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A Pillar: The front-most roof supports on a vehicle. The pillars that separates the windshield and the front doors.
ABS: An Anti-lock Braking System that activates during a brake application to automatically control the rotation of the wheels when an impending lockup is sensed.
Acceleration: The time rate of change in velocity of a body. Acceleration can be either positive or negative (deceleration).
Acceleration of Gravity: The acceleration of a falling body due to the force of gravity (32.2 ft / sec2).
Acceleration Scuff Marks: A tire mark caused by applying enough power to a wheel to make it spin.
Accident Reconstruction: See Crash Reconstruction.
Accident Scene: See Crash Scene.
Accident Site: See Crash Site.
Airborne: Projectile Motion involving a vehicle.
Antilock Brake Scuff: A skid mark created by an antilock brake vehicle during a deceleration which instigates the activation of the antilock brake system. Scuff marks are relatively volatile and can disappear very quickly if the roadway is exposed to traffic.
Articulated Vehicle: A vehicle with two or more wheeled units connected by joints. A tractor-trailer is an articulated vehicle.
B Pillar: The second roof support on a vehicle, counting back from the windshield A Pillar.
Banking: See Superelevation Rate
Baseline Offset: A crash scene measuring technique where the location of evidence is measured linearly from a fixed point such as a utility pole along the edge of the road. The offset measurement is determined by measuring the distance the evidence is from the baseline which is commonly the edge of the road.
Black Box: Colloquial term used to describe the automobile air-bag control module that has the ability to record crash data.
Brake Efficiency: The braking efficiency represents the percentage of braking capacity that the vehicle has available when a vehicle's brakes are working at less than full capacity, usually as a result of a defect.
C Pillar: The third roof support on a vehicle, counting from the windshield A Pillar.
Center of Gravity: The mean location of the gravitation forces (weight) acting on a body. Coincides with the Center of Mass in a uniform gravitational field.
Center of Mass: The mean location of all mass in a body. Coincides with the Center of Gravity in a uniform gravitational field.
Centrifugal Force: Same as Centripetal Acceleration.
Centripetal Acceleration: The seemingly outward force felt when cornering a vehicle. This acceleration is caused by the object continually changing direction while being pulled around a circular path.
Centrifical Force: Same as Centripetal Acceleration.
Centripetal Force: Same as Centrifugal Force.
Chord-Offset Method: A method for determining the radius of a roadway curve by laying a known length of a tape measure between two points on the curve and measuring the offset from the curve at the middle of the tape.
Closing Speed: The speed differential between two vehicles traveling on the same line. The sum of the two speeds for opposing vehicles. The following vehicle speed minus the lead vehicle speed for vehicles traveling in the same direction.
Coefficient of Friction: The skid resistance of a roadway surface expressed in tenths and hundredths of one G. The lower the number, the slicker the surface and the longer it will take a vehicle to stop at a given speed.
Collision Scrub: A short, usually broad, skidmark made during the engagement of vehicles in a collision.
Combined Speed: When the basic speed formula is used to calculate the equivalent speed loss over more than one surface. The results of each calculation must be squared, added together, and then the square root of this sum represents the actual speed of the vehicle at the beginning of the event.
Compass Method: A method for determining the degree of a roadway curve by measuring the change in compass heading through the curve and dividing 100 times that change by the length of the curve. The radius of the curve can then be calculated by dividing 5729 by the degree of curve.
Complex Reaction: A complex reaction time is usually referred to as a Perception-Reaction Time, which has four elements including: presentation, detection, identification, and reaction.
Conservation of Energy: The principle of physics that says the amount of energy in a closed system is constant regardless of the form of that energy.
Conservation of Momentum: The principle of physics that says the total momentum (Mass x Speed) of two colliding bodies is equal before and after collision.
Conspicuity: The quality of being conspicuous (e.g., an object that clearly contrast with its background).
Crash Reconstruction: The art and science of determining crash causes and events from known circumstances and available physical evidence.
