The Traffic Accident Reconstruction Origin Approach Angles Solution

Solution to the Approach Angles case "Ped Skid"
This is a solution to the "ped skid" Approach Angles problem. With the information provided in the case file, I believe that I have answered the primary question posed; "Was the suspect in excess of the 30 MPH speed limit when he locked the brakes?"
The Analysis
The problem established that the vehicle in question had in fact laid down the skid marks documented in the problem. They were described as starting as shadows, progressing to a "lighter mark", and then continuing to rest as a dark mark.
Including these "shadows", or as I would call them, "impending tire marks", in the total length laid down by each tire of the vehicle does not pose a problem in the analysis. This is due to the fact that during these impending tire marks, the vehicle is at it's maximum braking efficiency (near static friction) until the tires lose traction and begin to slide on the roadway at the locked wheel coefficient. By including these impending tire marks in the total slide distance, we are still giving every consideration to the driver of the vehicle by using the locked wheel coefficient of friction over the total skid distance, obtaining a lower speed.
In order to calculate an accurate coefficient of friction for the roadway, test skids were conducted with the use of a patrol car with it's ABS braking system disabled. If I were to conduct the skid tests, I believe I would have added a third test to confirm without a doubt the results of the first two. My calculations for the tests listed were as follows.
Test #1
Test Speed = 40 M/H Left Front = 56 feet Right Front = 58 feet Left Rear = 53 feet Right Rear = 56 feet
I first took the average of the skid lengths: 
To establish the coefficient of friction 
Test Speed = 40 M/H Left Front = 53 feet Right Front = 58 feet Left Rear = 56 feet Right Rear = 57 feet
I, once again, took the average of the skid lengths 
To establish the coeficient of friction 
The first step was to calculate the negative acceleration for the vehicle as it slid to a stop.. 
Left Front = 43 feet Right Front = 47 feet Left Rear = 50 feet Right Rear = 54 feet
Next, as I did with the test skids, I averaged the length of the collision skids: 
With these values, I then calculated the velocity of the vehicle at the start of the tire marks.

With the resultant velocity of 54.57 fps, I was able to calculate the speed of the vehicle at the start of the skids 
Thus ends my solution of this problem, with the answer to the initial question: The suspect was exceeding the speed limit for the location.
Mr Kemble opened a consulting business, "Southwest Collision Reconstruction" in 1997.
He can be reached at kemble7@westworld.com
Copyright ©
