**Glossary of Accident Reconstruction Terms**

To convey information accurately, it is essential that we talk the same language. The following practical and theoretical concepts are based on our experience. If you require further clarification or would like something added, please contact us. We have tried to use simple, and less technical wording in the following discussions so that a typical lay person might grasp the meaning of the terms. Some of the discussion may appear technically incomplete but this is necessary if we are to provide this introductory discussion to persons who are unfamiliar with these terms.

**DEFINITION OF TERMS**

**Weight & Mass**

Our weight is an expression of the earth’s gravitational pull on our mass. Our mass is the substance or matter of which we are made. In informal conversation, when we speak of “weight” we are really talking about a value that is a multiplication of the mass and “g” (acceleration due to gravity) and therefore we are describing a force. To speak the same language as accident reconstructionists, we must use their definition of weight, which is expressed in “newtons.” One newton is the weight value of a 1 kilogram mass multiplied by “g” or 9.81 m/s2.

This gets complicated because in North America we typically do not use the metric system in describing our weight. We say we weigh 150 pounds but we are really describing a force. When we try to convert this to the metric system we use the conversion factor of dividing by 2.2, such that 150 pounds is described as about 68 kilograms. What we don’t realize is that this conversion factor also transfers us from describing weight in the imperial system to mass in the metric system. In other words, the 68 kilograms is no longer a weight but it has been directly converted into the units of mass. Our actual weight in the metric system is really 68 kilograms times 9.81 or 667 newtons.

**Force**

Force is a description of the multiplication of mass by the acceleration experienced by that mass. In the case of our body standing on the ground, our feet apply a force to the ground equal to our mass of 68 kgs times the acceleration due to gravity (g) of 9.81 m/s2 and this is 667 newtons. A car with a mass of 2000 kgs applies a force to ground of 19620 newtons, but because it has four wheels, and assuming the mass is evenly distributed, each tire might apply a force of about 4905 kgs.

**Work & Energy**

Work is the result of what is done by a force applied over a specified distance. The seeming oddity is that even though an object might be heavy and is accompanied by a large acceleration, it will not perform any work if the force has not been applied over a distance. Thus, a large truck might be pressing against a stiff concrete wall but unless there is some crush to either the wall or the structure of the truck then no work has been performed.

You will hear experts refer to the equivalence of work and kinetic energy as the “Work-Energy Theorem.” The work performed is the same as the amount of kinetic energy that is dissipated.

**Speed & Velocity**

In conversation we use these terms interchangeably when talking about how fast we are going – but there’s more. “Speed” is the value that tells us how fast we are going but it does not tell us what direction. “Velocity” is similar to speed in that it tells us how fast we are going plus it tells us the direction in which we travel. It may not appear important to discuss direction of travel but momentum analysis dictates that this full description of motion is vital in accident reconstruction.

**Acceleration**

We often use words such as “accelerate” and “decelerate,” but to understand the language of accident reconstruction we must understand that there is only one word “acceleration” that exists in technical terms. There is no deceleration. If our acceleration is increasing (when we step on the gas pedal), we are experiencing positive acceleration. When our acceleration is decreasing, such as when we brake hard, we experience negative acceleration.

**Acceleration & Velocity**

Acceleration and velocity have a special relationship. Acceleration is simply a description of how quickly velocity changes. When our velocity changes quickly, we say we have a high value of acceleration. This is important when discussing air bags and at what collision severity we should expect them to deploy. Air bags deploy based on the acceleration evaluated by an air bag module, not necessarily because the vehicle sustained a large change in velocity. It just so happens that in most accident situations a change-in-velocity occurs rapidly; therefore we generally relate air bag deployment and velocity. But in some crashes the change-in-velocity occurs at a slower rate and results in a lower acceleration. The assessment of a lower acceleration by the logic in the air bag module may cause the air bag not to deploy, to deploy with less force, or to deploy at a later stage in the crash.

**NEWTON’S THREE LAWS OF MOTION**

These are discussed frequently in experts’ reports. They are:

- An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force.
- When a net force acts on an object, the acceleration that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass.
- Whenever one body exerts a force on a second body, the second body exerts an oppositely directed force of equal magnitude on the first body.

We can examine some common situations in accident reconstruction. The first law is appreciated when we travel around a curve. Our vehicle wants to travel in a straight line but a force is applied, from contact between the tires and road, which pushes our vehicle toward the inside of the curve. Although we are traveling at the same speed, our change in direction means that our velocity is changing (remember velocity has a directional component) therefore we are experiencing an acceleration.

