Wet-weather Crashes/Tire Skid Marks
When investigating a traffic crash on a wet surface, police officers should pay particular attention to the pavement surface condition for tell-tale signs of pre-impact braking skid marks. As a rule, the lubrication between the pavement and the skidding tire will not create enough surface friction to heat the asphalt and melt the tar/rubber combination, so typical dark skid marks will not appear on a wet road surface.
However, skid marks may appear as lighter, fainter streaks made by the “steam cleaning” process of the skidding tires on the wet pavement. By closely watching the pavement after the road dries, these white skid marks may be observed, measured and documented. Therefore, an indication as to whether a motorist braked or not before a collision may be made.
Yaw Mark Analysis
Tire yaw marks occur when a vehicle slides sideways while still moving forward. In a true yaw, where the vehicle’s rear is attempting to pass the vehicle’s front, each rear tire tracks outside the corresponding front tire. If the yaw mark is correctly identified and measured, a speed calculation of that vehicle can be made.
Officers must clearly identify that the vehicle is in a true yaw, meaning the yaw mark, which begins narrow and widens, has diagonal striations within its width and the rear tire is tracking outside the same front tire. Only the outside front tire yaw mark should be used (which closely resembles the vehicle’s center of gravity while in a yaw). The yaw mark is curved with a decreasing radius. The chord/middle ordinate measurements must be made just beyond the outside front/rear tire mark crossover point. A 30- to 50-foot chord length measurement is recommended.
Use extreme caution when measuring the middle ordinate because a slight error may significantly affect the calculated radius. All wheels must be on the same surface. Do not use post-impact yaw marks. Avoid the use of the yaw mark speed analysis on high CG vehicles and/or when extreme weight shift occurs.
When a vehicle runs off the road and enters a shoulder or median, look for and document the characteristics of the physical evidence at the beginning point where the vehicle ran off the road. For instance, measure the angle at which the vehicle left the road. Note if tire skid marks, scuff marks or furrow marks exist, or if the tires were simply rolling on the grass. Note the total length that the vehicle traveled off the road and the manner in which it was traveling (i.e. straight forward, yawing, off-tracking, etc.)
If the vehicle re-enters the pavement, note its angle back onto the pavement and how it was traveling. Real-world crash evidence indicates that drivers who are avoiding another vehicle and an object in the road will leave the roadway at an abrupt angle (more than five degrees), whereas fatigued or drowsy drivers run off the road at gradual angles (one to four degrees). Fatigued drivers usually do not attempt evasive action such as braking or steering until after traveling relatively long distances on the shoulder and/or median. If evasive action is taken, it’s typically done in an abrupt or sudden manner.
Photography at the Scene
Good photography is the most important method of documentation of the physical evidence at the crash scene. After observing and measuring significant tire marks, gouges, debris and vehicle/occupant final resting positions, photographs should be taken showing these items. Be sure to adjust the camera/flash attachment for optimum weather and lighting conditions.
Start photographing along the outer edges of the crash site and work inward to capture all significant physical evidence. Photograph the beginnings of tire marks in the direction that they were placed on the roadway. To illustrate where the point of impact occurred in the roadway (such as a tire mark direction change or a gouge mark), photograph the mark with a tape measure placed beside it to show its size and relationship to the pavement’s centerlines, edge lines or pavement edge.
If the crash occurred during inclement weather and/or night hours, take additional photographs after the pavement has dried or during daylight hours to capture evidence that might have been missed during the initial investigation. While photographs cannot replace needed measures, they can help explain the meaning of the measurements and where they are located in relation to other significant items at the crash scene.
Safety Belt Use/Non-use
It’s imperative to determine whether or not vehicle occupants were wearing safety belts at the time of the crash. Do not simply accept motorists’ statements; attempt to verify their statements through physical evidence, if possible. In serious crashes where vehicles are significantly damaged, sufficient energy forces are often exerted on the belted occupants so that obvious physical evidence on the belt systems creates excellent indicators of safety belt use.
Closely examine the belt webbing where it routes through the “D-ring” and at the latch plate for load forces such as burn marks, stretching, discoloration or brittleness. If the belt is made with a stitched loop design or a “replace me” tag, note if the stitching has been shredded or torn apart and if the tag is pulled out of its normal position. Note the condition of the metal-like “D-ring” and sliding latch plate for burn-abrasion marks made by the webbing. These are all excellent indicators that the safety belt was being worn at the time of the crash. Likewise, a close inspection of the interior contact points with the occupants’ obvious injuries may also determine if the belt was worn during the collision.
In rollover crashes, investigators should note and match the damage to the vehicle with the contact or touchdown points on the highway/pavement surfaces. These points can yield valuable information on the placement, location and path of travel of the vehicle during its crash sequence. In some instances on level surfaces, the trailing side of a vehicle in a rollover will sustain the most surface-to-vehicle contact damages. This is due to the compression and extension of the vehicle’s tires, shock absorbers and springs during the sudden weight shift associated with a rollover, which causes the vehicle’s leading side to tuck underneath the vehicle’s mass.