As Trace Evidence
Every traffic accident leaves some physical signs of what occurred. By studying, recording and photographing these signs, the investigator will have valuable information with which to supplement witness statements and his conclusions. Vehicle trace evidence includes debris, damage, tiremarks, scratches and gouges. All of this combined will establish the accident's chain of events.
Vehicle debris is detached material from a vehicle. It results when the vehicle strikes or is struck by something else. The primary use of vehicle debris is to determine the point of impact. In hit-and-run cases, vehicle debris can be used to help identify the suspect vehicle and help determine the vehicle's direction and actions after the impact.
Types of vehicle debris are:
Underbody debris, such as rust, dirt, mud.
Parts of the vehicle, such as chrome, glass, paint marks.
Vehicle fluids, such as water, gas, oil.
Cargo, either solid or fluid.
Road materials, such as dirt, tar, asphalt.
Body fluid from victims, such as blood.
Clothing from victims.
Damage includes, that occurring to the vehicles and to the roadway or fixed objects along the roadway. Damage can provide information concerning the chain of events, color of vehicles, damaged areas on vehicles, and may indicate the final resting position of vehicles. Vehicle damage may be either contact or induced, as discussed in Chapter 9.
Scratches and Gouges
These are masks made into the roadway by parts of the vehicle. They assist in determining the speed of the vehicle and the chain of events.
Tire marks are the marks left at the scene of an accident by the vehicles involved. They are caused by rotating or sliding tires. The table below shows the types of marks.
Tire marks are the prime means of estimating minimum speed of vehicles, and the path of vehicles before, during and after impact.
Prints--Tires leave prints on a road surface after rolling through a wet substance. This print will reveal the tire tread pattern, cuts, bald spots, etc.
Imprints--Tires rolling through soft materials such as mud, dirt or sand leave imprints.
Scuff Marks--Scuff marks are left on the road by tires that are sliding or scrubbing while the wheel is still turning. There are several types of scuffs.
Decelerating scuffs--These occur at the beginning of skidmarks after the brake has been applied enough to slow the wheels so they drag, but before the wheels are locked.
Side scuffs--These marks are left by a tire sliding sideways, without braking. Close striations are a characteristic of side scuffs. The striations are narrow, parallel, light grooves, scratches or stripes.
Combination scuffs--These are combinations of sliding with decelerating or side scuffs. They can be caused by a vehicle braking and steering at the same time, or steering and applying power at the same time.
Flat tire scuffs--These are marks left on the road by the edges of an under inflated or greatly overloaded tire. There are two distinct marks for each tire, leaving a flat tire scuff.
When a wheel is not rotating and the vehicle is in motion, the tire is sliding. There are five types of marks left by a sliding tire. These are skidmarks, yaw marks, a combination of skid and yaw marks, skip skids and gap skids.
Skid marks--These marks can appear as a straight or broken line. They occur when brakes are locked and the vehicle continues moving in the original direction. There is a distinct difference between front tire and rear tire skid marks. Front tire marks appear dark on the outer edges and light in the center; while rear tire marks are darker in the center and light on the outside edge.
Yaw marks--These are caused by a turning movement in the tires while the vehicle is still traveling in the same general direction. They are normally wider than skid or scuff marks. However, they may be narrower when just the edge of a tire is in contact with the road surface.
Combination marks--Sliding tires can show a combination of skid and yaw marks. As with all sliding marks, there is a lack of signs of steering and tread pattern.
Skip skidmarks are caused when a braking wheel jumps over a rut, bump or hole. They may also be left by rear wheels which are jumping on the springs. Skips are usually one or two feet (.4 to .5 meters) long, but may be longer if the vehicle was traveling at a high rate of speed.
Skips and Gap Skids
Gap skidmarks are caused when the pressure on the brakes is released. When pressure is reapplied, the marks continue. Gaps are rarely less than 10 feet (3 meters) in length. When measuring a skidmark with gaps, the gap lengths are not included in the total measurement.
Measuring Tire Marks
Skidmark measurements must include:
- Darker portion of the skidmarks caused by friction on pavement.
- Light gray or dark shaded area called the shadow.
- Any uncolored portion of road surface that shows scratches caused by material between the tire and road surfaces.
If photographs of the marks are needed, they should be taken first. Next, the dark portions and the shadow should be marked with chalk. The shadow is viewed by looking at the skid from a low angle, such as with your eyes approximately one foot above the ground. Measurements are then made. Skip skids are included in the measurements--gap skids are not included.
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