the direction of travel is 1.2 s; delay time of operation of the brake actuator - 0.1 s; deceleration rise time - 0.25 s; steady-state deceleration - 4.9 m / s 2.
SOLUTION:
In the current traffic situation, the safe speed of the VAZ-2110, according to the condition of visibility in the direction of travel, is 41.5 km/h:
The formulas indicate:
t 1 = 1.2 s -- the reaction time of the driver when orienting in the direction of travel;
t 2 \u003d 0.1 s - the delay time of the brake actuator;
t 3 \u003d 0.25 s - the rise time of the deceleration;
ja \u003d 4.9 m / s 2 - steady-state deceleration;
Sv \u003d 30 m - visibility distance in the direction of movement.
EXAMPLE #7.
Set the critical speed of the VAZ-2110 car on a turn according to the condition of cross slip, if the turning radius is 50 m, the coefficient of cross adhesion is 0.60; road cross slope angle - 10°
SOLUTION:
In the current traffic situation, the critical speed of the VAZ-2110 car on a turn according to the cross-slip condition is 74.3 km / h:
The formula indicates:
R \u003d 50 m - turning radius.
f Y \u003d 0.60 - the coefficient of transverse adhesion.
b \u003d 10 ° - the angle of the transverse slope of the road.
EXAMPLE #8
Determine the critical speed of the VAZ-2121 car on a turn with a radius of 50 m according to the condition of overturning, if the height of the center of gravity of the car is 0.59 m, the track of the VAZ-2121 car is 1.43 m, the coefficient of transverse roll of the sprung mass is 0.85 .
SOLUTION:
In the current traffic situation, the critical speed of the VAZ-2121 car on a turn according to the rollover condition is 74.6 km/h:
The formula indicates:
R \u003d 50 m - turning radius.
hц = 0.59 m - the height of the center of gravity.
B \u003d 1.43 m - track of the VAZ-2121 car.
q \u003d 0.85 - the coefficient of transverse roll of the sprung mass.
EXAMPLE #9
Determine the stopping distance of the GAZ-3102 car in ice conditions at a speed of 60 km/h. Vehicle load 50%, delay time of the brake actuator operation - 0.1 s; deceleration rise time - 0.05 s; adhesion coefficient - 0.3.
SOLUTION:
In the current traffic situation, the stopping distance of the GAZ-3102 is approximately 50 m:
The formula indicates:
t 2 \u003d 0.1 s - the delay time of the brake actuator;
t 3 \u003d 0.05 s - the rise time of the deceleration;
j \u003d 2.9 m / s 2 - steady-state deceleration;
V \u003d 60 km / h - the speed of the GAZ-3102 car.
EXAMPLE #10
Determine the braking time of the VAZ-2107 car at a speed of 60 km/h. Road and technical conditions: rolled snow, delay time of operation of the brake drive - 0.1 s, deceleration rise time - 0.15 s, adhesion coefficient - 0.3.
SOLUTION:
In the current traffic situation, the braking time of the VAZ-2107 is 5.92 s:
The formula indicates:
t 2 \u003d 0.1 s - the delay time of the brake actuator.
t 3 \u003d 0.15 s - the rise time of the deceleration.
V \u003d 60 km / h - the speed of the VAZ-2107 car.
j \u003d 2.9 m / s 2 - deceleration of the VAZ-2107 car.
EXAMPLE #11
Determine the movement of the KamAZ-5410 car in a braked state at a speed of 60 km/h. Road and technical conditions: loading - 50%, wet asphalt concrete, adhesion coefficient - 0.5.
SOLUTION:
In the current traffic situation, the movement of the KamAZ-5410 vehicle in a braked state is approximately 28 m:
j \u003d g * φ \u003d 9.81 * 0.50 \u003d 4.9 m / s 2
The formula indicates:
j \u003d 4.9 m / s 2 - steady-state deceleration;
V \u003d 60 km / h - the speed of the KamAZ-5410 car.
