ACTAR Collision Analysis & Reconstruction 5 — Questions and Answers
Question 1: In reconstruction of a run-off-road rollover, the 'trip mechanism' most commonly involves which of the following?
- Tire blowout causing sudden loss of steering
- The tire contacting a curb, soft soil, or other tripping element that induces roll (Correct answer)
- Excessive braking causing rear-wheel lockup
- Overcorrection by the driver without any roadway feature involved
Correct answer: The tire contacting a curb, soft soil, or other tripping element that induces roll
The most common rollover trip mechanism is the tire encountering a curb, soft shoulder, or similar tripping feature that redirects the vehicle's lower structure while inertia continues it laterally.
Question 2: A reconstructionist determines two vehicles' post-impact velocities using momentum analysis. What additional step is needed to find their pre-impact speeds?
- Measure the total crush depth of each vehicle
- Add back each vehicle's energy loss during the collision using the coefficient of restitution or crush energy (Correct answer)
- Measure the length of post-impact tire marks only
- No additional step is needed; post-impact and pre-impact speeds are equal
Correct answer: Add back each vehicle's energy loss during the collision using the coefficient of restitution or crush energy
Pre-impact speeds require accounting for energy dissipated during the collision, which is done using crush energy or the coefficient of restitution applied to post-impact velocities.
Question 3: Which scene documentation method is best suited for capturing three-dimensional spatial data of a large crash scene rapidly and accurately?
- Hand sketching with tape measure
- Total station survey
- 3D laser scanning (LiDAR) (Correct answer)
- Aerial drone photography without ground control points
Correct answer: 3D laser scanning (LiDAR)
3D laser scanning (LiDAR) captures millions of spatial data points rapidly, providing a highly accurate, georeferenced three-dimensional record of the entire crash scene.
Question 4: When analyzing a pedestrian throw distance to estimate vehicle speed, which formula is most commonly referenced?
- Searle's formula using pedestrian throw distance and drag factor (Correct answer)
- The skid-speed formula S = √(30df)
- The critical-speed scuff formula v = √(grf)
- The momentum formula p = mv
Correct answer: Searle's formula using pedestrian throw distance and drag factor
Searle's formula (and related variants) correlates vehicle pre-impact speed to the pedestrian's throw distance and the friction between pedestrian and road surface.
Question 5: An adjustable drag sled test on the crash scene pavement returns values of 0.68, 0.71, and 0.73. What value should the reconstructionist use for skid-speed calculations if conservatism favoring the driver is required?
- The highest value, 0.73
- The average value, 0.707
- The lowest value, 0.68 (Correct answer)
- The median value, 0.71
Correct answer: The lowest value, 0.68
Using the lowest drag factor value is conservative in the driver's favor because it yields the lowest calculated speed for a given skid distance.
Question 6: What does 'delta-V' represent in the context of a vehicle collision?
- The difference in vehicle weights
- The change in velocity experienced by a vehicle during the collision (Correct answer)
- The maximum speed before braking
- The angle between two vehicles at impact
Correct answer: The change in velocity experienced by a vehicle during the collision
Delta-V is the vector change in a vehicle's velocity from just before to just after the collision, representing the severity of the crash experienced by that vehicle.
Question 7: In a reconstruction involving a vehicle that vaulted off a roadway embankment, which projectile motion parameter must be measured from the scene to calculate launch speed?
- The vehicle's weight distribution
- Horizontal range and vertical drop of the vault trajectory (Correct answer)
- The coefficient of restitution at landing
- The tire pressure at the time of the crash
Correct answer: Horizontal range and vertical drop of the vault trajectory
Horizontal range and vertical drop from the launch point to the landing point are the essential measurements for applying projectile motion equations to calculate launch speed.
ACTAR Collision Analysis & Reconstruction 5
In reconstruction of a run-off-road rollover, the 'trip mechanism' most commonly involves which of the following?