Traffic accident reconstruction

BOGDAN ĩÓŁTOWSKI

University of Technology and Life Sciences

Summary

This article presents traffic accident reconstruction. There is a possibility to de-termine speed at the moment of starting braking by a driver on the basis of analytic methods and software. The problems with an appropriate selection of parameters and coefficient value in traffic accident reconstruction are discussed. Moreover, the necessity to use devices that record vehicle parameters is described, so that one could easily lead traffic accident reconstruction and characterize real causes of the accident.

Keywords: traffic accident, speed, traffic accident reconstruction 1. Introduction

The problem with speed estimation at the moment of a car accident on the basis of informa-tion after the accident is one of the primary tasks that should be solved in traffic accident recon-struction. The possibility range of traffic accident reconstruction depends, first of all, on the amount and quality of material proofs.

So far, it is rare that material proofs are sufficient to fully and clear-cut reconstruct a traffic accident.

The analysis of a traffic accident in which all basic data were confirmed by an expert opinion couldn’t be described as reconstruction because it relates to a hypothetical version of a car acci-dent. It could be created many times due to the change of numerical value of data. Such an analy-sis couldn’t be a reliable baanaly-sis to formulate categorical conclusions.

So only reconstruction on the basis of real data that relate to a specific traffic accident could give details about its causes.

2. Speed estimation by analytical method

Speed estimation is one of the most important elements in traffic accident reconstruction. In case of a dangerous situation the basic manoeuvre is to start braking. During the above-mentioned action wheels leave the prints on the road. Such wheels turn with smaller angular velocity that should be the basis of its translational motion.

The characteristic thing of these wheel prints is a visible outline of the tread pattern and the fact it is stretched.

During an increase in braking speed, the wheel is locked, so it doesn’t turn but it slides on the road by full skid.

The prints of locking on the bituminous surface are characterized by elongated lines that come from the tread pattern, by full illegibility of the wheel pattern. Hence, correct identification and

measurement of the wheel print length have a big influence on speed estimation before an acci-dent.

Thus, of all possible speed estimations, clear-cut reliable proofs are wheel prints on the road. However, the most reliable doesn’t mean without problems that should be solved by this method. An appearance of wheel prints on the road is always preceded by an increase in braking power on the wheels, which is connected with brake delay value; in this phase visible wheel prints don’t appear.

In effect, visible wheel prints, which are a basis to calculate the speed, are only a part of the real braking distance. The problem is how shorter than the braking distance the wheel prints are, or how get to an appropriate length of the braking distance from the visible length of the wheel print.

When the length of the wheel print and type and condition of the surface are known, we could calculate car speed at the beginning of braking, and use the following equation [1]:

Vo =

2

⋅

a

⋅

S

2

t

a

⋅

Vo [m/s] – car speed at the moment of turning on the braking system by a driver, Ss[m] – length of wheel prints,

a[m/s2_{] – estimated value of traffic conditions}

means value of full brake delay, tn [s] – delay time from zero to a full value of a.

The speed Vo, which is calculated with the equation above is speed that a vehicle had at the beginning of braking, i.e. at the moment when a driver started pressing the brake pedal.

In the above-mentioned equation there are variables that an expert witness should calculate on the basis of material proofs and traffic conditions at the moment of an accident. The length of wheel prints in the accident place is described by material proofs. The measurements in the acci-dent place should be very accurate.

By dimensioning in a coordinate system, as a general rule, we don’t measure the distance be-tween real points, but bebe-tween their projections on an appropriate line, i.e. road edge. A relatively small mistake of point projection can result in wrong wheel print length.

The second parameter that an expert witness is obliged to describe is brake delay. Brake delay is one of the primary parameters to calculate car speed before a car accident. This parameter depends on:

• surface condition,

• type and status of solid tyres, • status of braking system, • status of suspension system.

The brake delay value is calculated in the following way [1]: a = μs· g

where:

μs – skid value of grip by locked wheels on appropriate surface,

g – gravitational acceleration (g=9,81 m/s2_).

Brake delay is constant and it has got a value the so-called full delay i.e. the middle value of brake delay which takes place in appropriate conditions after speed develop of braking.

In practice, it is the middle value of brake delay that is visible in the wheel print length. The problem is to select this value.

So, in order to describe the brake delay value, adhesion coefficient should be calculated on the basis of available tables. It should be noticed that there are large differences for the same surface condition. There aren’t often any justifications why one expert witness gives a different value than another.

Paradoxically, when expert witnesses give other values in traffic accident reconstruction, it leads to different opinions about the causes of a car accident.

Fig 1. Tables of adhesion coefficient in literature Source: [1].