The second law describes what happens when a net force is applied to an object. The greater the force applied, the greater its acceleration; however, the mass (inertia) of an object also has an influence. The larger the mass, the less acceleration will be caused by the same force. This can be appreciated by considering the effect of striking a tennis ball with your hand versus striking a large truck. You will impart a reaction (acceleration) to the ball but no visible reaction in the truck due to its much larger mass.

Newton’s third law can be appreciated by considering that the force we applied to striking the tennis ball caused an equal and opposite force to be applied to our hand. The harder we strike the tennis ball the greater the force will be against our hand and in an opposite direction to which we applied the force to the ball.

**“G” FORCE**

This refers to the force experienced by all of us on the planet, imposed by the earth’s gravitational pull. We are pulled toward the centre of the earth by a force equal to 9.81 metres per second squared. This is a rate of acceleration.

**KILOMETRES PER HOUR (KM/H), METRES PER SECOND (M/S)**

Often, accident reconstructionists talk about speed or velocity in terms of metres per second rather than in kilometres per hour. This can be confusing. What do metres per second mean in real numbers?

Let’s say we’re traveling at 50 km/h. If we want to know how fast that is in m/s we divide km/h by 3.6. So, 50 km/h is about 13.89 m/s. And when we are given a speed or velocity in m/s we can multiply it by 3.6 to get to km/h.

Reconstructionists like to discuss motion in metres per second because we are often asked to assess motion over short time and distances, such as immediately prior to an impact.

**CO-EFFICIENT OF FRICTION & DRAG FACTOR**

Somehow, we know experts use these values to calculate speed but what do they mean?

Vehicles generally slow down or accelerate because a force exists between a tire and road surface. We want to know how large that tire force is and how quickly we can slow down or speed up.

The best way we have to describe that tire force is to reference it to the acceleration due to gravity – or its “g” force. 9.81 metres per second squared equals one “g.” In terms of the tire force, we say that a typical tire engaged in maximum braking on a typical dry asphalt road surface will generate a force equal to about 70% of the “g” force.

So, if you really want to know, take 70% of 9.81 m/s2 and you get about 6.87 m/s2. That means that during every second of maximum braking you will lose about 6.87 m/s of velocity. Therefore, if you are traveling at 50 km/h (13.89 m/s) and you slam on your brakes, after one second you will have lost about 6.87 m/s and your new speed will be 7.02 m/s. We can multiply this by 3.6 and get about 36 km/h. Therefore, after one second of maximum braking on a dry road we can reduce our speed from 50 to 36 km/h.

This is the long way of introducing the concept of a co-efficient of friction. Often it is indicated by the letter “f” and used in the form “f=.7.” This really means that the tire force available to cause positive or negative acceleration is about 70% of the acceleration due to gravity.

The “Drag Factor” is similar to the “co-efficient of friction.” But whereas discussion of the “co-efficient of friction” is with respect to a level surface, the “Drag Factor” is the final force that includes such influences as the road grade. If the road that you are sliding on is a steep downgrade then the tire force will be slightly less to cause negative acceleration. In other words, you cannot slow down as quickly, but you should be able to attain a higher positive acceleration because the slope of the road is helping to increase your velocity.

The most common use of these two terms is in a slide-to-stop formula. For example, a car produces 10 metres of skid marks and comes to a stop and we want to know its original speed. Using the skid distance and the drag factor (say .7) in a formula we can estimate the vehicle’s speed was about 42 km/h. Although this is simple to apply, there are many problems that can occur if the expert does not understand the foundation of the formula or the physical evidence that is being interpreted.

**COLLISION SEVERITY, CHANGE-IN-VELOCITY, AND DELTA-V**

These concepts are essentially the same – they are used when discussing what happens to a vehicle and its occupants as a result of an impact. The most common use of the terms is in discussion of occupant injuries.

People are injured not necessarily because their vehicle experiences a change in velocity but because that change in velocity occurs rapidly. In fact, we experience a tremendous change in velocity any time we land in an airplane. Although the airplane was flying at 800 km/h and came to a stop on the runway, it changed its velocity over many minutes. Contrast this with a change in velocity of a car, traveling 50 km/h, involved in a frontal impact that occurs in 1/10th of a second. The change in velocity of the car is much greater in a short time span and the acceleration experienced by the occupants is much greater resulting in injury. So, the time in which the change in velocity occurs is vital.

You often hear the term “Delta-V” in various corners of a reconstructionist’s conversation. It’s simply that the Greek letter “Delta” has been commonly used to refer to “change in” something. When you see the term “Delta-V,” it simply means “change in velocity.” In typical accident situations, a Delta-V of 50 km/h can be defined as a severe collision that could potentially cause fatal injuries if not for safety features designed in our vehicle interiors.

