ACTAR Collision Analysis & Reconstruction 4 — Questions and Answers
Question 1: In a two-vehicle in-line collision where one vehicle rear-ends another, which post-collision indicator best helps establish the struck vehicle's departure speed?
- The length of the striking vehicle's skid marks
- The struck vehicle's post-impact tire marks and point of rest (Correct answer)
- The striking vehicle's post-impact position
- The air bag deployment data from the striking vehicle
Correct answer: The struck vehicle's post-impact tire marks and point of rest
The struck vehicle's post-impact tire marks and final rest position, combined with scene drag factor, allow calculation of its post-impact departure speed.
Question 2: Which of the following best describes the 'coefficient of restitution' (e) in a collision?
- The ratio of total post-impact kinetic energy to pre-impact kinetic energy
- The ratio of relative separation speed to relative approach speed along the line of impact (Correct answer)
- The ratio of post-impact vehicle weights
- The ratio of crush depth to vehicle length
Correct answer: The ratio of relative separation speed to relative approach speed along the line of impact
The coefficient of restitution equals the relative separation velocity divided by the relative approach velocity, indicating how elastic the collision was.
Question 3: When using photogrammetry to reconstruct a collision scene, what is the primary advantage over traditional tape measurements?
- Photogrammetry does not require calibration
- It allows accurate measurements to be extracted from photographs after the scene has been cleared (Correct answer)
- It eliminates the need for a total station
- It provides drag factor measurements automatically
Correct answer: It allows accurate measurements to be extracted from photographs after the scene has been cleared
Photogrammetry enables precise measurements to be extracted from calibrated photographs long after physical evidence has been removed from the scene.
Question 4: In a T-bone intersection collision, the reconstructionist uses the 'time-distance' method. What does this analysis determine?
- The coefficient of friction at the scene
- Each driver's position and speed at critical moments before impact (Correct answer)
- The structural damage severity of each vehicle
- The post-collision trajectory of debris
Correct answer: Each driver's position and speed at critical moments before impact
Time-distance analysis plots each vehicle's position against time to determine speeds, positions, and sightlines at critical pre-crash moments.
Question 5: A vehicle slides to rest on a level surface leaving 80 feet of skid marks. If the drag factor is 0.70, what was the vehicle's speed at the beginning of the skid?
- 41 mph (Correct answer)
- 47 mph
- 52 mph
- 58 mph
Correct answer: 41 mph
Using S = √(30 × d × f) = √(30 × 80 × 0.70) = √1680 ≈ 41 mph.
Question 6: Which type of collision analysis accounts for the rotation of vehicles after impact in addition to their translational motion?
- Energy methods only
- Linear momentum analysis only
- Combined linear and angular momentum analysis (Correct answer)
- Skid-mark analysis only
Correct answer: Combined linear and angular momentum analysis
Combined linear and angular momentum analysis accounts for both translational and rotational motion that vehicles exhibit after off-center impacts.
Question 7: What is the significance of the 'engagement distance' in a collision reconstruction?
- The total stopping distance of the striking vehicle
- The distance over which the two vehicles were in contact during the collision (Correct answer)
- The distance each vehicle traveled after the collision
- The sight distance available to each driver
Correct answer: The distance over which the two vehicles were in contact during the collision
Engagement distance is the length of the contact zone between the two vehicles during collision, relevant to crush analysis and impact duration estimation.
ACTAR Collision Analysis & Reconstruction 4
In a two-vehicle in-line collision where one vehicle rear-ends another, which post-collision indicator best helps establish the struck vehicle's departure speed?