Glossary of Terms

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The following are some of the terms you may come across. Please note that these are taken from a variety of sources. If any errors are observed, please feel free to inform us. Please also note that the descriptions are specifically shortened for simplification. Feel free to use the internet to study these terms in greater detail.

 

ACCELERATION time rate of change of velocity; change of velocity divided by time; a vector quantity measured in feet per second per second (ft per sec²) or meters par second per second (m per sec²) or expressed as a decimal fraction of acceleration of gravity; a = µ*g. Positive acceleration increases the velocity of a body, negative acceleration (deceleration) reduces the velocity of a body. This is the change in velocity with respect to time. Linear acceleration is measured in feet per sec², or often more conveniently, in units of g's. One g is the acceleration caused by gravity. 0.8 g is heavy braking for a car. A 30 mph crash into a rigid barrier when the un-deformed part of the car goes from 30mp h. to zero in a distance of 2 feat produces an average deceleration (i.e., negative acceleration) of 15 g.

AIS (ABBREVIATED INJURY SCALE): an integer scale developed to rate the severity of trauma; the AIS scales include l minor, 2 moderate, 3 serious, 4 severe, 5 critical and 6 maximum; currently untreatable. The scale does not explicitly denote a fatal injury. It was developed by the American Association for Automotive Medicine.

ANGLE COLLISION: a collision between two traffic units approaching on separate roadways or other paths that intersect.

ANGULAR VELOCITY: is thus a measure of the change of rotation with respect to time. Angular acceleration is the rate of change of angular velocity, usually measured in radians per second.

AT SCENE INVESTIGATION: examining and recording results of the accident and obtaining additional information at the scene of a traffic accident which may not be available later and which supplements data obtained for the accident report The information obtained is factual as far as possible.

BAC (BLOOD ALCOHOL CONCENTRATION): result of a test for alcohol in the blood. The relative proportion of ethyl alcohol in the blood stated in terms of the number of milligrams of alcohol in 100 millilitres of blood (weight/volume). In nearly all states, 0.10 percent BAC is considered the limit for legal intoxication.

BANK: the degree to which the outside edge of a roadway is higher than the inside edge at a specified point on a curve; the change in elevation per unit distance across the roadway from the inside of a curve to the outside edge super elevation. Bank is designated in feet (meters of rise, positive bank, or fall, negative bank, per foot (meter) of level distance from the inside to the outside of the curve. Bank may also be designated by rise as a percent of level distance.

BIOMECHANICS: the study of energy and forces and their relation to the motion, deformation or equilibrium of human and other living entities. The analysis of the action of forces on biological materials or biological systems.

BRAKING DISTANCE: the distance through which brakes are applied to slow a vehicle; the shortest distance in which a particular vehicle can be stopped by braking from a specified speed on a particular surface; the distance from application of brakes to collision.

CAUSE ANALYSIS: the effort to determine, from whatever information available, why the accident occurred, that is, the complete combination of factors that caused the highway transportation system to malfunction at the time and place of the accident with resultant injury and damage; the fifth and final level of traffic accident investigation. Cause analysis has been referred to as determining "indirect" or conditional factors.

CENTRIFUGAL FORCE: the force of a body in motion which tends to keep it continuing in the same direction rather than following a curved path.

CHIP: a short, deep gouge; a hole in pavement made by a strong, sharp pointed metal object under great pressure, usually without striations.

CHOP: a broad shallow gouge, even and regular on the deeper side and terminating in scratches and striations on the opposite, shallower side; a depression in pavement made by strong, sharp metal edge moving sideways under heavy pressure.

CHORD: a straight line connecting the ends of an arc or two points on curve A chord is never greater than the diameter of a circle of which the arc is a part.
CLOSING VELOCITY: the velocity of approach of two vehicles (measured at each centre of gravity) taken along a line between the centers at initial contact.

COEFFICIENT OF FRICTION: a number representing the resistance to sliding of two surfaces in contact; the drag factor of a vehicle or other object sliding on a roadway or other surface which is level; the force parallel to a surface required to keep an object sliding on that surface in motion, divided by the force of the object against that surface; measured by the Greek letter Mu.

COEFFICIENT OF RESTITUTION: is the ratio of the rebound velocity to the impact velocity of two objects and measures the amount of bounce. When two cars collide they may bounce apart with no deformation (coefficient = 1.0) or they may deform to a given extent (coefficient §1). The difference between the dynamic crush and static crush COR = [DYN CR – STA.CR]. The measured values are typically between 10 and 20%.