EXAMPLE #12
On the road with a width of 4.5 m, there was a head-on collision of two cars - a ZIL130-76 truck and a GAZ-3110 Volga passenger car.
During inspection of the scene of the accident, brake marks were recorded. The rear tires of the truck left a skid mark 16 m long, the rear tires of the passenger car - 22 m. about 200 m. At the same time, the truck was located at a distance of about 80 m from the collision site, and the car was 120 m.
Establish the technical capability to prevent a collision of cars for each of the drivers.
For research accepted:
for car ZIL-130-76:
for GAZ-3110 car:
SOLUTION:
1. Stopping path of cars:
cargo
Passenger
2. The condition for the possibility of preventing a collision with a timely response of drivers to an obstacle:
Let's check this condition:
The condition is met, therefore, if both drivers correctly assessed the traffic situation and at the same time made the right decision, then the collision could have been avoided. After the cars stopped, the distance between them would be S = 200 - 142 = 58 m.
3. The speed of cars at the moment of the beginning of full braking:
cargo
passenger car
4. The distance traveled by vehicles skidding (full braking):
cargo
passenger car
5. Movement of cars from the collision site in a braked state in the absence of a collision:
cargo
passenger car
6. The condition for the possibility of preventing a collision for car drivers in the current situation: for a truck
The condition is not met. Consequently, the driver of the ZIL-130-76 car, even with a timely response to the appearance of the GAZ-3110 car, did not have the technical ability to prevent a collision.
for a passenger car
The condition is met. Consequently, the driver of the GAZ-3110 car, with a timely response to the appearance of the ZIL-130-76 car, had the technical ability to prevent a collision.
Conclusion. Both drivers did not react in time to the appearance of danger and both braked with some delay. (S "y d = 80 m > S" o = 49.5 m: S "y d = 120 m > S" o = 92.5 m). However, only the driver of the GAZ-3110 car in the situation that had developed had the opportunity to prevent a collision.
EXAMPLE 13
A LAZ-697N bus moving at a speed of 15 m/s hit a pedestrian walking at a speed of 1.5 m/s. The pedestrian was hit by the front of the bus. The pedestrian managed to pass 1.5 m along the bus lane. The total movement of the pedestrian is 7.0 m. The width of the carriageway in the accident zone is 9.0 m.
For research accepted:
SOLUTION:
Let's check the possibility of preventing a collision with a pedestrian by avoiding a pedestrian in front and behind, as well as emergency braking.
1. Minimum safety interval when passing a pedestrian
2. Dynamic corridor width
3. Maneuver coefficient
4. The condition for the possibility of performing a maneuver, taking into account the traffic situation when passing a pedestrian:
behind
front
Pedestrian bypassing is possible only from behind (from the back).
5. Lateral displacement of the bus required to bypass a pedestrian from the back:
6. The actually required longitudinal movement of the bus to move it to the side by 2.0 m
7. Removing the car from the place of collision with a pedestrian at the time of the occurrence of a dangerous situation
6. Condition of safe pedestrian detour:
The condition is met. Therefore, the bus driver had the technical ability to prevent the pedestrian from being hit by a detour from the back.
7. Bus stop push length
Since S oud \u003d 70 m > S o \u003d 37, b m, the safety of the pedestrian crossing could also be ensured by emergency braking of the bus.
Conclusion. The bus driver had the technical ability to prevent a collision with a pedestrian:
a) by passing a pedestrian from the back (at a constant speed of the bus);
b) by emergency braking from the moment the pedestrian begins to move along the carriageway.
EXAMPLE 14.
As a result of damage to the tire of the front left wheel, a ZIL-4331 car suddenly drove onto the left side of the carriageway, where a head-on collision with an oncoming GAZ-3110 car occurred. The drivers of both cars applied the brakes to avoid the collision.
The question was put to the expert's permission: did they have the technical ability to prevent a collision by braking.
