Crash Scene Investigation

Activity Home Teacher's Guide Glossary Credit & Thanks


Approach Speed:  
The speed of a vehicle before impact.

Approach Angle: 
Also known as pre-collision angle.  The angle that a car was traveling prior to an accident.

Crush Evaluation:  
The degree to which a car is deformed after a crash.  The crush evaluation is expressed in inches from which the impacted area differs from the expected size of the car, as given by the manufacturer.  Example:  a particular type of car should have a front hood of 48 inches from back to front.  After a crash the hood is now only 36 inches long, giving a crush measurement of 12 inches.

Departure Angle:  
Also known as post-collision angle.  This refers to the angle at which a vehicle deviated from the direction it was going after a crash.   In calculating this angle, it is assumed that the car would have continued straight, had the accident not occurred.

Drag Factor:  
The degree of resistance to acceleration or deceleration of a moving object.  For the purposes of crash scene reconstruction, the drag factor is simply the friction of the road on tires.  Drag factor can also be referred to as friction coefficient or surface resistance factor.    

Percent Braking:    
The percent of the brakes that can be calculated to have been used during an accident.  If the driver applied fully functional brakes, this would be equivalent to 100% braking.  If the driver failed to apply the brakes at all, this would be 0% braking.  The percent braking is determined for each vehicle in an accident depending on what occurred and what is known about the crash.

Post-collision Speed:    
The speed of a vehicle after impact

A mark left on the road by some part of a car that comes in contact with the surface of the road and leaves an imprint or gouge behind.

A mark left by tires on a road, typically from rapid deceleration.  Fresh skid marks have small pieces of the tire present.

Test Skids:   
Skids performed by law enforcement after a serious accident.  The troopers or police officers will use their own vehicles on the same road in similar weather conditions to determine the drag factor of the road.  This drag factor will then be used in calculations of pre- and post-collision speeds.  In most instances, the officers are aided by an on-board computer that helps in determining the drag factor of the road where the accident took place.