Crash Scene: The aftermath of a crash where the vehicles and the people involved are still present.
Crash Site: The location of a crash after the vehicles have been moved.
Critical Curve Speed: A theoretical maximum vehicle speed on a roadway curve assuming the vehicle is tracking the exact radius of the roadway curve at a steady speed and is using all of the available friction between the tires and the roadway.
Debris: Loose material scattered on the road as the result of a crash including: vehicle parts, fluids, and contents of the colliding vehicle.
Deceleration: The time rate of change in velocity manifest in braking or slowing of a body.
Direct Damage: Damage to a vehicle caused by direct contact with another vehicle or object.
Drag Factor: The acceleration or deceleration rate of a vehicle expressed as a fraction of the acceleration of gravity. The horizontal force needed to produce acceleration divided by the weight of the body. With all wheels locked, the Drag Factor and the Coefficient of Friction have the same value.
Drag Sled: A weighted portion of a car tire pulled across the roadway surface with a fish scale used to determine the Coefficient of Friction of a road surface.
Dynamics: A branch of mechanics which involves the study of why objects move. Refers to the motion of bodies caused by applied forces.
ECM: Electronic control module, a.k.a. engine control module.
Equivalent Barrier Speed: A surrogate measure for the impact speed required to create a given damage pattern when impacting a barrier. Used to compute the energy dissipation associated with crash damage to a vehicle.
EBS: Same as Equivalent Barrier Speed.
Evasive Action: Any action such as steering, braking, or acceleration taken to avoid a collision.
Event Data Recorder: A device used for recording data associated with a crash. These are usually integrated into to the engine control module or airbag electronic control module.
Fall: The trajectory of a vehicle when its velocity carries it without ground contact from its take-off from a point at a steep drop off until the point where it strikes the ground.
Flat Tire Scuff Mark: A mark made on the pavement by an under-inflated tire.
Flip: A dynamic (commonly mistaken for a rollover) that occurs when a vehicle slides to a point where its velocity is suddenly decreased by impact with a low object such as a curb or a ground furrow below its center of gravity, which results in the ensuing lift off and rotation. Usually considered in a sidewise direction. An endwise flip is usually called a Vault.
Fog Line: The solid white line on the right edge of a roadway.
Forensic Map: A crash scene or site diagram produced using electronic surveying type equipment.
Furrow Mark: A depression left by a skidding or scuffing tire in loose material such as snow or soil. This kind of mark is often an indicator of the beginning of a flip.
G-Force: A term used when accelerations are expressed as a multiple of the acceleration of gravity. The acceleration of gravity equaling 1G or 32.2 feet per second squared.
Gap Skid Mark: Skid mark made by application and release of the brakes.
Gore: Is the paved area between a highway and an exit ramp.
Gouge Mark: A pavement scar made by a crash-involved vehicle.
Grade: The rate of change in elevation of a roadway or vehicle path. Usually expressed as ft/ft. or as a % and is positive for upgrades and negative for downgrades.
Gravity: The acceleration rate at which objects accelerate towards the earth. This acceleration rate is equal to 32.2 feet per second squared.
Hot Shock: Hot shock occurs when a crash force is applied to a vehicle causing its headlights, taillights, brake lights, etc. to become deformed as a result of being incandescent at the time the force is applied.
Human Factors: The study of how humans behave physically and psychologically in relation to particular environments, products, or services.
Hydroplaning: Occurs when a layer of water builds between the tires of a vehicle and the road surface.
Indirect Damage: Same as Induced Damage.
Induced Damage: Crash damage other than direct impact damage caused by the vehicle bending, twisting, and crushing.
Jackknife: A dynamic that occurs in a combination vehicle when the vehicle towing a trailer brakes more heavily than the trailer. This braking imbalance will cause the rear of the towing vehicle to move right or left depending on the external forces applied to it. Jackknifing can also be caused by an extreme Oversteer condition in the towing vehicle.
Kinematics: A branch of mechanics that studies how objects move without regard to the cause.