Initial data:
- roadway - asphalted, wet, horizontal profile;
- distance from the collision site to the start of the ZIL-164 turn to the left - S = 56 m;
- the length of the braking track from the rear wheels of the GAZ-3110 - = 22.5 m;
- the length of the braking track of the ZIL-4331 car before the impact - = 10.8 m;
- the length of the braking track of the ZIL-4331 car after the impact to a complete stop - = 3 m;
- the speed of the ZIL-4331 car before the incident -V 2 = 50 km/h, the speed of the GAZ-3110 car is not set.
The expert accepted the following values of the technical quantities required for the calculations:
- deceleration of vehicles during emergency braking - j = 4m/s 2 ;
- reaction time of drivers - t 1 = 0.8 s;
- delay time of operation of the brake drive of the GAZ-3110 car - t 2-1 = 0.1 s, of the ZIL-4331 car - t 2-2 = 0.3 s;
- the rise time of the deceleration of the car GAZ-3110 - t 3-1 = 0.2 s, the car ZIL-4331 t 3-2 = 0.6 s;
- the weight of the GAZ-3110 car - G 1 \u003d 1.9 tons, the weight of the ZIL-4331 car - G 2 \u003d 8.5 tons.
B. M. Tishin,
non-state forensic expert in the field of autotechnical expertise,
candidate of technical sciences
(Saint Petersburg)
The braking and stopping distances calculated by the methods available in expert practice are based on the assumption that the vehicle speed is equal throughout the entire braking process. The paper proposes a method for the refined calculation of the braking and stopping distances of vehicles, taking into account the speed reduction at all stages of the braking process. The distances calculated by the refinement method give a result 10–20% less than using the methods available to experts today.
Keywords: calculation method; braking distances; stop way; equality of speeds; speed reduction; error of results; slowdown; movement time.
T 47
LBC 67.52
UDC 343.983.25
GRNTI 10.85.31
VAK code 12.00.12
To the question of the refined calculation of the braking and stopping distance of the vehicle in the analysis of road accidents and the production of auto-technical examinations
B.M.Tishin,
non-state forensic expert in the field of autotechnical expertise
(city Sankt-Petersburg)
The distances of the braking and tracks stopping, calculated by the methods available in expert practice, are based on the assumption that the speed of the vehicle is equal throughout the braking process. In the work the technique of the refined calculation of distances of a brake and stopping way of vehicles, taking into account speed reduction at all stages of process of braking is offered. Calculated distances by the refinement method give a result of 10 ÷ 20 % less than the methods available to experts today.
keywords: calculation technique; braking distances; stopping way; equality of speeds; reduction in speed; error in results; slow down; driving time.
_____________________________________
The most objective indicator by which one can judge the speed of movement before braking is the traces left by the tires of the vehicle on the road surface.
The speed of the vehicle before braking in expert practice is calculated by the formula:
Here:
Steady deceleration when braking the vehicle;
Standard deceleration rise time;
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- the length of the measured braking track before the vehicle stops.
This formula takes into account the fact that when you press the brake pedal, there is a gradual increase in deceleration, and therefore, the formula takes into account the change in speed during the increase in deceleration as an average value at the initial deceleration "0" and the final - "".
However, a change in the speed of movement during the braking process occurs not only during the increase in deceleration, but also during the operation of the brake actuator and during the movement of the vehicle, when the driver decides that braking is necessary, stops the fuel supply and moves his foot from the fuel pedal to the brake pedal. . At this time, the vehicle moves under the action of inertia, overcoming the resistance to the movement of the vehicle, depending on the driving conditions and the resistance to forced scrolling of the engine crankshaft from the wheels through the transmission, if the gear on the gearbox (gearbox) is not turned off, since the crankshaft speed decrease sharply after the fuel supply is cut off, and the wheels continue to rotate for some time, practically at the same speed.
Currently, the presence of an anti-lock wheel device (ABS) in the brake system does not allow the wheels to block during intensive (emergency) braking. Therefore, there are no signs of braking, as such, on the road surface. This provision is enshrined in GOST R 51709-2001, clause 4.1.16: “Automotive vehicles equipped with anti-lock braking systems (ABS), when braking in running order, (taking into account the mass of the driver), with an initial speed of at least 40 km/hour, must move within the traffic corridor without visible signs of drift and skidding, and their wheels must not leave skid marks on the road surface until the ABS is turned off when the speed corresponding to the ABS cut-off threshold is reached (no more than 15 km/hour). The functioning of the ABS signaling devices must correspond to its good condition.
