ACTAR Speed Calculations 3

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?

Accepted Answer

0.67

Answer

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?

Accepted Answer

To determine the distance the vehicle traveled before the driver began braking

Answer

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?

Accepted Answer

The ramp or departure surface angle at the point of departure

Answer

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?

Accepted Answer

Gravitational acceleration (32.2 ft/s²) divided by 2, converted to mph units

Answer

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?

Accepted Answer

Critical speed formula using the yaw mark radius

Answer

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.

(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test

(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test

ACTAR Speed Calculations 3