Kinetic Energy: The energy possessed by a moving body. Equal to one half its mass times the square of its speed.
Lane Line: A dashed or solid line dividing two lanes traveling in the same direction.
Lateral Acceleration: Same as Centripetal Force.
Linear Momentum: The product of a body's mass and velocity.
Locked Wheel Skid: When a non-ABS vehicle is heavily braked and its wheels come to a complete stop before the vehicle comes to a complete stop. The vehicle will be sliding on its locked wheels and creating very dark skid marks.
Mass: On the earth's surface mass is the weight of object divided by the acceleration of gravity.
Maximum Engagement: The damaged state of two colliding vehicles at the point of maximum intrusion.
Mechanics: A branch of physics that involves the study of motion. Crash reconstruction is effectively applied mechanics.
Median: The area between the two sides of a divided highway, which may be paved or unpaved.
Median Crossover: Paved portion of a median that allows vehicles from roads intersecting a divided highway access to both sides of the highway.
Middle Ordinate: The radial measurement from the center of a chord to its circular arc.
Minimum Speed: A reconstruction of vehicle speeds often is a study of the boundaries of the speed that can be calculated with the available information. A reconstruction opinion is often a conclusion as to which speed within the boundaries of all calculated speeds is the most probable. Law enforcement reconstructions used in criminal cases should use the lowest speed within the boundaries for a given vehicle so that a conclusion can be drawn that that is the lowest probable speed for the vehicle and all other solutions are greater than that speed. This is done in order to meet the standard of reasonable doubt in criminal cases.
Newton's Laws: Three definitions which make up the foundation of mechanics.
Newton's First Law of Motion: This is Newton's Law of "inertia" which tells us that force is required to cause a body to move or change direction.
Newton's Second Law of Motion: This law tells us that the acceleration of an object is proportional to the net force applied to the object and inversely proportional to its mass.
Newton's Third Law of Motion: This law tells us that when an object exerts a force on another object that the second object exerts a force equal and opposite to the force applied.
Normal Force: That component of force exerted on a body that is perpendicular to the surface of contact.
NTSB: National Transportation Safety Board.
Occupant Kinematics: The study of the motion of the occupants inside of a vehicle during a collision sequence.
Off Tracking: A dynamic that occurs in a turning combination vehicle whereby the trailer tires track a different radius than the towing vehicle tires. The trailer tires usually track a smaller radius except at high speeds.
Overdriving Headlights: Driving at a speed that is too high to allow stopping a vehicle for an object in the road that is first presented at the distance of headlight illumination.
Oversteer: When a vehicle is cornering, the path of the axle and the direction the tires are not equal. The difference between the direction of the tires and the path of the vehicle is called a slip angle. When the rear tires of the vehicle side slip more than the front tires, an oversteer condition is experienced. This characteristic results in a tendency to steer toward the inside of a roadway curve, especially at higher speeds. An oversteering vehicle when driven through a curve has a tendency to continually decrease the radius that it is traversing. This dynamic causes the driver to have to reduce steering input through a turn, which is often uncomfortable for most drivers.
Perception Reaction Time: The time needed by a driver for seeing a hazard, recognizing that hazard, deciding on an evasive action, and initiating that action.
Pitch: A rotational dynamic of a vehicle, when the vehicle rotates around its Y- axis.
Point of Impact: The point where two colliding vehicles first touch.
Point of Perception: The point where a driver first recognizes a hazard.
Point of Presentation: The earliest point where a driver could detect the presence of a hazard.
Point of Rest: The place where a colliding vehicle stops after collision.
Post-Impact Speed: The speed at which each vehicle separates from the other vehicle after impact.
Pre-Braking Speed: The speed of a vehicle immediately prior to braking.
Pre-Impact Speed: Usually refers to the pre-braking speed of a vehicle just before impact. It is the same as the impact speed when no pre-impact braking is done.
Projectile Motion: The arced trajectory of an object projected into the air at a given initial speed and angle.