COLLISION SCRUB: a short; usually broad, skid mark or yaw mark made during engagement of the vehicles in a collision. A collision scrub may be connected to skid marks or yaw marks made before or after collision but has characteristics that differentiate it from such marks.

CONSERVATION OF ENERGY: the principle of physics which states that the amount of energy in a closed system is constant regardless of the changes in the form of that energy. Used in accident investigation in estimating speed of a vehicle from the distance required to stop. Energy can neither be created nor destroyed. Hence in considering a collision the kinetic energy before equals the kinetic energy afterwards plus the work done during the collision in deforming structures.

CONSERVATION OF MOMENTUM: the principle of physics which states that the total momentum of two bodies colliding with each other is the same before collision as after collision. This principle states that the overall momentum before and after collision should remain the same, i.e.
mlvl + m2v2 = mlv3 + m2v4
This principle can therefore be used as an aid to calculating speed of vehicles just before impact. If speeds after impact can be assessed from tire marks and other evidence.

CONTACT DAMAGE: damage to a vehicle resulting from direct pressure of same foreign object in a collision or rollover It is usually indicated by striations, rub off of material, or puncture. Compare with induces damage.

CRUSH: the amount of exterior deformation of car involved in a crash. Thus a car in a standard barrier crash may have 28 inches of frontal crush if it is relatively "soft". A "hard nosed" car may only exhibit 18 inches of frontal crush in the same impact.

CRUSH ENERGY: the amount of energy used up in crushing vehicle structures in a collision.

DEBRIS: loose material strewn on the road as the result of a traffic accident; dirt, liquids, vehicle part., clothing, and other materials from the involved traffic units at the scene of the traffic accident.

DECELERATION: rate of slowing; negative acceleration.

PRINCIPLE DIRECTION OF FORCE: the direction of the principal force sustained by a vehicle during an impact and resulting in the vehicle damage (see "Collision Deformation Classification" SAEJ224, Society of Automotive Engineers) This differs from point of impact which describes an area or location of damage.

DISCRIMINATORY REACTION: a reaction to perception of an unfamiliar hazard or other situation which required additional information than that immediately available to make a decision or which presents several choices of possible evasive tactics. Time required is 1.5 sec or more, possibly much more.

DRAG FACTOR: a number representing the acceleration or deceleration of a vehicle or other body as a decimal fraction of the acceleration of gravity; µ=a/g; the horizontal force needed to produce acceleration in the same direction divided by the weight of the body to which the force is applied. When a vehicle slides, with all wheels locked, the coefficient of friction and the drag factor have the same value.
DRIBBLE: liquid debris, from a vehicle or its cargo, that drops to the ground, often leaving a trail, if the vehicle la moving, from a spatter area to a puddle.

DRIVER PERCEPTION REACTION DISTANCE: the distance travelled by a vehicle during the time that elapses from the instant the driver detects a hazard until he applies the brake or makes an evasive manoeuvre.

DRIVER PERCEPTION REACTION TIME: the interval of time lapse between the instant a driver first perceives a reason to stop or manoeuvre his vehicle and the instant he takes action.

DYNAMIC CRUSH: the maximum exterior deformation suffered by a vehicle in impact with another vehicle or object and, due to the elastic properties of car bodies, will be slightly greater than the post impact measurement (see"static crush"). The maximum deformation that a vehicle would experience during the event. The dynamic crush is often about 10 20% greater than the static crush. The difference being the "bounce" or coefficient of restitution.

EQUIVALENT BARRIER SPEED: a much confused concept. Most commonly used to describe the equivalent impact speed into a flat rigid barrier to produce the same amount of crush as is showing on a case vehicle. Somewhat differently it can be defined as the speed needed in a rigid barrier impact to produce the same crush energy as is assessed in a case vehicle. The concept of equivalent barrier speed is only satisfactory when conditions of a given collision approximate to the pattern of damage which occurs in the standard barrier test. The more a struck object varies from a rigid barrier, the leas satisfactory is the E.B.S. For example, a car hitting a Styrofoam covered barrier at 30 m.p.h. would exhibit less crush than would occur if there was no energy absorbing foam on the barrier. In an extreme case the foam could be four feet thick and absorb most of the car's kinetic energy leaving the car looking as if it had hit a rigid "equivalent barrier" at 5 m.p.h., when in fact it was travelling at 30 m.p.h.

