4Skid 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.
5Skid MarksThis picture shows skid marks that begin just past a seam in the pavement.Does this show where the car began braking?
6ABS and Skid MarksAnti-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.
7Yaw MarksYaw 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.
8Yaw marks are always curved. There are often striations at an angle to the length of the mark.
9Tire 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.
10This 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.
11Measuring Skid MarksIf all four wheels lock at the same time, the car will skid in a straight line.The marks from the rear wheels willoverlap the marks from the front wheels.
12Skid marks from the front wheels have two thin lines on the outer edges. Rear wheel skid marks have a dark streak in the center.
13Measuring Skid MarksIf 4 marks are found, they should be measured individually.To get the average skid distance, add the four measurements together and divide by four.
14Skid Marks ContinuedIf 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.
15Drag 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 thehardest for tires to slide on?
16Examples of Drag Factor Asphalt: to 0.90Gravel: to 0.80Ice: to 0.25Snow: to 0.55
17Braking EfficiencyEach 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.
18For 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?
19You Have Arrived!!You now have the three variables required to complete the minimum skid speed formula.
20In this formula:S = speed in miles per hour30 = a constant valueD = skid distance in feetf = drag factor for the road surfacen = braking efficiency in decimal form
21ExampleA 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?
2236.7 miles per hourIt 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.
23SummaryOne 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.