The Traffic Accident Reconstruction Origin -Approach Angles Solution-

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Pedskid Problem

This hypothetical collision involves a 1983 Chevrolet Caprice. The collision vehicle has struck a child on a residential street. The child is seriously injured. The major question that has emerged is how fast was the car going.

As the investigating officer you can see four skid marks in the roadway. You are convinced that the skids you are looking at came from the Caprice in question. The following facts lead you to that conclusion:

  • All four skid marks lead back to the rest location of the Caprice.
  • All tires, but more noticeably the front tires, show signs of speckling on their treadface.
  • As the car is moved away you look at the tread face that was on the bottom of the tire at rest. As it rotates off the ground you see an elliptical area that is darker than the remainder of the tire. This area also shows lighter scratch like marks oriented front to back (or back to front) within this spot. You recognize this place on the tire as a flat spot.

In an attempt to document the length of these skid marks you back away from the car and look down its path. Backing away you can see the marks change from their dark character to a lighter mark, then disappear. You conclude that the skid marks begin as shadows, but it is difficult to tell just exactly where they begin.

You ask your assistant to stand near the end of the shadows and you continue to walk back away from the car. Twenty or thirty feet farther you stop turn around and kneel down to the ground. From this distant, low viewpoint the obvious tire marks are even more easy to see. But, more importantly, it is now easy for you to see all four skids and where the shadow marks begin. You direct your assistant to place his foot beside the beginning of all the marks. Of course it is not as easy for him to see where they begin, so you have to direct him. Once he has his foot in the right place you tell him OK, and he marks the spot.

Now it is easy for you to measure all four of the skids. Here are the lengths you measure:
Left Front = 43 feet Right Front = 47 feet
Left Rear = 50 feet Right Rear = 54 feet

Now you must find a drag factor. The serious nature of the injuries is enough to convince your supervisor that estimating a drag factor from a table is not good enough. You want an accurate drag factor for this road on this day.

You run your skid tests as follows:

You disable the ABS system of your patrol car by pulling the fuse from the fuse panel. You are happy that your speedometer has been recently calibrated because you know that drag factor calculated form skid tests requires an accurately known speed. You accelerate up to 45 MPH. You take your foot off the accelerator and poise both feet over the brake pedal. You make one last check through the windshield to be sure the car is pointed in a direction you want it to go. When you see the speedometer reach 40 MPH you slam on the brakes with both feet. Your patrol car skids to a halt in a cloud of smoke.

In a manner similar to the one you used to find the shadow and skid length of your suspects vehicle you mark and measure the lengths of each of the patrol cars skid marks here are the recorded lengths:
Left Front = 56 feet Right Front = 58 feet
Left Rear = 53 feet Right Rear = 56 feet

So as to provide confidence in your first test result you run a second test. This second test, also run at 40 MPH yields the following results:
Left Front = 53 feet Right Front = 58 feet
Left Rear = 56 feet Right Rear = 57 feet

Was the suspect in excess of the 30 MPH speed limit when he locked the brakes?

Approach Angle Solution

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