**CRITICAL SPEED & YAW**

These terms are often used when reconstructionists talk about calculating the speed of a vehicle, which has lost directional control, often while driving on a curve.

The roadway can only provide a maximum amount of tire force, say 70% of g. But if you’re traveling too quickly around a curve, there may not be enough tire force available to keep your vehicle in the curve and your vehicle slides toward the outside of the curve. (See Newton’s first law)

Generally, the rear tires slide out and the vehicle begins to rotate about its vertical axis. This type of rotation is referred to “Yaw.” For example, rotation about the longitudinal axis is called “roll” and rotation about the lateral axis is called “pitch.” When a vehicle cannot make a turn and starts sliding because it is traveling too quickly we say its speed is beyond the critical speed or maximum speed of the curve. We know this occurs because such a vehicle will produce special tire marks called “yaw” marks, which are curved and have striations, or diagonal lines with the cross-section of the tire mark.

When reconstructionists see these marks they can perform a critical speed calculation that tells them the minimum speed the vehicle had to be traveling. They do this by finding the radius of the curved mark and its middle ordinate and then applying a standard formula. There is considerable controversy with using this technique – not because it is invalid, but because too often the reconstructionist does not determine whether what he sees is truly a critical speed yaw mark without influence from other confounds.

Equally, other calculations need to be performed so the reconstructionist can be certain the physical evidence has not been misinterpreted. Braking can affect calculations as can travel on different surfaces. Confirmation is needed that a consistent reduction in speed along the tire mark has occurred. Impact induced rotation cannot be used. There are many practical issues that make the application of this valid, theoretical concept prone to misuse.

**SPEED CALCULATION FROM PEDESTRIAN/CYCLIST THROW DISTANCE**

This methodology uses the concept of the earth’s gravitational pull and projection angle to determine how fast a struck object might have been traveling upon take off. Consider the artillery units in a battle who calculate the angle of their barrel and the muzzle velocity to predict where a cannon ball should fall. This is a similar procedure.

What is important to remember is that anyone thrown into the air is not slowed while flying (well, minimally if you take air resistance into account). But once they strike the ground they tumble or slide and reduce their speed. This tumbling or sliding is estimated according to the drag factor. We should appreciate that, although an object does not lose speed while in the air, the amount of speed lost upon reaching the ground will depend on how high the object was thrown. The higher the object is thrown the harder will be its initial contact with the ground and the greater speed loss. If a pedestrian strikes his head on asphalt or a curb his/her injuries could easily be as great from this contact as from the initial impact by the vehicle.

**MOMENTUM ANALYSIS**

Traditionally, this has been the primary method of speed calculation. Momentum is the product of the mass (weight) of an object and its velocity. So, if we have a car that weighs 2000 kilograms and its velocity is 10 metres per second, we say that its momentum is 20000 kilogram metres per second or “kg/ms.”

The Principle of Conservation of Momentum states that in the circumstance of a collision of two motor vehicles, the total pre-impact momentum will be equal to the total post impact momentum. This doesn’t simply involve multiplying the weight and velocity of vehicles. (Note from our earlier definition of Momentum that it has a direction and this direction is extremely important in momentum analysis.)

By drawing the direction at which two vehicles come into impact and by drawing the direction at which they leave the area of impact there is a procedure to determine which vehicle’s pre-impact motion caused the change in angles of departure. This needs further discussion but just remember that momentum analysis is dependent on good evidence at the collision scene.

A common error made by reconstructionists is that they simply draw a line between the vehicle’s position from impact to rest and state that this is the angle of departure. In fact, other factors can cause a vehicle to come to rest at unusual locations. Another common problem is that momentum analysis treats the impact phase of a collision as a black hole in which nothing takes place. Sometimes this assumption may be acceptable and sometimes not. Momentum analysis is best used in 90 degree angle collisions such as impacts at intersections. Momentum analysis can be sensitive to error when vehicles approach each other in a co-linear fashion such as in a head-on collision. Sophisticated computer programs such as PC-Crash have ways of reducing these problems.

**CRUSH ANALYSIS**

This term is frequently applied by accident reconstructionists to describe their procedures to estimate how much energy was dissipated by the crushed structure of a vehicle.

Although the exact value of dissipated energy is difficult to quantify, the speed calculation is not particularly sensitive to inaccuracies of energy dissipation and is often used in conjunction with other analyses to reach a reasonable speed estimate. This method is more accurate than a momentum analysis in situations where two vehicles are approaching each other from opposite directions such as in a head-on collision. This method is used in the well known computer reconstruction program called CRASH and its derivatives. The CRASH program not only assesses the energy from crush but simultaneously performs a momentum analysis that is then compared to the crush analysis.

