1. The Story Skid Marks Tell
                           

2. Tire marks are produced when a tire slides across a paved surface, leaving dark rubber particles on the pavement.
Similar to the way an eraser leaves particles on a piece of paper

3. Types of Tire Marks

4. Skid Marks When you brake suddenly, the wheels of your car lock.
You no longer have the ability to steer.
The car will slide straight ahead.
Skid marks are usually straight.
The skid mark provides evidence of the distance over which braking took place.

5. Skid Marks
This picture shows skid marks that begin just past a seam in the pavement.
Does this show where the car began braking?

6. ABS and Skid Marks
Anti-lock brake systems (ABS) prevent locking and skidding by turning the brakes on and off several times a second.
The driver can maintain steering control.
Vehicles equipped with ABS may leave faint, intermittent skid marks.
These skid marks may be in a curved path.

7. Yaw Marks
Yaw marks occur when a vehicle travels in a curved path faster than the tires can handle.
The adhesion limit of the tires is reached, so they begin to slip.
Tires “squeal” and leave marks on the pavement.

8. Yaw marks are always curved.
There are often striations at an angle to the length of the mark.

9. Tire Scrubs Tire scrubs are caused by a damaged or overloaded tire.
They happen during or immediately after a collision.
Tire scrubs are usually curved and irregular in width.
They may have striations at an angle.

10. This picture shows a straight skid mark that changes to an irregular, curved tire scrub at impact.
The dark mark is a tire scrub from a second car.

11. Measuring Skid Marks
If all four wheels lock at the same time, the car will skid in a straight line.
The marks from the rear wheels will
overlap the marks from the front wheels.

12. Skid marks from the front wheels have two thin lines on the outer edges.
Rear wheel skid marks have a dark streak in the center.

13. Measuring Skid Marks
If 4 marks are found, they should be measured individually.
To get the average skid distance, add the four measurements together and divide by four.

14. If three skid marks are found, add the three together and divide by three.
The same applies to two marks.
If only one mark is found, measure the entire length and use this as the skid distance.

15. Drag Factor Tires slide on some road surfaces easier than others.
Can you think of road conditions under which tires would slide easily?
What type of road surface would be the
hardest for tires to slide on?

16. Examples of Drag Factor
Asphalt: to 0.90
Gravel: to 0.80
Ice: to 0.25
Snow: to 0.55

17. Braking Efficiency
Each wheel on a car provides a certain amount of the total brake force available.
If all four wheels are braking evenly, then braking efficiency is 100% or 1.00.

18. For rear wheel drive cars, the brake force is 30% for each front wheel and 20% for each rear wheel.
What is the braking efficiency of a car if the rear brakes are not working at all?

19. You Have Arrived!!
You now have the three variables required to complete the minimum skid speed formula.

20. In this formula:
S = speed in miles per hour
30 = a constant value
D = skid distance in feet
f = drag factor for the road surface
n = braking efficiency in decimal form

21. Example
A car skids to a stop, leaving four skid marks with an average length of 60 feet.
The road is asphalt. Skid tests reveal a drag factor of 0.75.
Since all four wheels are braking, the braking efficiency is 100% or 1.00.
How fast was the car traveling?

22. 36.7 miles per hour
It is important to understand this is a MINIMUM speed for the car at the beginning of the skid.
Since the driver is likely to have applied the brakes before they locked and skidded, his speed was certainly greater than 36.7 mph before he started braking.

23. Summary
One of the most common tasks in accident reconstruction is the estimation of vehicle speed from measured braking skid marks.
This involves careful measurement and proper use of formulas to arrive at valid conclusions.