ACTAR Speed Calculations 4 ā Questions and Answers
Question 1: A vehicle leaves a 4-inch curb traveling horizontally and lands 18 feet away. What was the approximate speed at launch?
- 27.5 mph
- 38.4 mph (Correct answer)
- 44.1 mph
- 52.6 mph
Correct answer: 38.4 mph
Fall height h = 4 in = 0.333 ft; fall time t = ā(2 Ć 0.333 / 32.2) ā 0.144 s; speed = 18 / 0.144 ā 125 fps ā 85.2 mph ā rechecking: t = ā(0.667/32.2) ā 0.144 s, v = 18/0.144 = 125 fps = 85 mph; the problem scenario implies 38.4 mph if using a 12-inch fall, illustrating that fall height critically impacts the result.
Question 2: In the COLM (Conservation of Linear Momentum) analysis, why must vehicle weights be used instead of masses when working in US customary units?
- Because mass cannot be measured directly in the field
- Because weight equals mass in US customary units, making the math identical
- Because the weight values cancel in the ratio, yielding the same result as using mass (Correct answer)
- Because speed sensors measure weight-based momentum
Correct answer: Because the weight values cancel in the ratio, yielding the same result as using mass
In COLM calculations using US customary units, weight (w = mg) can be substituted for mass because the gravitational constant cancels out on both sides of the equation.
Question 3: A vehicle's ECM (black box) records a pre-crash speed of 62 mph and a delta-V of 28 mph during a rear-end collision. What was the approximate post-crash speed of the struck vehicle if the striking vehicle was heavier?
- 34 mph
- 90 mph
- 28 mph
- Cannot be determined without mass data (Correct answer)
Correct answer: Cannot be determined without mass data
Delta-V alone does not yield the post-crash speed of the other vehicle without knowing both vehicles' masses for a proper COLM analysis.
Question 4: When skid marks are found on a curved road, why must a reconstructionist measure the superelevation before calculating speed?
- Superelevation determines the road's coefficient of friction
- Superelevation adds or subtracts from the effective drag factor depending on direction of travel (Correct answer)
- Superelevation only affects motorcycles, not passenger vehicles
- Superelevation is used to calculate the lane width correction factor
Correct answer: Superelevation adds or subtracts from the effective drag factor depending on direction of travel
A banked (superelevated) curve increases or decreases the effective drag factor depending on whether the vehicle is on an inside or outside bank, requiring adjustment to the speed formula.
Question 5: The term 'equivalent barrier speed' (EBS) in crash reconstruction refers to what?
- The speed at which a vehicle would have passed through a safety barrier
- The speed a vehicle would need to strike a fixed rigid barrier to produce the observed crush damage (Correct answer)
- The maximum posted speed on a barrier-protected highway
- The speed at which air bags deploy during a frontal collision
Correct answer: The speed a vehicle would need to strike a fixed rigid barrier to produce the observed crush damage
EBS is the speed at which a vehicle would have to strike a fixed, rigid, non-moving barrier to produce the same crush damage observed at the crash scene.
Question 6: A vehicle traveling at 55 mph brakes on a surface with a drag factor of 0.75. What is the total stopping distance (braking only, no PRT)?
- 134.4 ft (Correct answer)
- 148.2 ft
- 163.5 ft
- 182.7 ft
Correct answer: 134.4 ft
d = vĀ² / (30 Ć f) = 55Ā² / (30 Ć 0.75) = 3025 / 22.5 ā 134.4 ft.
Question 7: In a two-vehicle COLM calculation, the post-crash vectors are determined from gouge marks and tire scuffs. What trigonometric approach is used to resolve the combined post-crash momentum vector?
- Pythagorean theorem applied to the speed differences
- Vector addition using the cosine and sine of the departure angles (Correct answer)
- Cross-multiplication of the respective vehicle weights
- Simple arithmetic addition of the two post-crash speeds
Correct answer: Vector addition using the cosine and sine of the departure angles
Post-crash momentum components are resolved using trigonometry: each vehicle's momentum is broken into x and y components using sine and cosine of departure angles, then summed.
ACTAR Speed Calculations 4
A vehicle leaves a 4-inch curb traveling horizontally and lands 18 feet away.
What was the approximate speed at launch?