Skid Marks and Accident Reconstruction – The Skid Mark Equation

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# Skid Marks and Accident Reconstruction – The Skid Mark Equation - PowerPoint PPT Presentation

Skid Marks and Accident Reconstruction – The Skid Mark Equation. Officer S. Chavarria #513 Chandler Police Department 602-565-8721. LG#5: Demonstrate VELOCITY and SPEED as a rate of CHANGE OF POSITION OVER TIME Slamming on the brakes locks your brakes. What forces create a skid mark.

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### Skid Marks and Accident Reconstruction – The Skid Mark Equation

Officer S. Chavarria #513

Chandler Police Department

602-565-8721

LG#5: Demonstrate VELOCITY and SPEED as a rate of CHANGE OF POSITION OVER TIMESlamming on the brakes locks your brakes. What forces create a skid mark

Skid length
• Determines the speed the vehicle was traveling
• Determine who was at fault or if traffic violations occurred
ESTIMATE SPEED
• To estimate speed when brakes were applied you will need the following:
• Measure the skid marks
• Estimate drag coefficient
• Determine braking efficiency
WHAT DO YOU NEED?

S1: SPEED

d1: Average

Distance

of Skid

Marks

f: Drag

Coefficient

STEP 1
• Measure the skid marks
• If there are several measure all of them
STEP 2
• Find the average of the distances by adding each skid and dividing the total by the number of skid marks
STEP 3
• Estimate the drag coefficient based on the surface on which the car was driving
• The drag coefficient represents how much the road contributes to the stopping car
• The lower drag coefficients represent slicker roads.
STEP 3 (cont.)
• EXAMPLES OF DRAG FACTORS:
• Asphalt: 0.50 to 0.90
• Gravel: 0.40 to 0.80
• Ice: 0.10 to 0.25
• Snow: 0.10 to 0.55
STEP 4

Estimate the speed by taking the square root of the product of 30 times the average skid distance times the drag coefficient

Measurements must be taken in feet

• Skid marks from a vehicle with anti-lock brakes will leave dashed skid mark but you should still measure from the start of the dashes