1. Tab 7
Speed Estimates

2. Speed Estimates Skid Marks Sliding Objects Scuff Marks Linear Momentum
Airborne
Kinetic Energy

3. Determining Slide to Stop Speed
Energy of Motion
(Kinetic Energy) =
Energy to stop
(Work Energy)
Kinetic energy = the energy needed to keep a body in motion (Newton’s 1st Law)
Work energy = the energy needed to stop it (Newton’s 3rd Law)
½ M V2 = Fd

4. Through algebraic substitution from one side of the equals sign to the other, the above formula becomes:

5. Three basic speed formulae

6. Three basic speed formulae
Speed from skid marks

7. Three basic speed formulae
Distance from speed and factor

8. Three basic speed formulae
Factor from speed and distance (test skids)

9. Traffic Template 101

10. This page will be needed many times in the next couple days
Time for another test
On a clean sheet of paper, draw a vertical line from top to bottom in the middle of the sheet
This page will be needed many times in the next couple days

11. Distance (feet)

12. Feet per Second
(Velocity)
Miles per Hour
(Speed)

13. Drag Factor or Acceleration Factor
Acceleration Rate
(FPS2)

14. How many feet per second will a vehicle travel at 30 mph?
FPS = Speed (1.47)
FPS = 30 (1.47)
FPS = 44.1
Calculate using the Template
Metric: 1 Km/hr = 1000 meters in 3600 sec.
1000 m/3600 sec = conversion factor

15. First, refer to the “SPEED” side of the center column

16. Second, using the page you made with the line, place the line horiz
Second, using the page you made with the line, place the line horiz. across the center column at 30 MPH

17. Third, on the FPS side of the center column, read the cor-responding Feet per Second opposite 30 MPH

18. 30MPH = 44 FPS

19. Use your template to calculate the answer.
If a vehicle travels 110 feet in one second, how many miles will it travel in one hour?
Use your template to calculate the answer.

20. 110 FPS = 75 MPH

21. Use your template to calculate.
What would be the safe following distance at 60 MPH, assuming a 2-second following distance?
Use your template to calculate.

22. 60 MPH = 88 FPS X 2 seconds = 176 feet

23. Use your template to calculate.
A car left 100 feet of skid marks on a dry, traveled, asphalt surface. How fast was it going when it began to brake?
Use your template to calculate.

24. First, decide what you’ll use as the drag factor,
let’s say .70
Second, place your line (horiz.) on the drag factor column at .70

25. Fourth, read the SPEED in the center column
Third, place the other end of the line on the 100-ft mark on the distance (left) column
Fourth, read the SPEED in the center column
~46 MPH
(Remember, MPH, not FPS)

26. Use your template to calculate.
A pickup skidded for 75 feet on a wet, traveled, asphalt road surface. How fast was it going when it began to brake?
Use your template to calculate.

27. Drag factor = 0.60
Speed = ~36 MPH

28. Use your template to calculate.
How far will a car traveling 30 MPH slide to a stop on a surface with a drag factor of .70?
Use your template to calculate.

29. Drag factor = 0.70
Speed = 30 MPH
Distance = 43 feet

30. Use your template to calculate.
How far will a car traveling 60 MPH slide to a stop on a surface with a drag factor of .70? (If we double the speed, will the distance double, too?)
Use your template to calculate.

31. Drag factor = 0.70
Speed = 60 MPH
Distance = 172 feet

32. If we double the speed, we quadruple the distance (so long as the factor remains equal)
Similarly, if we double the speed, the Kinetic energy quadruples. This is due to the term “v2” in the formula KE = ½ Mv2

33. If you know two out of three, you can calculate the third:
If you know the factor and the speed, you can calculate the distance.
If you know the distance and the factor, you can calculate the speed.
If you know the distance and the speed, you can calculate the factor.