FALL: the motion of a vehicle falling through the air when its velocity carries it, without ground contact, beyond the point where the surface on which the vehicle wee travelling slopes abruptly downward or where an upward elope ends; the path of any body or of loose material dislodged from a moving vehicle and continuing through the air until it strikes the ground. A falling vehicle has little rotation. Compare with flip.

FENDER VAULT: the kinematics of the impacted pedestrian where the pedestrian is positioned near either side of the vehicle at impact, the torso and head pivot toward the hood, and the torso and head fall off the side of the fender.

FURROW: a channel in loose or soft material, such as snow or soil, made by a skidding or a scuffing tire or some other part of a moving vehicle.

GOUGE: a pavement scar deep enough to be easily felt with the fingers; chip, chop, or groove.

GRADE: the change in elevation in unit distance in a specified direction along the centre line of a roadway or the path of a vehicle; the differences in level of two points divided by the level distance between the points. Grade is designated in feet per foot (meters per meter) of rise or fall per foot (meter) of level distance or in rise or fall as a percent of the level distance. Grade is positive (+) if the surface rises in the specified direction and negative if it falls in that direction.

GROSS BIOMECHANICS: the overall result of forces applied to a biological system as a whole which define the macroscopic response of the system. For example, the overall motion of the arms, legs, torso and head as a result of impact forces.

GROSS VEHICLE WEIGHT (GVW): the maximum capacity of a vehicle, including the weight of the base vehicle, all added equipment, driver and passengers, and all cargo loaded into or onto the vehicle. Actual weight maybe less than or greater than GVW. hatching, shading consisting of closely spaced parallel lines in a drawing.

INDIRECT DAMAGE: the deformation not associated directly with collision force but occurs as a concomitant result of direct damage.

INDUCED DAMAGE: damage to a vehicle other than contact damage. Often indicated by coupling, distortion, bending, and breaking. Induced damage includes damage done by another part of the same vehicle. Compare with contact damage.

INTERNAL BIOMECHANICS: the response of the interior of a biological system to externally applies forces. For example, the amount of rib deflection resulting from impact.

INTRUSION: the extent to which elements of the vehicle structure intrude into the passenger compartment space as a result of impact forces. the reduction in an interior dimension, due to door, foot-well or bulkhead intrusion for example.

KINEMATICS: the motion of a body defined by displacement, velocity and acceleration but not including the influences of mass and force.

KINETIC ENERGY: the amount of energy represented by a moving mass. Energy and work are essentially the same thing, so one foot pound force is the amount of work tone (or energy used) to raise a mass of one pound one foot. Thus a 2000 lb. car moving at 30 m.p.h. has a kinetic energy of about 60,124 foot pounds. Lend use, designates the accident location as urban or rural. Classifications used are those of the Federal Highway Administration.

MAXIMUM ENGAGEMENT: greatest penetration of one body, such as a vehicle, by another collision; instant of greatest force between objects in collision; time and place of this occurrence; position of the bodies with respect to each other at this instant.

MIDDLE ORDINATE: the perpendicular distance between an arc and its chord at the middle of the chord.

MOMENTUM: mass or weight times velocity or speed; wv. Momentum is a vector quantity. momentum, this is defined as mass multiplied by velocity. Thus the momentum of a 2000 lb. car travelling at 30 m.p.h. is:
Momentum = 2000 x 44
32.2
= 2733 lbs. ft/sec.

NEWTON'S LAWS

First Law A body continues in a state of rest or of uniform motion in a straight line unless an externally applied force causes a change. ,

Second Law the rate of change of momentum is proportional to the force producing the change and it takes place in the direction of the applied force. Stated differently. Force is proportional to the mass multiplied by the acceleration.
F =ma
Third Law To every active force there is always an equal and opposite reactive force.

OCCUPANT KINEMATICS: the study of the motions of vehicle occupants, especially in the crash environment. The motion of a vehicle occupant during a crash situation.

OCCUPANT STRETCH: the distance travelled by the occupant's appendages after restraint system loading is achieved. This distance varies for different appendages and is most important for the head. Occupant stretch can be as little as O cm for an airbag restraint system or 50 cm (20 in) for some child car seats.

OVERDEFLECTED: a condition of a tire in which the pressure on the road is greater at the edges of the tread than in the middle; an overloaded or under inflated tire condition.

