Analysis of a collision can involve several issues including:
- Vehicle speeds
- Impact severity (delta-V)
- Pedestrian, bicycle, and motorcycle collisions
- Mechanical failure
- Wheel failures
- Driver actions – Perception-Response Time (PRT)
- Collision sequence
- Collision avoidance
- Damage match-up
- Night-time reconstructions
- Seat belt use, benefits and air bags
- Occupant motions and direction
- Interior occupant contact zone
- Crash testing (low speed)
- Check and interpret Police analysis and reports
- Computer simulations and animations
Analyzing the crash is highly dependent on the type of crash, the data available and the answers being sought. For example, in a left-turn crash, the speed of the oncoming car is often questioned – “was it speeding”? In a highway rollover, did the tire fail and cause the rollover, or did the tire fail during the rollover? In a pedestrian collision, was the car speeding and was the pedestrian reasonably visible? These are just a few types of questions we can answer with the appropriate data.
First off, by plotting a scaled diagram of the accident site, the collision sequence can be determined and measurements taken to calculate speeds using energy and momentum methodologies. The Point of Impact (POI) is normally determined by evidence on the roadway such as impact gouges or tire marks. The damage to vehicles cannot specify a POI alone, although it may indicate whether one vehicle was out-of-control, which can then suggest a more likely scenario.
Generally, for a two vehicle crash, a momentum analysis is performed to determine the impact speeds of vehicles. Physics dictates that the momentum into a crash is equal to the momentum out of a crash such as in the case of two pool balls on a pool table. PC CrashTM software is ideal for this type of scenario, although a simple manual momentum analysis can be completed as well. For this type of analysis, the following must be known or estimated:
- Vehicle masses including occupants and cargo
- Vehicle inbound angles and vehicle departure angles
- Post-impact travel distances of both vehicles
- Friction between the vehicles and ground surface
The data from EDRs (Event Data Recorders) is becoming more detailed with each new model year. This data can provide insight into the last few seconds before a crash including wheel speed and braking. Note that EDRs only record configured information, which means that if a wheel is locked while skidding up to impact, the EDR will record a zero speed, but this is wheel speed, not vehicle speed.
The damage to vehicles is normally measured and termed “crush”. The extent of crush to each vehicle can then be compared to similar crash tests in order to quantify the crash severity, which is normally quantified in terms of energy absorbed in the crash. This type of analysis is typically based on empirical data, but a computer analysis can be completed if enough details about the vehicle structure are known. Once the amount of energy absorbed is known, it can be correlated to a speed loss or delta-V from impact, and thereby considered with other crash details to determine an impact speed.
Although rare, it is possible for a vehicle failure or defect to contribute or cause a crash. Transport Canada recalls can be checked to see if any recalls are associated with the crash circumstances. When necessary, our offices work with automotive technicians to examine the vehicles to assess whether a component failed prior to the crash, or as a result of the crash.
Visibility is a large part of collision reconstruction. While documenting a collision site, a visibility or line-of-sight assessment is undertaken. If the collision occurred on a straight and flat portion of roadway, then sightlines are probably not an issue. However, if upon our arrival at the site, a curve or hill is in the immediate vicinity of the collision area, then visibility may have been restricted. Furthermore, object lighting becomes an issue during dawn, dusk and night time conditions. In this case, a night time reconstruction may be necessary to assess the visibility under these conditions such as a pedestrian impact at night. Note that during vehicle examinations, the vehicle lamps can be examined for evidence of being on or off at impact (e.g. hot shock or cold shock).
Lastly, our offices are commonly asked to determine whether or not an occupant was seat belted. Loading marks will manifest on seat belt hardware for a belted occupant if the collision is of sufficient severity and the occupant is of sufficient mass. In this case, by closely examining the seat belt hardware, the condition of the vehicle interior (for occupant damage) and considering the occupant’s injuries, we can determine whether an occupant was seat belted or not.