The same circumstance does not allow setting the vehicle speed before braking according to the above formula, which takes into account the speed change during the deceleration build-up time.
Therefore, the speed of movement before braking is established by the investigation, the court, experts by other methods, when the change in speed during the increase in deceleration is not taken into account.
According to GOST R 51709-2001, the braking distance is understood as the distance traveled by the vehicle from the beginning to the end of braking.
The brake diagram given in GOST R 51709-2001 in Appendix "B" is shown in fig. 1.
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Rice. 1. Braking diagram: lag time of the braking system; deceleration rise time; deceleration time with steady deceleration; brake system response time; steady slowdown of ATS; H and K - the beginning and end of braking, respectively.
The start of braking is the point in time at which the vehicle receives a signal to apply braking. Designated with a dot "H" in Appendix "B".
The end of braking is the point in time at which the artificial resistance to the movement of the vehicle has disappeared or it has stopped. Denoted by the dot "K" in Appendix "B".
Annex "G" (GOST R 51709-2001) states that it is allowed to calculate the braking distance in meters for the initial braking speed based on the results of checks of the deceleration indicators of the vehicle during braking according to the formula (Appendix "D"):
where: - the initial braking speed of the vehicle, km/hour;
Lag time of the braking system, With;
Deceleration rise time, With;
steady slowdown, m/With 2 ;
In Appendix "D", the first term of the braking distance expression is equated to the expression in which "A" is a coefficient characterizing the response time of the braking system.
In the same appendix, a table of values of the coefficient "A" and the standard steady-state deceleration for various categories of vehicles is given.
This calculation method is used when recalculating the braking distance standards.
Table E. 1
| ATS | Initial data for calculating the standardstopping distancePBX in equippedcondition: |
||
| A | m /With 2 |
||
| Passenger and utility vehicles | M1 | 0,10 | 5,8 |
| M2, M3 | 0,10 | 5,0 |
|
| Cars with a trailer | M1 | 0,10 | 5,8 |
| Trucks | N1 , N2, N3 | 0,15 | 5,0 |
| Trucks with a trailer (semi-trailer) | N1 , N2, N3 | 0,18 | 5,0 |
Based on the standard values of the coefficient "A", for vehicles of categories M1, M2, M3, the braking distance increases by 10% of the initial speed. For vehicles of categories N1, N2, N3 without a trailer - by 15% of the initial speed. For automatic telephone exchanges of categories N1; N2; N3 with a trailer or semi-trailer - by 18% of the initial speed.
The initial speed is substituted into km/hour.
In the practice of analyzing accidents or in the production of autotechnical examinations, to determine the effectiveness of braking, it is not the braking distance due to the technical parameters of the vehicle that is taken, but the stopping distance of the vehicle, due to both the technical parameters of the vehicle and the psychophysiological capabilities of the driver.
According to the definition given by Professor S. A. Evtyukov, the stopping distance is the distance necessary for the driver to stop the vehicle by braking at the initial braking speed when driving in specific road conditions. The stopping distance consists of the distance traveled by the vehicle during the driver's reaction to danger, the delay of the brake drive and the increase in deceleration during emergency braking, as well as the distance traveled by the vehicle with a steady deceleration up to its complete stop.
As can be seen from the definitions of braking and stopping distances, they differ from each other by the distance that the vehicle travels during the reaction time of the average driver.
In expert practice, the stopping distance is calculated based on the average driver's reaction time standards, by types of traffic situations, the standard delay time of the brake drive and the increase in deceleration by vehicle categories and types of brake drives.
where: - driver reaction time, selected by an expert in tables of differentiated values of driver reaction time, in accordance with meteorological and road conditions.