OVER STEER: a characteristic of a motor vehicle load that results in a tendency to swerve toward the inside of a curve, especially at high speed. Motor vehicles with more weight on the rear wheels than on the front and with too little pressure in the rear tires are likely to over steer.

PASSIVE RESTRAINT: an occupant restraint system that does not require any action on the part of the user for its activation or use.

PEDESTRIAN TRAJECTORY: the vertical angle and velocity of a pedestrian ca used by an impact. The trajectory determines the distance the pedestrian is thrown. Pedestrian trajectory is also used as the term describing pedestrian impact kinematics. Six impact trajectories have been defined as (13):

1. Wrap trajectory
2. Forward projection
3. Fender vault
4. Roof vault
5. Somersault
6. Run over

PERCEPTION: comprehending a situation, such as a hazard, by means of the senses and the mind; in highway transportation, comprehending the risk of starting or continuing a movement; the time and place at which perception occurs.

PHOTOGRAMMETRY: the use of photographs for making maps.

PILLAR: a vehicle roof support structure. The pillars between the windshield and the front seat aide windows are the A pillars, those between the front and rear seats are the B pillars, and those at the back of the rear seats are the C pillars. There are left and right A, B, and C pillars. The roof support structure at the back of a station wagon is the D pillar (See Figure 7).

PITCH: one of the three principle axis of a vehicle (See Figure 2). It is the forward/ rearward motion experienced in a vehicle upon brake application. The front of a vehicle will "nose" down upon braking effort this is in the pitch axis direction.

PROFESSIONAL RECONSTRUCTION: the effort to determine, from whatever information is available, how the accident happened. Reconstruction used to be referred to as determining "behavioural" or "mediate" causes of an accident. Reconstruction is the fourth level of traffic accident investigation. It involves studying results of the accident, considering other circumstances, and applying scientific principles to form opinions relative to events of the accident which are otherwise unknown or are a matter of dispute.

RESTRAINT SYSTEM: manually operated restraint systems include shoulder belts, lap belts, lap and shoulder belt combinations, or child safety seats. Automatic restraint systems include a relatively small number of "passive" belts and air cushion systems.

RESTRAINT SYSTEM LOADING: the process of engaging the restraint system, removing slack from the restraint system, and having the restraint system apply a restraining force to the vehicle occupant.

RESTRAINT SURVIVAL DISTANCE (RSD): The distance available between a restrained vehicle occupant in the normal, seated position and the interior surfaces of the vehicle in the direction of the crash forces. When the vehicle slows to a stop during impact it achieves its total deceleration during the crush. Thus all slowing will take place over the time required to produce the dynamic crush. The restrained occupant will not begin to slow at the same time as the vehicle due to the slack in the restraint system. The occupant will keep moving at the pre-crash velocity as the vehicle begins to slow until all the slack and stretch is eliminated from the restraint system. The restraint system will then hold the occupant back and cause him to slow down. If the slack in the restraint system is removed before the vehicle comes to a complete stop, then the occupant will be able to slow along with the vehicle during ride-down (See Figures 7 and 8).

RETROGRADE AMNESIA: a loss or impairment of memory of events preceding injury. In a traffic accident, a person who has been unconscious for a considerable time or who has lost much blood at the scene or the accident may have no recollection of events immediately preceding the crash.

RIDE DOWN: the allowing of the vehicle occupant that occurs while the vehicle is still being crushed in an impact. An unrestrained occupant will typically not experience ride down. For a restrained occupant, the amount of ride down achieved is determined by the time required to fully load the restraint system.

ROLL: the motion of a vehicle which has been retarded at the ground level while the remainder of the vehicle continues moving forward without leaving the ground; rollover. One of the three principle axis of a vehicle (See Figure 2). It is the right/left motion (as perceived by the occupants) experienced upon sitting in a car. That is the vehicle will "rock" going down on the occupants side as one enters into a vehicle.

ROLLING RESISTANCE: horizontal force required to keep a vehicle in motion on a level surface with the engine disconnected from the wheels and with no brake application; drag factor produced by friction within the vehicle and deformation of the tires and road surface. Compare with wind resistance.

ROLLOVER: an accident situation where the vehicle rolls at least 90 degrees. The term rollover is also sometimes used to describe a pitch over. See roll.