**COMPUTERIZED ACCIDENT RECONSTRUCTION AND SIMULATION PROGRAMS**

A number of computerized programs have been developed to study motor vehicle accidents. The earliest and most commonly used are those developed for the U.S. National Highway Traffic Safety Administration (NHTSA) in the early 1970’s – SMAC and CRASH.

SMAC was the main simulation program to be used for NHTSA safety programs while CRASH was intended as a pre-processor to help evaluate the conditions at impact. The CRASH analysis was to be used as input into the SMAC program. Since those early days the programs have been enhanced by private vendors and re-named such as EDSMAC, WINCRASH, M-SMAC, and so on. Other three dimensional programs such as the Engineering Dynamics program HVE is based on the original code of SMAC and CRASH but has been advanced to another level.

A completely different approach, based on momentum analysis, was brought from Europe in the form of the PC-Crash program.

Other programs exist that are an assemblage of standard formulae combined with some logic. Some use Excel spreadsheet formats while others incorporate additional features such as the study of perception/response in add-on programs, such as DRIVE3.

There has been misunderstanding and abuse of computerized analysis that has been detrimental to the accident reconstruction community. From our viewpoint there has been an equal amount of misuse by experts, unintentionally and knowingly, while at the same time there has been an attempt to disparage this analysis by those experts who do not understand their function, or who have personal gain to do so. Such programs can be used by ignorant individuals who simply have developed an understanding of how to fill-in all the required input elements without introducing any obvious error warnings. Equally, some have attempted to refer to such programs as simple “black boxes” or “garbage-in-garbage-out” so as to confuse an issue under litigation. In both cases a qualified expert should be able to debunk such notorious actions by referring directly to the program’s inputs and outputs and by providing detailed descriptions of the calculations performed by the program.

We continue to point out that sophisticated logic based on CRASH, SMAC, and that of PC-Crash provide enough output of the program’s calculations that the potential of misuse is lessened. We also note that those using hand calculations or personalized methods of analysis, often fail to check whether their work is consistent or in agreement. Later, other experts who must assess such calculations have a harder time rooting out miscalculations if they are performed by a manual or personalized methodology that does not provide consistent and standard output describing those calculations.

**EVENT DATA RECORDERS**

As air bags were introduced in the late 1980’s, so too have a variety of sensors and monitors, not only to assess the function of the restraint systems but also to obtain data on the functioning of other systems in the vehicle.

Diagnostic tools have been developed to scan these systems for malfunctions. The most common experience you will encounter is when your car is not running properly and your automotive technician plugs a scan tool into your diagnostic link connector (DLC) and searches for fault codes. In the world of accident reconstructionists there are similar modules and scan tools.

While manufacturers have had the ability to scan event data recorders for a number of years, private reconstructionists have not had this ability until 2000 when the Vetronix Corporation, in agreement with General Motors and its suppliers, made the Crash Data Retrieval (CDR) Tool Kit available. This kit initially allowed downloads from modules in General Motors’ vehicles. Soon, Ford came into a similar agreement. Vetronix had been working for several years to obtain agreements with other manufacturers and suppliers to make their systems downloadable with the CDR tool kit. An Internet chat group of hundreds of reconstructionists was formed of all those who possess the tool kits and training acknowledged by Vetronix to interpret the data. Yearly conferences are held with reconstructionists performing downloads. Equally, many accident reconstructionist organizations in the U.S. and Canada are performing crash tests to study how these modules record data in crashes where speed and other factors are known.

More recently Bosch Corporation has taken over Vetronix’s business and most vehicle manufacturers have encoporated crash event data recorders that can be accessed either by the manufacturer or through the Crash Data Retreival tool kit.

As always, nothing is a panacea, and with new tools come new opportunities for misuse and misinterpretation. Although this technology appears extremely promising to achieving justice and improving vehicle safety, it is threatened by potentially misguided privacy legislation, as well as by those who would want to abuse our privacy by using our personal information without our consent.

Our position is that event data recorders must be available in all motorized vehicles. But it is also our position that the owners and drivers of those vehicles must have full knowledge of the information that is being gathered, they have full control to shut off that recording, and that they have full control over who uses that data. We are equally firm that not enough is being done to educate the public about the limited time frame of data that we as accident reconstructionists require. We would like to record the final 10 seconds before a crash, the information obtained during the actual crash, as well as a similar 10 seconds of data after the crash. We have no interest in knowing the driver’s driving habits, where they go, or anything beyond the short time frame around impact.