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It is generally known that a vehicle's speed can be determined from skid marks. But whenever you hear a traffic accident investigator or reconstructionist talk about speed from skid marks, they use the term "minimum" or "at least" when quantifying the calculated speed. There is a reason for this. It is very likely the police officer that investigated the accident scene, while all the evidence was fresh and in place, could not see the entire length of the actual skid or the skid mark he could see does not reveal the entire distance the car traveled while the brakes were being applied. In short, the car was braking for a distance greater than the visible skid distance and the calculated speed from the skid marks must be lower than the speed the car was really going when the driver applied the brakes. You may hear terms like "impending skid, "shadow skid" or "visible skid" in conjunction with the minimum skid speed terms. These descriptions of skid marks are important. For the case at hand, assume a car is proceeding along a four lane, undivided road in an industrial area. The road surface is a concrete asphalt mix. The road is level, no major surface defects and dry at the time of the accident. As the car was proceeding in the right lane, a bicyclist entered the road from a driveway to the driver's right. The driver saw the bicycle entering his lane of travel and applied the brakes hard in an attempt to avoid a collision. Unfortunately, the car was unable to stop in the available distance and a collision occurs. The speed limit in the area is 35 mph. The bicyclist claims the driver was going well in excess of the speed limit. The driver says he was not speeding. The police are called and an investigation is conducted of the scene. The investigating officer is presented with a scene as in Photo 1. The car is at the position of rest after striking the bicycle. Skid marks are visible from the car going into the collision. They can be clearly seen leading up to the wheels of the car. The officer measures each of the skid marks. Based on the length of the skid marks, he uses the basic skid speed formula, after having conducted skid tests to determine a drag factor, and reports he has calculated a speed of 34.5 mph for the car when the skid begins. (For a more detailed article on the basic skid speed formula, click here.)
Photo 1 In Photo 1, you can clearly see the skid marks begin just past a seam in the pavement. To the left is a skip center line dividing the two right lanes of the four lane road. The officer has measured the skid marks as you see them in this photo. But is this really where the car began braking? If the distance the car traveled while the brakes were being applied, wouldn't the speed be higher than the distance measured by the police officer? Yes. A skid mark does not necessarily show the entire "braking distance," that is the full distance the car traveled while the brakes were slowing the car down. There very often is a distance of "shadow" or "impending" skid where the car is slowing but the wheels have not completely stopped rotating or the tires are not yet leaving a visible mark. To see this, examine Photo 2.
Photo 2 Photo 2 is the same as Photo 1. For Photo 1, the tape measure, orange cones and a chalk shot mark were digitally removed. This was done to present the skid marks as they are normally found in accident scenes. In reality, the skid marks were produced as part of a skid test with the vehicle equipped with a chalk bumper gun. With this device, we can see where the car was when the brakes were first applied and where the skid marks become visible. In Photo 2, in the lower left corner, is the end of a tape measure. To the right of this tape measure, and slightly past the end, there is a bright, orange chalk spot. Orange traffic cones have been placed at ten foot intervals, beginning with end of the tape measure nearest the bottom of the photo. Two orange lines to the left of the tape measure show the positions of each axle when the brakes were applied. The chalk mark was produced by the chalk bumper gun. The gun hangs on the side of the test car and fires a small piece of chalk to the ground when a pressure switch on the brake pedal is depressed as the brakes are applied. This provides a reference mark on the road, that can be measured to the gun's position on the car and the actual braking, or skid, distance can be measured. Notice the visible skid marks do not become visible for a considerable distance beyond the point where the brakes were applied. The end of the tape measure is set to the position of the rear wheels at the moment the brakes were applied while the chalk bumper gun was near the center of the car.
Photo 3 Photo 3 is of the car with the bumper gun installed. The bumper gun is just behind the driver's door.
Photo 4 Photo 4 is the bumper gun. A .25 caliber rifle chalk load cartridge is held in a metal block just below the center of the device. The "hammer" is released by a solenoid inside the blue housing. The wires provide a ground, connect to the vehicle for electrical power and to the pressure switch attached to the brake pedal. At the instant the driver's foot depresses the brake pedal, the chalk cartridge is fired, leaving a distinct mark on the pavement as seen in Photo 2. What happens during braking is that the wheels do not lock up instantly. There is a short time period, even during a panic brake application, when the wheels are slowing but not quite locked. And it takes time for the contact patches of the tire to achieve a high enough temperature to begin leaving a visible skid. This time period, or distance, varies according to the type of road surface and sometimes to the composition of the tires. During this period, the car is slowing, just not leaving a distinct skid mark that can be found by the investigating officer. The on-scene investigator must work with what he can see, that is, the visible skid mark. Over the distance of the shadow or impending skid, the car is losing speed. Since the amount of speed lost during this period cannot be readily quantified at the accident scene, the calculations to determine a speed from the skid marks is a minimum speed. This is to say the car was going at least this fast since it could not have left a skid mark of this length had it been going slower. It almost certainly was going faster than this. By performing skid tests at the scene, resulting in visible skid marks of the same length as found at the accident scene, and using a chalk bumper gun to record the actual skid distance and speed during the tests, it can be shown with great certainty how fast the car was actually going to leave visible skid marks of that length. If you have a case where the calculated speed from skid marks is at issue, contact Harris Technical Services. We can perform a skid speed demonstration for use at trial with photographs and videotape as part of the reconstruction. You can see a skid speed demonstration video on-line by clicking here. |
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