ROOF VAULT: the kinematics of the impacted pedestrian where the head and torso pivot down toward the hood and the legs rotate up over the torso. The pedestrian then flips up and over the roof of the vehicle falling to the ground behind the vehicle. (13)

ROTARY MOTION: the equations of motion relating distance, velocity, time and acceleration in a linear system have their exact equivalents in rotary motion. These are angular displacement (degrees or radians), angular velocity and angular acceleration. The equivalent of mass when rotation is occurring is more complex however. It is the Moment of Inertia. Consider a uniform rod in comparison to a dumbbell. Each la the sane weight but the distribution of that weight is different. If both objects are rotating about their geometric centers at the same speed, they clearly have different moments of inertia as a result of the different distributions of weight. The moment of inertia can be measured or calculated for all objects and that parameter (I) replaces mass'' in calculations involving angular equations. These are equations relating to angular momentum and energy of rotation.

SCAR: any mark on a road, on the ground, or on a fixed object such that surface material is removed or displaced; durable sign on the road of the location of a traffic accident.

SCRAPE: a broad area of a hard surface covered with many scratches or striations made by a sliding octal part without great pressure.

SCRATCH: a light and usually irregular scar made on a hard surface, such as paving, by a eliding octal part without great pressure.

SCUFFMARK: a friction mark on a pavement made by a tire which is both rotating and slipping; acceleration scuff, yaw marks, flat tire marks.

SIDE SLIP ANGLE: the angle between the heading direction and the velocity direction of the vehicle. This is because when the pre collision skidding occurs, the heading direction of the skidding vehicle is generally not aligned with its velocity direction at the point of impact.

SKIPSKID: a braking skid mark interrupted at frequent regular intervals; the skid mark made by a bouncing wheel on which brakes keep the wheel from turning. Compare with gap skid.

SOMERSAULT: the kinematics of the impacted pedestrian where the pedestrian is struck by the front of a vehicle and then tumbles to the ground ahead of the vehicle. (13)

SPATTER: the collection of marks on the road made by liquid from the vehicle or its cargo squirted from containers on the vehicle by force of collision. Spatter areas are irregular in shape and often consist of many small spots.

STATIC CRUSH: The deformation as measured on a vehicle after the event has occurred. This is the damage a reconstructionist would measure on a subject vehicle in a salvage yard (See Figure 3).

STRIATIONS: narrow, light, parallel stripes or streaks usually made by friction or abrasion on the roadway or on vehicle parts.

TRACK: the distance on the ground between the centre of the tire tread on one side of the vehicle to the centre of the tire tread on the opposite aide.

TRACK WIDTH: the distance between the front wheels and/or the distance between the rear wheels. (See figure 6)

TRACTION FORCE: the adhesive friction of a body, such as a tire, on a surface. such as a roadway, that keeps the body from clipping on the surface. Traction breaks and slipping begins when another force, such as a braking force and centrifugal force, exceeds traction force.

UNDERSTEER: a characteristic of a motor vehicle as loaded that results in a tendency to swerve toward the outside of a curve. Motor vehicles with more weight on the front wheels than on the rear or with too little pressure in front tires are likely to under-steer at high speeds.

VECTOR: a quantity that has both magnitude and direction such as force, velocity, acceleration, momentum; represented graphically by an arrow indicating direction and, by its magnitude.

WHEEL BASE: the distance from the centre of the rear wheels or, if there Is a tandem axle, the distance to the midpoint between the two tandem axles. The distance between the front axle and rear axle (See Figure 6).

WRAP DISTANCE the distance measured from the ground below the front bumper of a vehicle up over the front face of the vehicle, around the hood edge, and back along the hoot to the point of head impact The wrap distance is typically within a few centimetres of the pedestrian's height

WRAP TRAJECTORY the kinematics of the impacted pedestrian where the torso and head pivot over and onto the hood during impact The pedestrian achieves the velocity or the vehicle and may stay with the vehicle until the vehicle comes to a stop (*1)

YAW: a sidewise movement of a vehicle in turning movement of a vehicle in another direction than that in which it is headed; sidewise motion produced when centrifugal force exceeds traction force Often the result of over reaction or exceeding the critical speed Sometimes revealed by tire marks on the roadway One of the three principle axis of a vehicle (See Figure 2) This is experienced when the rear end of a vehicle breaks loose as it tries to feat the front end Often experienced by drivers on snow/ice, loose gravel/sand.

YAWMARK: a scuffmark made while a vehicle is yawing; the mark made on the road by a rotating tire which is slipping in a direction parallel to the axis of the wheel