Practice Test Geeks(ACTAR) Accreditation Commission for Traffic Accident Reconstructionist Practice Test
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Speed Calculations Flashcards

7 cards from real ACTAR practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

Read the first 7 Speed Calculations flashcards as text

  1. A vehicle leaves a 4-inch curb traveling horizontally and lands 18 feet away. What was the approximate speed at launch?

    Answer: 38.4 mph

    Fall height h = 4 in = 0.333 ft; fall time t = √(2 × 0.333 / 32.2) ≈ 0.144 s; speed = 18 / 0.144 ≈ 125 fps ≈ 85.2 mph — rechecking: t = √(0.667/32.2) ≈ 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.

  2. In the COLM (Conservation of Linear Momentum) analysis, why must vehicle weights be used instead of masses when working in US customary units?

    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.

  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?

    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.

  4. When skid marks are found on a curved road, why must a reconstructionist measure the superelevation before calculating speed?

    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.

  5. The term 'equivalent barrier speed' (EBS) in crash reconstruction refers to what?

    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.

  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)?

    Answer: 134.4 ft

    d = v² / (30 × f) = 55² / (30 × 0.75) = 3025 / 22.5 ≈ 134.4 ft.

  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?

    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.