- normative and technical values of the braking parameters, taken by the expert according to the tables of experimentally calculated values of the braking parameters of vehicles in expert practice.
Both for calculating the stopping distance according to the formula given in GOST, and for calculating the stopping distance according to the formula used in the practice of expert calculations, assumptions are made: the initial speed of the vehicle before braking is taken equal to the speed when the brake pedal is pressed and when the movement starts in a decelerating state with a steady deceleration. That is, it is conditionally assumed that throughout the entire braking process until a steady deceleration occurs, the speed of the vehicle remains constant.
In fact, during the braking process, there is a constant decrease in speed both when driving during the reaction time of the driver, and when driving during the response time of the brake system. When calculating the braking and stopping distances in the above formulas, parameters are used that take into account the distances that the vehicle travels during the braking stages, but it does not take into account that the vehicle travels these distances at a constantly decreasing speed.
When the vehicle moves during the reaction of the driver, it travels a distance under the action of inertia, overcoming the force of rolling resistance along the actual road surface, and, if the gearbox is not disengaged when the brake pedal is pressed, then overcoming the force of resistance to movement from turning the crankshaft of the engine through transmission.
The rolling resistance force of a vehicle is generally determined by the product of the rolling resistance coefficient on the actual road surface and the vehicle's gravity:
When driving on a horizontal section of the track or when the slope - the rise can be neglected, 
The resistance to movement of a vehicle arising from the rotation of the engine crankshaft is very difficult to calculate analytically, therefore, in the practice of the theory of vehicle movement, the resistance to movement arising from the rotation of the engine shaft through the transmission is calculated using the empirical formula of Yu. A. Kremenets:
where is the working volume of the engine (displacement), in liters;
Vehicle speed before braking km/hour.
vehicle gravity, kg.
If the movement is not carried out in direct gear, then the gear ratio of the gearbox is entered into the numerator.
The complexity of taking into account these parameters lies in the fact that for each specific case it is necessary to calculate its own values of the deceleration that occurs when overcoming the resistance to movement. However, this also increases the accuracy of the calculations of the stopping and braking distances.
The deceleration of the vehicle when overcoming the resistance to movement is determined by the general deceleration formula:
where is the total value of the coefficient of resistance to movement.
In particular, it includes the coefficient of rolling resistance and the conditional coefficient of resistance from scrolling the engine shaft through the transmission - .
The coefficient is calculated by the general formula - the drag force divided by the vehicle's gravity.
The deceleration of the vehicle that occurs when driving during the reaction time of the driver:
During the reaction time of the driver, the speed decreases:
m/s
At the moment of the beginning of the response to the danger, the speed of the vehicle, and at the moment of pressing the brake pedal - 
m/s
Therefore, the entire time the vehicle is moving during the driver's reaction time should be considered as moving at an average speed:
Based on the presented calculation, by the time the brake system starts to operate, the vehicle speed will not
m/With
When the vehicle is moving during the operation of the brake system ( 
, the end of the movement is carried out with a speed:
m/With
The movement of the vehicle during the operation of the brake system is carried out at an average speed:
Decrease in speed for the time of operation of the brake system
Thus, by the time a steady deceleration occurs, the speed of the vehicle is equal to 
It is this speed that should be substituted into the term that determines the distance the vehicle travels during the movement with a steady deceleration to a stop or to a predetermined value.
The proposed method for taking into account the speed reduction allows us to propose another option for calculating the stopping and braking distances:
Despite the cumbersomeness of the proposed expressions, they are easy to calculate, since general conclusions are given here. By sequentially solving the values of the average speeds for the initial and final speeds, the calculation process is simplified.
Let us consider a specific braking event of a passenger vehicle of category , with the driver's reaction time to danger equal to 1 With, the delay time of the brake drive equal to 0.1 With, the rise time of the deceleration occurring on a dry asphalt pavement 0.35 With, with steady deceleration 6.8 m/With 2. Engine displacement 2 l, actual vehicle weight 1500 kg, the initial speed of the vehicle before braking 90 km/hour (25 m/With). The steady-state deceleration is taken without taking into account the influence of the ABS system.
