1. What data might a car leave behind at the scene of an accident?

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1 Bellwork It takes 8,460 bolts to assemble an automobile, and one nut to scatter it all over the road. Author Unknown 1. What data might a car leave behind at the scene of an accident? 1

2 5 9 ACCIDENT INVESTIGATION DATA OBJECTIVES Determine the minimum skid speed using the skid mark formula. Determine the minimum skid speed using the yaw mark formula. 2

Key Terms accident reconstructionist - a person with knowledge of both crime scene investigations and the mathematics that can help to explain the circumstances surrounding the accident skid mark - a

3 Key Terms accident reconstructionist - a person with knowledge of both crime scene investigations and the mathematics that can help to explain the circumstances surrounding the accident skid mark - a mark that a tire leaves on the road when its in a locked mode shadow skid mark - when the driver first applies the brakes and the skid mark is light anti lock braking system (ABS) - a system which most cars have that does not allow the wheels to continuously lock skid distance - a function of the number and lengths of the skid marks left at the scene of an accident skid speed formula drag factor - the pull of the road on the tires; it is a number that represents the amount of friction on the road surface contributes when driving braking efficiency - a number determined by an examination of the rear and front wheel brakes (0%-100%); the number is expressed as a decimal when used in the skid speed formula 3

4 Example 1 A car is traveling on an asphalt road with a drag factor The speed limit on this portion of the road is 35 mi/h. The driver just had his car in the shop and his mechanic informed him that the brakes were operating at 100% efficiency. The driver must make an emergency stop, when he sees an obstruction in the road ahead of him. His car leaves four distinct skid marks each 80 feet in length. What is the minimum speed the car was traveling when it entered the skid? Round your answer to the nearest tenth. Was the driver exceeding the speed limit when entering the skid? 4

5 Example 2 Melissa was traveling at 50 mi/h on a concrete road with a drag factor of 1.2. Her brakes were working at 90% efficiency. To the nearest tenth of a foot, what would you expect the average length of the skid marks to be if she applied her brakes in order to come to an immediate stop? 5

6 1. What information might skid marks give an accident reconstructionist? 6

7 yaw mark - curved tire marks that indicate the vehicle was slipping sideways while at the same time continuing in a forward motion chord - the line segment that connects two points on an arc or circle middle ordinate - a segment drawn from the center of a chord to the arc of a yaw mark; this perpendicular segment is used to find the radius of a circle 7

8 EXAMPLE 3 An accident reconstructionist took measurements from yaw marks left at a scene. Using a 43 foot length chord, she determined that the middle ordinate measured approximately 4 feet. The drag factor for the road surface was determined to be 0.8. Determine the radius of the curved yaw mark to the nearest tenth of a foot. Determine the minimum speed that the car was going when the skid occurred to the nearest tenth. 8

9 CHECK YOUR UNDERSTANDING Determine the minimum speed of a car at the point the brakes are immediately applied to avoid a collision based upon a yaw mark chord measuring 62.4 feet and a middle ordinate measuring 5 feet. The drag factor of the road surface is 1.2. Round your answer to the nearest tenth. 9

10 Homework: Page , #'s 1-3, 5, 7-15, SKIP 8c 10

Estimating the Speed of a Motor Vehicle in a Collision

Journal of Criminal Law and Criminology Volume 58 Issue 1 Article 14 1967 Estimating the Speed of a Motor Vehicle in a Collision Conrad K. Rizer Follow this and additional works at: http://scholarlycommons.law.northwestern.edu/jclc