Reaction Time: Refers to last two segments of the Perception-Reaction Time.
Reference Point: A permanent landmark used to measure and locate the position of important spots in a crash scene.
Resting Position: The final resting position of a vehicle after a collision. Usually measured at its wheel locations.
Rim Gouge: A scar on the pavement made by a wheel rim. Usually related to a missing or under-inflated tire.
Rollover: The sidewise circular motion of a vehicle which occurs when a vehicle is turned too abruptly and rolls onto its side.
Scientific Method: As it applies to crash reconstruction, the scientific method involves gathering measurable evidence about a crash and testing a hypothesis about its causes and circumstances by applying Newtonian physics and empirical data.
Scrape: An area of pavement covered by many scratches or striations made by sliding metal.
Scratch: A light scar made on the pavement, made by a sliding metal part.
Scrub Marks: A "Skid Mark" caused by a vehicle being redirected as a result of a collision. These marks generally look like irregular shaped smears and are characteristic of the point of impact.
Scuff Mark: A tire mark from a wheel that is both rotating and slipping: acceleration scuffs, yaw marks, flat-tire marks.
Shoulder: A drivable area adjacent to the traveled way of a roadway.
Roll: A rotational dynamic of a vehicle, when the vehicle rotates around the x-axis.
Side Scuff Marks: See Yaw Marks.
Skidmark: A mark from a tire that is sliding without rotation.
Skip Skid Marks: A braking skid mark interrupted at frequent regular intervals. A mark made by a bouncing braked wheel.
Stopping Distance: The total distance needed to perceive, react, and brake to a stop.
Superelevation: The cross slope (banking) on a roadway curve, expressed in ft/ft or as a percentage.
Swing Out: A dynamic that occurs in a combination vehicle when the trailer being towed brakes more heavily than the towing vehicle. This imbalance in braking will cause the trailer to move right or left depending upon the forces applied to it.
Tangent: A line that touches a curve at only one point and is perpendicular to the radius at that point. A term used to describe a straight section of roadway that leads up to a curve.
Tread Depth: A measurement the tread grooves on a tire usually measured in 32nds of an inch. Tires which are at the end of their useful life usually have between 2- and 4- 32nds of an inch of tread remaining. New tires typically have between 10- and 32- 32nds of an inch. Regulatory requirements for commercial trucks require that tires have no less than 4/32nds when used on the steer axle and no less than 2/32nds of an inch when used on other axles.
Triangulation: A method of location a point on the ground by measurement from two known reference points that are spatially related.
Tripped Roll: Same as Flip.
Track Width: The distance between the center of the tire tread on one side of the vehicle and the center of the tire tread on the opposite side. For dual-tired axles, the track width is measured between the centers of each dual assembly.
Understeer: A vehicle characteristic that results in a tendency to steer toward the outside of a roadway curve, especially at higher speeds. An understeered vehicle is more comfortable for most drivers than an Oversteered vehicle.
Vault: An endwise Flip.
Vehicle Identification Number: A 17 digit number mandated by Code of Federal Regulations chapter 49 section 565. It identifies many aspects of the vehicle including the Gross Vehicle Weight Rating, the country of manufacture, the year of manufacture, and an 8 digit serial number specific to the individual vehicle.
Wheel Base: For a 2-axle vehicle, this is the distance between the center of the front wheels and the center of the rear wheels. For vehicles with tandem rear axles, the distance is measured to midway between the rear axles.
X-axis: Is a rotational axis through the vehicle's center of gravity between its front and rear bumper.
Yaw: A rotational dynamic of a vehicle when the vehicle rotates around its X-axis. The sideways movement of a vehicle is produced when the centrifugal force exceeds the traction force.
Yaw Mark: A Scuff Mark made by a yawing vehicle.
Y-axis: The rotational axis of the vehicle through its center of gravity between the right and left side.
Z-axis: The rotational axis of the vehicle through its center of gravity between its top and bottom.

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