The deceleration in the process of vehicle movement during the reaction time is equal to:
m/s 2
where is the coefficient of rolling resistance on dry horizontal asphalt - 0.018.
Conditional coefficient of resistance to the crankshaft of the engine through the transmission:
Deceleration of the vehicle during the reaction time of the driver:
When driving, during the reaction time of the driver, the speed decreases:
Average speed during the reaction time of the driver:
Speed at the end of reaction time:
Steady-state deceleration during the braking system response time:
Decrease in speed for the time of operation of the brake system:
The average speed of movement during the time of operation of the brake system. 
Travel speed at the end of the brake response time:
It is this speed that should be substituted into the term that determines the distance the vehicle moves in the braking mode with a steady deceleration.
Calculate the stopping distance according to the formulas adopted in GOST and according to the proposed method:
According to the method of GOST R 51709-2001, Appendix "D":
According to the methodology allowed by Appendix "G", GOST R 51709-2001:
Which is, respectively, 19.8 and 16.6% of the braking distance, determined according to GOST R 51709-2001.
According to the method adopted in expert practice for calculating the stopping distance:
According to the proposed method of refined calculation:
Which is 11.6% of the braking distance calculated according to the accepted method:
The proposed method makes it possible to take into account the influence of a particular vehicle model and reduce the calculation error in a differentiated calculation of braking and stopping distances. This makes it possible to make a categorical conclusion about the presence or absence of the technical possibility of preventing traffic accidents on more reasonable calculations, and not on average standard parameters and the assumption of equality of the speed during the entire braking process until a steady slowdown occurs.
The formulas used in expert practice for calculating the braking and stopping distances give an overestimated result, exceeding 10%, in comparison with the proposed method of refined calculation. When calculating the braking and stopping distances of vehicles of categories N1 , N2 , N3 according to the proposed method, the difference in results compared to the methods used will increase, as the value of the coefficient "A" increases.
Literature:
1. Evtyukov S.A., Vasiliev Ya.V. Examination of road accidents: a Handbook. - St. Petersburg: DNA, 2006.
2. The use of differentiated values of the driver's reaction time in expert practice: Guidelines for VNIISE. - M., 1987.
3. Use in expert practice of extreme design values of vehicle braking parameters: Guidelines of VNIISE. - M., 1986.
4. Borovsky B. E. Road traffic safety. - L .: Lenizdat, 1984.
The indicators of the braking dynamism of the car are:
deceleration Jz, deceleration time ttor and braking distance Stor.
Vehicle deceleration
The role of various forces in decelerating a car during braking is not the same. In table. 2.1 shows the values of the resistance forces during emergency braking on the example of a GAZ-3307 truck, depending on the initial speed.
Table 2.1
The values of some resistance forces during emergency braking of a GAZ-3307 truck with a total mass of 8.5 tons
At a car speed of up to 30 m / s (100 km / h), air resistance is no more than 4% of all resistances (for a car, it does not exceed 7%). The influence of air resistance on the braking of a road train is even less significant. Therefore, when determining the deceleration of the car and the braking path, air resistance is neglected. Taking into account the above, we obtain the deceleration equation:
Jz \u003d [(tsh + w) / dvr]g (2.6)
Since the coefficient cx is usually much greater than the coefficient w, then when the car is braking on the verge of blocking, when the pressing force of the brake pads is the same, that a further increase in this force will lead to blocking of the wheels, the value of w can be neglected.
Jz \u003d (tskh / dvr)g
When braking with the engine off, the rotating mass coefficient can be taken equal to unity (from 1.02 to 1.04).
Deceleration time
The dependence of the braking time on the vehicle speed is shown in Figure 2.7, the dependence of the speed change on the braking time is shown in Figure 2.8.
