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The calculation of movement is the definition of the main parameters of the movement of a car and a pedestrian: speed, path, time and trajectory of movement.
When calculating the uniform movement of the car, the elementary relation is used
Where S A , V A And t à - respectively: the path, speed and time of the car.
Braking at a constant friction coefficient
If the driver braked during an accident, then the initial speed of the car can be determined quite accurately by the length of the tire slip track (trace) on the road that occurs when the wheels are completely blocked.
An experimental study of the braking process shows that due to a change in the coefficient of adhesion of tires to the road and oscillations caused by the presence of elastic tires and suspension elements, deceleration j during the braking process is complex.
Rice. 5.1. Braking diagram
To simplify calculations, we assume that during the time tн (deceleration build-up time) the deceleration increases according to the straight line law (section AB), and over time (time tу of steady-state deceleration) remains constant (section BC) and at the end of the period of full deceleration instantly decreases to zero (point C).
The deceleration of the car is calculated based on the conditions for the full use of the clutch by all tires of the car,
, m/s 2 (5.2)
Whereg = 9.81 m/s 2 ;
h - the coefficient of longitudinal adhesion of tires to the road, which is assumed to be constant.
Since the full and simultaneous use of the clutch by all tires of the car is relatively rare, a correction factor for braking efficiency is introduced into the formula Ke, and the formula becomes:
, m/s 2 , (5.3)
Value TO uh takes into account the correspondence of the braking forces to the adhesion forces and depends on the braking conditions. If all wheels were locked during braking, then TO uh choose depending on X .
Table 5.1
The value of k in the presence of traces of use
The most common way to determine the speed of the vehicle before the start of braking is presented by the formula available in all literary sources,
Where: j A - deceleration of the car, developed during its braking, depending on the type of vehicle, the degree of its loading, the condition of the pavement of the roadway, m / s 2;
t n - the rise time of the vehicle deceleration during its braking, which also depends on all of the above factors, as well as deceleration, and practically varies in proportion to the change in the vehicle load and the value of the coefficient of adhesion, s;
S - the length of the braking track of the car, counting to the axis of the rear wheels; if the trace remains from the wheels of both axles of the car, then the base of the car is subtracted from the size of the “skid” track L, m.
Braking and stopping distance of the car
Braking distance, stopping distance, braking trail, deceleration of the vehicle, etc. - the meanings of these terms often have to be referred to in order to objectively assess the driver's actions in a particular traffic situation.
The stopping distance of a vehicle is the distance that a car covers from the moment the driver begins to react to danger until it comes to a complete stop:
, m (5.5)
The stopping distance of a vehicle is the distance that a vehicle travels from the moment the brake pedal is pressed to the moment it comes to a complete stop:
, m. (5.6)
Thus, the stopping distance of the car is greater than its braking distance by the distance that the car overcomes during the driver's reaction time t 1 .
Driver reaction time t 1 . The value of the driver's reaction time (in autotechnical expertise) is the time interval from the moment the danger signal appears in the driver's field of vision until the start of the impact on the vehicle controls (brake pedal, steering wheel, accelerator pedal).
The reaction time of the driver is affected by all elements of the "driver - car - road - environment" (VADS) system, therefore it is advisable to differentiate the reaction time values depending on typical traffic situations characterized by certain combinations of interrelated factors of the VADS system. The reaction time varies widely - from 0.3 to 1.4 or more seconds.
So, when calculating the maximum allowable speed according to the conditions of visibility of the road, the minimum time of a simple sensorimotor reaction should be taken equal to 0.3 s. The same reaction time should be taken when determining the minimum allowable distance between passing vehicles.
In the case of any malfunctions of the vehicle during movement that affect traffic safety, as well as in case of physical intervention of the passenger in the process of driving the vehicle, the driver's reaction time can be taken equal to 1.2 s.
In case of traffic accidents at night, when the obstacle was hardly noticeable, it is allowed to increase the driver's reaction time by 0.6 s.
Delay time for actuation of the brake actuator t 2 . During this time, the free play of the brake pedal and the gaps of the brake system drive are selected. The value depends on the type of brake drive and its technical condition.
Hydraulic brakes work faster than pneumatic ones. The delay time of the hydraulic drive is taken t 2 = 0.2 - 0.4 s. Passenger cars during emergency braking t 2 = 0.2 s, and for trucks t 2 = 0,4 With. The delay time for the operation of a faulty hydraulic actuator (in the presence of air in the system or malfunction of the valves in the master brake cylinder) increases. If the brakes are activated from the second pressing the pedal, then it rises to an average of 0.6 s, and with three presses - up to 1.0 s.
The delay time of the operation of the pneumatic drive of the brakes varies within t 2 = 0.4-0.6 s, and its average value t 2 = 0.4 s. For road trains with a pneumatic drive, this time increases: with one trailer t 2 \u003d 0.6 s, and with two - t 2 = up to 1 s.
