RECONSTRUCTION USING VIDEO FOOTAGE
Using 3-D Computer Simulations and Analytical Techniques to accurately reconstruct and illustrate an event.
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THE CHALLENGE
How to develop an accurate reconstruction of municipal bus accidents when there are no skid-marks and little physical evidence.
THE SOLUTION
The lack of skid-marks and other physical evidence present challenges when determining point of impact, speed and acceleration. Fortunately, many of today’s buses are equipped with security cameras that record the passing of items outside the bus. These recorded videos can be used as the foundational basis for determining the factors needed to reconstruct the accident properly.
LACK OF PHYSICAL EVIDENCE
Accidents involving municipal buses often involve pedestrians or cyclists, and can be difficult to reconstruct due to the lack of physical evidence. Often there are no skid-marks, dents or scratches, which are needed to determine the velocity, acceleration, point of impact and trajectories of the involved parties. The large size of the bus, including oversized mirrors, widow pillars and other internal sight obstructions presents unique visibility challenges.
Fortunately many municipal buses are now equipped with security video cameras. While the primary purpose of these cameras is to record events inside the bus, the cameras also record what is passing by outside the bus windows. PSI has developed several techniques that can be used to create an accurate reconstruction of what occurred outside the bus, using the captured video from these cameras.
VIDEO PROCESSING OVERVIEW
First the video from the security cameras is separated into individual frames or pictures, which are enhanced digitally to improve clarity and picture quality. These still frames each depict the bus interior and exterior at a single point in time. The rate at which the cameras record the video must be determined in order to use the sequence of frames as a timer. The recording rate, or refresh rate, determines the smallest time interval that can be analyzed for the reconstruction. Typically, today’s security cameras record at a rate of 1.5Hz or 1 new picture every 2/3 of a second. Therefore, objects that pass by the bus and are visible in the video 2 frames apart occurred 2/3 x 2 = 1.33 seconds apart. This method can be used to determine the time between any event or object that is recorded by the video, such as the passing of trees or traffic signals.
SCENE MEASUREMENTS
After the individual pictures are digitized and enhanced and the recording rate is determined, the scene outside the bus during the critical time period is surveyed using a Total Station or 3D Scanning Laser. The data collected is used to create either a 2D or 3D Working Model of the scene, providing distances between objects seen passing by the bus in the security camera video.
Coupling these distances with the elapsed times determined in the previous step, it is now possible to calculate velocity and acceleration of the bus in the period prior to impact.
The working model can then be used as the basis for a compelling 3D animation of the event and illustrate what was available to be seen by the bus driver immediately prior to impact.
THE SCENARIO
The following example from a recent case illustrates how an accident can be reconstructed accurately and with proper foundation using the video from the onboard security camera.
A Los Angeles city bus collides with a bicycle rider while making a right turn in an intersection. The bike rider is crushed under the bus and suffers major injuries. The plaintiff’s attorney looked to PSI to help reconstruct the motion of the bus and create an animation of the incident. The defense alleges that the bike hit the bus and that the bus was acting in accordance with traffic rules. An accurate reconstruction is needed to determine who is at fault.
