What is Collision Reconstruction?

Collision reconstruction is the scientific process of investigating a collision and gathering all of the available information including such things as physical evidence, statements, injuries, and the damage to the involved vehicles, to determine the movement and relative positions and interaction of vehicles and persons before, during and after a collision event.  The detailed and complex process includes studying the human, environment and vehicle factors during all three phases of the collision.  It can, and usually does, include a quantitative analysis that follows the laws of physics to determine the impact velocities of one or more of the involved vehicles.  Impact velocities aid the Investigator in determining the relative positions of the involved parties as they move through the collision sequence.  When an accurate reconstruction has been completed, the cause of the collision can be determined with accuracy and fault may be assigned.

What are the phases of a Collision Event? 

The collision event is separated into three phases: pre-collision, at-collision and post-collision

  • Pre-collision may begin well in advance of the actual striking of a vehicle with another vehicle, object or pedestrian. This phase of the collision sequence can start seconds, minutes, hours, or even months prior to the actual impact. It continues until the initial contact with another vehicle, object or pedestrian occurs (but before any damage or injury takes place). There may be a significant pre-collision history that is a major contributing factor to the collision.  For example, a vehicle owner who knowingly allows the brakes on his car to deteriorate over a period of time to the point the brake system is nearly inoperative, yet loans the vehicle to an unsuspecting friend to tow a large boat trailer . 
  • The at-impact phase begins when the vehicle comes into physical contact with another vehicle, object or pedestrian and damage and/or injuries begin to occur.  The at-impact phase typically occurs in a very short time span (approximately 0.2 seconds) and ends when the vehicle separates from the impact.
  • The post-impact phase begins at separation (see at-impact above) when internal and external forces no longer have an influence on the damage and injuries to the involved vehicles, objects and humans and ends when the vehicle and its occupants come to rest.  Similar to the pre-collision phase, this final phase of the collision sequence can last seconds, minutes, hours, or even months after the crash occurs.

What are Human Factors?

Human factors are the actions of the driver, occupant or pedestrian involved in a collision. They include their actions before, during and after the collision event. For example, it would be important in a reconstruction to determine if the driver of an involved vehicle applied their vehicle's brakes or input steering prior to the collision. Physical evidence at the collision scene may not be demonstrative of these activities. A driver's knowledge of their vehicle's handling characteristics, mechanical condition, or roadway environment are also human factors. Additionally, any pre-collision medical conditions may be important in determining the cause of a collision. Human factors are determined by a thorough investigation that may include interviews with the involved drivers and occupants, relatives, rescue personnel and witnesses.

What are the Environmental Factors?

Environmental factors are the physical characteristics of the collision environment. Weather, roadway surface contaminants, roadway construction, grade, super elevation, sight distance restrictions and signing are all examples of environmental factors.

What are Mechanical Factors?

Mechanical factors include the physical condition of the involved vehicles and all of their systems before, during and after the collision. Knowing in detail the mechanical condition of each involved vehicle aids the investigator in thoroughly understanding how and why a vehicle reacted a certain way during a collision event. It is critical to determine what conditions were present prior to the collision versus what deficiencies were collision induced. Additionally, the investigator needs to know how the vehicle was removed from the scene to determine if tow out procedures caused or amplified an existing condition.

Can the speed of a vehicle be determined by the skid marks it leaves on the road?

Yes - but tire friction (skid) marks left by an involved vehicle are only one piece of a complicated reconstruction "puzzle."  A minimum velocity can be calculated if a vehicle is traveling at a certain velocity and its driver applies the brakes and skid marks are produced. The actual velocity at the start of braking can only be determined from skid marks alone if the vehicle comes to a complete stop without colliding with any other object.  If the vehicle impacts another vehicle or object, then even though the skid marks end, the vehicle still possessed kinetic energy. That kinetic energy must be combined with that energy converted during the skidding event to predict the velocity of the vehicle at the start of the skid marks.

Can the speed of a vehicle involved in a collision be determined from the damage it sustained?

The damage a vehicle sustains in a collision is a reflection of the severity of the collision. By analyzing the depth and location of the damage an estimate of the speed change the vehicle experienced to produce the damage pattern can be determined. The speed change experienced by the vehicle and the vehicle's impact velocity will not be the same in most collisions.

How is momentum used to estimate impact velocities?

The principle of the conservation of linear momentum can be used to predict the impact velocities because momentum in a collision system is considered to be conserved. That is, the pre-impact momentum in the system will be equal to the post-impact momentum.  If an investigator can determine the post-impact velocities of the involved vehicles, their respective entry and exit angles and their weight at the time of the collision he/she can determine the post-impact momentum of the collision system. Since the pre and post-impact momentum are equal the pre-impact momentum can be predicted and consequently, the pre-impact velocities. The ability to predict the impact velocities in this manner is largely dependent on the quality and collection of the physical evidence at the collision scene and the ability to closely examine the involved vehicles.

What is a critical speed scuff mark?

A critical speed scuff (CCS) mark is a mark left by a rotating tire that is slipping sideways and diagonally to its direction of travel. Unlike the longitudinal striations commonly observed in locked wheel tire friction marks, CCS marks have noticeable diagonal striations indicative of the direction of sideslip.  The CCS mark will generally be thinner in width than the true width of the tire and widen as the vehicle rotates from in-line to a broadside orientation.  All CCS marks are curvilinear but not all arched marks are true CCS marks. Velocity of the involved vehicle can be predicted by making specific measurements of the mark however, it is critical that the mark be interpreted as a CCS and not a mark left by a broadside vehicle or a locked wheel skid mark.

What is friction and how is it used in collision reconstruction?

Friction is defined as the resisting force to motion between two surfaces in contact. In collision reconstruction it is generally of one of three types.                                                       

  • Static: the friction when the object (tire) is just starting to slide across the roadway surface
  • Dynamic: the friction once the tire has begun to slide
  • Rolling: the resisting forces when a tire is rolling with no braking applied

The friction value between a tire tread face and a roadway surface is used to estimate the kinetic energy converted while a vehicle is accelerating or decelerating. The vehicle can be experiencing the maximum amount of friction available or if uneven braking is occurring, some percentage of the maximum.