Figure 2.7 - Dependence of indicators
Figure 2.8 - Brake diagram of the braking dynamism of the car on the speed of movement
The braking time to a complete stop is the sum of the time intervals:
to=tr+tpr+tn+tset, (2.8)
where tо is the braking time to a complete stop
tr is the reaction time of the driver, during which he makes a decision and puts his foot on the brake pedal, it is 0.2-0.5 s;
tpr is the response time of the brake mechanism drive, during this time the parts move in the drive. The interval of this time depends on the technical condition of the drive and its type:
for brake mechanisms with a hydraulic drive - 0.005-0.07 s;
when using disc brakes 0.15-0.2 s;
when using drum brake mechanisms 0.2-0.4 s;
for systems with pneumatic drive - 0.2-0.4 s;
tn - deceleration rise time;
tset - the time of movement with steady deceleration or the time of braking with maximum intensity corresponds to the braking distance. During this period of time, the deceleration of the car is almost constant.
From the moment of contact of the parts in the brake mechanism, the deceleration increases from zero to that steady value, which is provided by the force developed in the brake mechanism drive.
The time spent on this process is called the deceleration rise time. Depending on the type of car, road condition, traffic situation, qualification and condition of the driver, the state of the brake system tb can vary from 0.05 to 2 s. It increases with an increase in the vehicle's gravity G and a decrease in the friction coefficient u. In the presence of air in the hydraulic drive, low pressure in the drive receiver, oil and water ingress on the working surfaces of the friction elements, the value of tn increases.
With a working brake system and driving on dry asphalt, the value fluctuates:
from 0.05 to 0.2 s for cars;
0.05 to 0.4 s for hydraulic trucks;
from 0.15 to 1.5 s for trucks with pneumatic drive;
from 0.2 to 1.3 s for buses;
Since the deceleration rise time varies linearly, we can assume that in this time interval the car moves with a deceleration equal to approximately 0.5 Jzmax.
Then the decrease in speed
Dx \u003d x-x? \u003d 0.5 Jsttn
Therefore, at the beginning of deceleration with steady deceleration
x?=x-0.5Jsettn (2.9)
With a steady deceleration, the speed decreases according to a linear law from x?=Jsettset to x?=0. Solving the equation for time tset and substituting the values x?, we get:
tset=x/Jset-0.5tn
Then stopping time:
to=tr+tpr+0.5tn+x/Jset-0.5tn?tr+tpr+0.5tn+x/Jset
tr+tpr+0.5tn=ttotal,
then, assuming that the maximum intensity of braking can be obtained, only with the full use of the friction coefficient uh we obtain
to=tsum+х/(цхg) (2.10)
Braking distances
The braking distance depends on the nature of the deceleration of the vehicle. Denoting the paths covered by the car during the time tr, tpr, tn and tset, respectively Sp, Spr, Sn and Sst, we can write that the full stopping distance of the car from the moment the obstacle is detected to a complete stop can be represented as a sum:
So=Sp+Spr+Sn+Sset
The first three terms represent the path traveled by the car during the time ttot. It can be presented as
Stot=xttot
The path traveled during the steady-state deceleration from the speed x? to zero, we find from the condition that in the section Sst the car will move until all its kinetic energy is spent on doing work against the forces that impede movement, and under known assumptions only against the forces Ptor i.e.
mх?2/2=Sset Rtor
Neglecting the forces Psh and Psh, one can obtain the equality of the absolute values of the inertial force and the braking force:
РJ=mJset=Рtor,
where Jst is the maximum deceleration of the car, equal to the steady one.
mх?2/2=Sset m Jset,
0.5х?2=Sset Jset,
Sust \u003d 0.5x? 2 / Jst,
Sust \u003d 0.5x? 2 / cx g? 0.5x2 / (ch g)
Thus, the braking distance at maximum deceleration is directly proportional to the square of the speed at the beginning of braking and inversely proportional to the coefficient of adhesion of the wheels to the road.
Full stopping distance So, the car will
So \u003d Stot + Sset \u003d xttot + 0.5x2 / (tx g) (2.11)
So=xtsum+0.5x2/Jset (2.12)
The value Jset can be set empirically using a decelerometer - a device for measuring the deceleration of a moving vehicle.