To analyze adhesion coefficient, we should give a definition of individual coefficients [2]: - stick adhesion coefficient – it is the maximal coefficient value by a wheel partly slipping

20–30%; on the surface there aren’t any visible prints of sticking,

- slip adhesion coefficient – it takes place by locked wheel, i.e. when relative slipping is 100%; there are many visible wheel prints on the surface.

The third variable is the time from zero to full value of braking vehicle. In literature, the above-mentioned coefficient has the following value: - for a hydraulic system – 0,4s

- for a pneumatic system – 0,6s

So, speed estimation by analytical method, in which the values are taken from literature, can cause discrepancies with reference to the real speed at the beginning of braking.

3. Speed estimation basis of software

Computer simulations can be treated as a simple experiment. This experiment is carried out by an initial configuration of selected objects and by a manoeuvre, which decide about their disloca-tion; the given arrangement is saved.

So, in simulated calculations it is possible by trail-end-error method to get the speed that leads to wheel prints with the wished length. It is recommended to use software, first of all, in difficult

traffic accidents i.e. with a different condition of tyre friction on the surface (when there is sand on the road) or when a tyre is damaged during an accident. It should be noticed that software shouldn’t be used uncritically. Although they are protected, the experience and knowledge of a user have got big meaning. A wrong key in the input parameters could lead to wrong output parameters, so we should know and understand the software well.

Fig 2. Graph of vehicle parameter basis in V-SIM program

It should be mentioned that speed estimation before an accident by analytical method, as well as computer software in traffic accident reconstruction can make mistakes, which may result from wrong coefficients and input parameters.

4. Vehicle parameter basis in data parameters devices – „black boxes”

For traffic accident reconstruction the main problem is the lack of the most important informa-tion about the car accident occurrence. In the air force for over 50 years the devices that record selected parameters – the so-called „black boxes” – have been used. These records significantly facilitate the access to information about a plane crash, which, as a consequence, leads to the description of its causes. With currently used electronic system, there are no technical problems to install similar devices in automobiles.

Electronic steering units have been already in common use (i.e. motor steering, ABS/ESP steering and steering of an airbag system) and thanks to them we can get the information about a traffic accident. For many years similar devices to “black boxes” have been available. These are autonomic systems and they have to record the parameters of car moving, driver’s behaviour and surrounding. Their aim is to facilitate traffic accident reconstruction; it should be noticed that to describe vehicle data parameters before an accident it is obligatory to use vehicle data parameter devices in the actual time, and then to read them to make clear-cut traffic accident reconstruction [3].

The range of possible parameters of a car “black box” is characterized in Haddon Matrix (Fig. 3.).

Fig. 3. The so-called Haddon Matrix describes information useful in traffic accident reconstruc-tion

Source:[3].

The symbol „√” means knowledge of some parameters that describe matrixes; an empty field is a lack of direct information. Under „b” additional information with reference to „a” are written. It should be concluded that common use of “black boxes” in automobiles enables to describe clear-cut parameters, which also enables speed estimation at the beginning of braking by a driver. 5. Conclusion

Correct conduct of traffic accident reconstruction requires collecting appropriate proofs that are connected with its causes. The example of describing car speed at the moment of switching on the braking system by adriver shows that selecting right coefficients has got an influence on the result of parameters with the use of analytical method, as well as computer software.

It should be concluded that recording devices that are used in automobiles enable clear-cut traffic accident reconstruction and characterization of its real causes.

Bibliography

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REKONSTRUKCJA WYPADKU DROGOWEGO Streszczenie

W artykule przedstawiono tematykĊ dotyczącą rekonstrukcji przebiegu wypadku drogowego. Przedstawiono moĪliwoĞci okreĞlenia prĊdkoĞci w chwili rozpoczĊcia manewru hamowania przez kierującego pojazdem w oparciu o metody analityczne oraz przy wykorzystaniu programów komputerowych. Podniesiono równieĪ problemy związane z właĞciwym doborem parametrów i wartoĞci współczynników w procesie rekonstrukcji przebiegu wypadku drogowego. Ponadto zwrócono uwagĊ na potrzebĊ stosowania w pojazdach urządzeĔ rejestrujących parametry ruchu pojazdu umoĪli-wiających w sposób jednoznaczny dokonania rekonstrukcji i okreĞlenia rzeczywi-stych przyczyn zaistnienia wypadku.

Słowa kluczowe: wypadek drogowy, prĊdkoĞü pojazdu, rekonstrukcja wypadku drogowego

*This paper is a part of WND-POIG.01.03.01-00-212/09 project. Mariusz ĩyła

Bogdan ĩółtowski

University of Technology and Life Sciences Faculty of Mechanical Engineering