The determination of the point of impact between the vehicle and the pedestrian can be sometimes complicated due to a lack of evidence at the scene of the accident. In many instances, the vehicle trajectory from the point of impact to point of rest is indeterminate due to a lack of evidence. The reaction of the vehicle driver to the impact may be post-impact and therefore there are no tire marks to mark an approximate area of impact.
In the instances where there are tire marks large variations in the perception/reaction times for various drivers of various ages/experience/fatigue levels/etc. make any definitive determination of the point of impact relative to the start of visible tire marks unreliable.
To demonstrate some of the variability possible in the location and extent of tire marks, a brief review of related driver perception/reaction studies is in order:
In 1974, Rice & Dell’Amico (Ref. 34) found that for a surprise intrusion (a barrel being thrown in front an unsuspecting vehicle during testing at the Calspan test facilities) that:
The Mean reaction time was 0.65 seconds ( time between ejection of the barrel and first evidence of driver avoidance action, either steering or braking) ( Range of response times was 0.40 to 1.70 seconds )
In 75 % of the cases the first reaction was pure braking
Out of 34 tests, only one individual successfully avoided the object.
In 1989, Olson (Ref. 35) reported that for “surprise” events that the perception/reaction time ranged from 0.80 to 1.8 seconds with an apparent approximate 85% percentile of 1.4 seconds.
In 1990, Araki (Ref 36) studied response times for individuals using as a test device a driving simulator. The recorded responses to a pedestrian suddenly darting across the road were that:
90% (29 out of 32 drivers) acted to avoid the pedestrian
A review and bibliography related to perception/reaction time values and studies can be found in reference 37.
Another item to consider is the meaning of tire mark evidence. The investigator must realize that the different braking systems on various vehicles may begin leaving marks at different times after the application of the brakes. For example, a common misconception is that due to the increased use of anti-lock brakes on vehicles that much of the normal pre-impact tire mark evidence will be reduced. Depending on the design of the braking system used in conjunction with the antilock braking system, the antilock brake will begin making marks in a manner quite similar to standard braking systems. However, with the anti-lock brakes the vehicle will continue to respond to steering even while full braking is applied. The pulsing action of the antilock braking system does not prevent the tires from heating up and leaving rubber deposits on the roadway during heavy braking.
Also with respect to braking, the accident reconstructionist should be aware that generally the speed of the vehicle at the time of brake application is generally as much as 10% greater than the speed of the vehicle once it begins leaving tire marks (Ref. 38) ( This if of particular importance when considering traveling speed from skid to stop calculations based on skidmark length).
Therefore, to attempt to determine the approximate point of impact in a pedestrian/vehicle impact, a detailed examination of the accident scene should be performed. The first task should be to determine if there are any “scuff” marks and/or other items of evidence to indicate the location of the initial point of impact. Shoe “scuff” marks are left when a shoe with weight on it is rapidly accelerated to the speed of the vehicle. The shoe may leave a scrape or transfer of materials at the point of impact. Likewise for pedestrians in other than standing positions, (kneeling, laying, etc.) transfers of fibers from clothing may occur on the ground as the occupant is accelerated to the velocity of the vehicle.
The path that the vehicle traveled should be carefully examined to determine if there are any items of evidence to indicate a point of impact.
Many times pedestrians may jump up in the air at the instance of impact and therefore not leave any “scuff” marks. In these instances the investigator should try to determine the probable pre-impact path of the pedestrian from witnesses to the accident and/or the scene layout.
The investigator should then locate and measure the locations of any other debris, the location of any landing areas, and the location of the final positions of rest of the pedestrian and vehicle.
The speed of a pedestrian is determined by the distance traveled, which may include a distance traveled rolling and/or sliding on the ground subsequent to landing. The speed required to “throw” vs. the speed required to “throw” and slide to a given distance may vary substantially. Therefore if there is documentable evidence to determine the actual landing area then it should be included in the investigative file for use in the reconstruction.