Speed and Sequence Analysis

Speed and Sequence Analysis

The most often asked question of us is to determine vehicle speeds. Determining the speeds of the vehicles is not only important just to know the speeds, but for further analysis. For example, the speeds allow us to reconstruct the pre-impact sequence leading up to a crash. The location on the roadway where the driver started to perceive and react to a hazard can be reconstructed. From this, crash avoidance potential can be explored. For example, would the crash have been avoided if the vehicle had been traveling slower when the driver perceived and reacted to the crash?

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:

  1. Vehicle masses including occupants and cargo
  2. Vehicle inbound angles and vehicle departure angles
  3. Post-impact travel distances of both vehicles
  4. Friction between the vehicles and ground surface

If the accident is a single vehicle rollover, a speed calculation can be completed in this case as well using scene information such as the tumble distance. Regardless of the type of accident, physics and/or empirical data can be used to determine a vehicle’s speed.

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.

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