ACTAR Speed Calculations 3 — Questions and Answers
Question 1: A vehicle skids to a stop leaving marks of 80 feet on a 5% downgrade with a drag factor of 0.72. What adjusted drag factor should be used?
- 0.77
- 0.72
- 0.67 (Correct answer)
- 0.62
Correct answer: 0.67
On a downgrade, subtract the grade factor: adjusted f = 0.72 − 0.05 = 0.67.
Question 2: What is the purpose of the 'perception-reaction time' (PRT) in reconstruction speed calculations?
- To calculate the speed at which brakes were first applied
- To determine the distance the vehicle traveled before the driver began braking (Correct answer)
- To measure the total stopping distance from perception to rest
- To estimate the coefficient of friction during the reaction phase
Correct answer: To determine the distance the vehicle traveled before the driver began braking
PRT is used to calculate the distance a vehicle traveled at its pre-braking speed before the driver physically applied the brakes, which must be added to the braking distance.
Question 3: In a vault trajectory calculation, the vehicle's front wheels leave the ground first. Which variable primarily determines the launch angle for the airborne phase?
- Vehicle weight distribution
- Road surface friction
- The ramp or departure surface angle at the point of departure (Correct answer)
- Vehicle center of gravity height
Correct answer: The ramp or departure surface angle at the point of departure
The angle of the departure surface (ramp or curb) at the point where the vehicle becomes airborne determines the launch angle for projectile calculations.
Question 4: A reconstructionist uses the '30d' speed formula (v = √30df). The '30' in this formula is derived from which constants?
- Gravitational acceleration (32.2 ft/s²) divided by 2, converted to mph units (Correct answer)
- The average tire contact patch area in square inches
- The standard drag factor for dry asphalt
- The conversion factor from km/h to ft/s
Correct answer: Gravitational acceleration (32.2 ft/s²) divided by 2, converted to mph units
The constant 30 comes from g/2 in fps units (16.1) converted for use with mph speeds: 2 × 16.1 / (1.467)² ≈ 30.
Question 5: Which method is most appropriate for estimating speed when only tire scuffs (not full skid marks) are visible at the scene?
- Time-distance analysis using witness statements only
- Energy methods using crush depth and vehicle stiffness
- Critical speed formula using the yaw mark radius (Correct answer)
- Vault trajectory calculations
Correct answer: Critical speed formula using the yaw mark radius
Tire scuffs in a curved pattern are yaw marks, and the critical speed formula v = √(f × g × R) is used to determine the minimum speed that caused the yaw.
Question 6: A pedestrian is struck and their body lands 35 feet from the point of impact. Using the throw-distance formula, what is the estimated vehicle speed?
- 10.9 mph
- 20.2 mph (Correct answer)
- 30.6 mph
- 40.1 mph
Correct answer: 20.2 mph
Using v ≈ √(10.93 × D) = √(10.93 × 35) = √382.55 ≈ 19.6 mph ≈ 20.2 mph (Searle's formula approximate).
Question 7: When reconstructing a rollover, an investigator uses the critical rollover speed formula. What does the variable 't' represent in v = √(g × t / h)?
- Time of rollover in seconds
- Track width of the vehicle (Correct answer)
- Coefficient of static friction for tipping
- Total wheelbase length
Correct answer: Track width of the vehicle
In the rollover critical speed formula, 't' is the vehicle's track width (axle width), which along with center-of-gravity height 'h' determines the tipping threshold.
ACTAR Speed Calculations 3
A vehicle skids to a stop leaving marks of 80 feet on a 5% downgrade with a drag factor of 0.72.
What adjusted drag factor should be used?