Deceleration rise time t n. The time of deceleration increase is the time from the beginning of the appearance of deceleration or from the moment the linings come into contact with the brake drums until the moment the vehicle starts moving with the established maximum deceleration or until the linings are completely pressed against the brake drums, and in the event of the formation of brake marks - until the formation of the latter on the roadway .
During emergency braking until the wheels lock, this time practically changes in proportion to the change in vehicle loading and the value of the coefficient of adhesion.
The deceleration rise time depends mainly on the type of brake actuator, the type and condition of the road surface, and the mass of the vehicle.
So, if the initial speed of the car is known V a, then the speed V Yu , corresponding to the beginning of full deceleration can be found by assuming that during t at the car is moving at a uniform rate with constant deceleration 0,5 j.
, m/s. (5.7)
Technical ability to prevent accidents
When analyzing the circumstances of a traffic accident after determining the stopping distance of the car S O it is necessary to define:
Car removal ( S a) from the place of collision at the moment when there was a danger to traffic;
The time required to stop the car, i.e. the time for the stopping distance ( t o);
Pedestrian time ( t P ), which he spends on moving from the place of danger to the place of collision;
Time ( 
), during which the braked vehicle moved before the collision.
The time of movement of a pedestrian to the place of collision is determined by:
, s, (5.8)
Where:S n - the path of a pedestrian from the place of occurrence of a dangerous situation to the place of collision, m;
V n - the speed of the pedestrian, determined either from tabular data or experimentally, km / h.
If the time of movement of the pedestrian to the place of impact is less than or equal to the total reaction time of the driver and the response time of the brake actuator ( t n t 1 + t 2 + 0.5t n = T ), then the pedestrian will be in the lane of the car, while braking has not yet occurred. In this case, there is no technical possibility to prevent a collision, regardless of the value of the vehicle speed.
If t a > T, then the analysis is carried out in the following sequence:
Determine the distance S a between the car and the place of collision at the moment of danger to traffic;
Compare the distance S A with stopping distance of the vehicle S o .
If the stopping distance of the car (S O ) less distance ( S a), then a conclusion follows about the technical possibility of avoiding an accident, otherwise the driver does not have one.
To determine the distance S a VNIISE recommends the following formulas:
In the event of a collision before braking
, m, (5.9)
Where L oud- distance from the place of impact of the car to its front part, m;
In the event that a braked vehicle after a collision continued to move to a stop,
, m (5.10)
, m, (5.11)
Where 
- the distance that a car travels after a collision until it comes to a complete stop.
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. Determine the possibility of preventing a collision with a pedestrian by avoiding a pedestrian or emergency braking.
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 ;
- drivers reaction time - 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.
n1.doc
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 reaction time of the driver is understood as the time interval from the moment the driver receives a signal of danger until the driver begins to act 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 driver attention.
If the driver reacts to the signal more slowly than other drivers, therefore, he must be more careful when driving a vehicle 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.
After each traffic accident, the speed of the vehicle before and at the moment of impact or collision is necessarily determined. This value is so important for several reasons:
- The most frequently violated point of the traffic rules is the excess of the maximum permissible speed, and thus it becomes possible to determine the probable culprit of the accident.
- Also, the speed affects the braking distance, and hence the ability to avoid a collision or collision.
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Determination of vehicle speed by braking distance
The braking distance is usually understood as the distance that a vehicle travels from the start of braking (or, to be more precise, from the moment the brake system is activated) to a complete stop. The general, non-detailed formula, from which it is possible to derive a formula for calculating the speed, looks like this:
Va = 0.5 x t3 x j + √2Sy x j= 0.5 0.3 5 + √2 x 21 x 5 = 0.75 +14.49 = 15.24m/s = 54.9 km/h where: in the expression √2Sy x j, where:
- Va is the initial speed of the vehicle, measured in meters per second;
- t3– vehicle deceleration rise time in seconds;
- j– steady vehicle deceleration during braking, m/s2; note that for wet pavement - 5 m / s2 according to GOST 25478-91, and for dry pavement j = 6.8 m / s2, hence the initial speed of the car with a “skid” of 21 meters is 17.92 m / s, or 64 .5km/h
- Su- the length of the brake track (skid), also measured in meters.
The process of determining speed during an accident is described in more detail in a wonderful article. Accounting for potential deformation when determining the vehicle speed at the time of an accident. You can download it in PDF format. Authors: A.I. Money, O.V. Yaksanov.
Based on the above equation, we can conclude that the stopping distance is primarily affected by the speed of the car, which, with the other known values, is easy to calculate. The most difficult part of calculating this formula is the exact determination of the coefficient of friction, since its value is influenced by a number of factors:
- type of road surface;
- weather conditions (when the surface is wetted with water, the coefficient of friction decreases);
- tire type;
- tire condition.
