The Potentially Counterintuitive Effects of Witness Evidence

Perception response, 1 blue car, 1 red car

Witness evidence following a car accident can be a tricky thing. Some witnesses recall considerable detail, whereas others recall very little. A witness may accurately recall portions of an event, but then confuse the sequence of events or inaccurately estimate distances and times. Some witnesses may even alter or exaggerate their evidence to try to minimize or eliminate their fault. These inaccuracies or exaggerations can have significant and often counterintuitive effects when incorporated into a reconstruction of the collision/event, and an engineer can help you understand their full effect.

For instance, take a pedestrian collision. Physical evidence in these collisions can be sparse: there is often no evidence of the point of impact, and if the vehicle has anti-lock brakes, there may be no roadway evidence of braking. The pedestrian often recalls little or nothing, and thus the driver’s evidence with respect to their speed, their reactions and the pedestrian’s speed can play a major role in the collision reconstruction and their potential to have avoided the pedestrian.

Vehicle Speed

A driver who strikes a pedestrian may underestimate his actual pre-braking speed. While travelling below the speed limit may seem to benefit the driver, it can actually increase their potential to avoid a collision. The lower a vehicle’s speed, the shorter the distance it travels during the driver’s perception-response time and the shorter the distance required to stop (Figure 1). Thus a vehicle speed estimate that is too low may shift liability onto the driver if the pedestrian was visible for a long period of time before the collision.

Perception response, 1 blue car, 1 red car

Figure 1: The difference in distance travelled during perception-response time and braking to a stop for a vehicle travelling at 50 km/h (red car) and a vehicle travelling at 40 km/h (blue car)

Braking Before Rather than After Impact

Without tire marks or black box data, the only evidence with respect to braking may come from the driver. The location where braking starts, combined with the vehicle’s rest position, can be used to calculate the vehicle’s initial speed and to compare the driver’s response to that of a typical driver in a similar situation. Braking well before impact may indicate a high initial vehicle speed, and braking too close to impact or after impact may indicate an inattentive driver.

Swerving Before Rather than After the Impact

Swerving plays a larger role now that most vehicles are equipped with anti-lock brakes (ABS). An ABS-equipped vehicle can be steered even during maximum braking, and therefore its lane position at rest may not be its lane position at impact. The lane in which impact occurs will determine how far a pedestrian travelled to reach impact and how long the pedestrian was on the road. For example, if the driver swerved away from the crossing direction of a pedestrian before impact, using the vehicle’s rest position to infer its lane position at impact could exaggerate the duration a pedestrian was on the road before impact.

Evidence of a swerve before impact also establishes that the driver had time to react. If the time to impact indicates an unusually long perception-response time, then questions may arise regarding driver attention and distraction.

Pedestrian Speed

A pedestrian’s speed can be particularly difficult to establish in car-to-pedestrian collisions and the effect of a pedestrian’s speed on the collision dynamic can be counterintuitive. For example, a fast-moving pedestrian leaves a driver less time to react, but needs less time to clear the vehicle’s path. As a result, even a brief interval of braking or only a slightly lower vehicle speed may be enough for the pedestrian to clear the vehicle’s path. Conversely, a slow-moving pedestrian leaves a driver more time to brake, but needs more time to clear the car’s path. In this latter case, the driver must stop short of impact to avoid a collision. Since stopping short means that the driver must detect the pedestrian well before impact, a slow-moving pedestrian can sometimes be more difficult to avoid than a fast-moving pedestrian.

These few examples show that inaccurate or exaggerated witness evidence can have counterintuitive effects when incorporated into a reconstruction of the collision. A forensic engineer can help you understand the full effect of a witness’ evidence on their reconstruction of a particular collision.

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