For an accurate calculation result, it is also necessary to take into account the features of the braking system of a particular vehicle, for example:
- material, as well as workmanship of brake pads;
- diameter of brake discs;
- functioning or malfunction of the electronic devices that control the braking system.
Brake trail
After a fairly quick activation of the brake system, prints remain on the road surface - brake marks. If the wheel is completely locked during braking and does not rotate, solid marks remain (sometimes called the “skid mark”), which many authors urge to consider as the result of the maximum possible pressure on the brake pedal (“brake to the floor”). In the case when the pedal is not fully pressed (or there is some kind of defect in the brake system), there are as if “blurred” tread prints on the road surface, which are formed due to incomplete blocking of the wheels, which retain the ability to rotate during such braking.
stopping way
The stopping distance is the distance that a certain vehicle travels from the time the driver detects a threat to the stop of the car. This is precisely the main difference between the braking distance and the stopping distance - the latter includes both the distance that the car covered during the time the brake system was activated, and the distance that was covered during the time it took the driver to recognize the danger and react to it. Driver reaction time is affected by the following factors:
- position of the driver's body;
- psycho-emotional state of the driver;
- fatigue;
- some diseases;
- alcohol or drug intoxication.
Determination of speed based on the law of conservation of momentum
It is also possible to determine the speed of the car by the nature of its movement after a collision, and also, in the event of a collision with another vehicle, by the movement of the second car as a result of the transfer of kinetic energy from the first. Especially often this method is used in collisions with stationary vehicles, or if the collision happened at an angle close to a straight line.
Determination of vehicle speed based on the obtained deformations
Only a very small number of experts determine the speed of the car in this way. Although the dependence of vehicle damage on its speed is obvious, there is no single effective, accurate and reproducible method for determining the speed from the obtained deformations.
This is due to the huge number of factors that affect the formation of damage, as well as the fact that some factors simply cannot be taken into account. The following can influence the formation of deformations:
- the design of each particular vehicle;
- features of cargo distribution;
- the life of the car;
- the quantity and quality of body work carried out by the vehicle;
- metal aging;
- vehicle design modifications.
Determination of speed at the time of collision (collision)
The speed at the time of the collision is usually determined from the brake wake, but if this is not possible for a number of reasons, then approximate speed figures can be obtained by analyzing the injuries sustained by the pedestrian and the damage resulting from the collision with the vehicle.
For example, the speed of a car can be judged by the features of a bumper fracture.- an injury specific to a car collision, which is characterized by the presence of a transverse splinter fracture with a large bone fragment of an irregular rhomboid shape on the side of impact. Localization when hit by a car bumper - the upper or middle third of the lower leg, for a truck - in the thigh area.
It is generally accepted that if the speed of the vehicle at the moment of impact exceeded 60 km/h, then, as a rule, an oblique or transverse fracture occurs, but if the speed was below 50 km/h, then a transverse fragmental fracture is most often formed. In a collision with a stationary car, the speed at the moment of impact is determined based on the law of conservation of momentum.
Analysis of methods for determining the speed of a car in an accident
Following the brake
Advantages:
- relative simplicity of the method;
- a large number of scientific papers and compiled methodological recommendations;
- sufficiently accurate result;
- the ability to quickly obtain the results of the examination.
Flaws:
- in the absence of tire tracks (if the car, for example, did not slow down before the collision, or the features of the road surface do not allow measuring the skid mark with sufficient reliability), this method is impossible;
- does not take into account the impact of one vehicle during a collision on another, which can.
According to the law of conservation of momentum
Advantages:
- the ability to determine the speed of the vehicle even in the absence of signs of braking;
- with careful consideration of all factors, the method has a high reliability of the result;
- ease of use of the method in cross-collisions and collisions with stationary vehicles.
Flaws:
- the lack of data on the mode of movement of the vehicle leads to an inaccurate result;
- compared with the previous method, more complex and cumbersome calculations;
- the method does not take into account the energy spent on the formation of deformations.
Based on the resulting deformities
Advantages:
- takes into account the energy costs for the formation of deformations;
- does not require brake marks.
Flaws:
- doubtful accuracy of the results obtained;
- a huge number of factors taken into account;
- often the impossibility of determining many factors;
- lack of standardized reproducible determination methods.
In practice, two methods are most often used - determining the speed on the trail of braking and based on the law of conservation of momentum. When using these two methods at the same time, the most accurate result is ensured, since the methods complement each other.
The remaining methods for determining the speed of a vehicle have not received significant distribution due to the unreliability of the results obtained and/or the need for cumbersome and complex calculations. Also, when assessing the speed of a car, the testimony of witnesses to the incident is taken into account, although in this case it is necessary to remember the subjectivity of the perception of speed by different people.
To some extent, analysis of video from surveillance cameras and video recorders can help to understand the circumstances of the incident and, as a result, get a more accurate result.
