The first part of this article may be simplistic but hang in there for a while. When you are driving on patrol, you are managing time and distance. We measure time and distance using the car’s speedometer, which indicates speed measured in miles per hour (mph), the time it takes to cover a given distance. It’s a natural unit of reference that everyone is familiar with in driving discussions. But for Emergency Vehicle Operations Course (EVOC), training may not be the best reference for measuring time and distance. When driving through an exercise, the student does not have a mile or an hour to decide. Stuff happens in feet and seconds. It would be a good skill to have to convert MPH to Feet/Sec (FPS).
CONVERTING MPH TO FPS
If you are moving at the rate of 40MPH, how many feet do you travel in a second? At 40 miles per hour, you are traveling 58.8 feet per second. To convert miles per hour to feet per second, multiply the miles per hour figure by 1.47. There are 5280 Feet in a mile and 3600 Seconds in an hour. If you divide 5280 Feet by 3600 Seconds (5280 FT /3600 SEC.), you get 1.47.
30 mph = 44.1 ft/sec
40 mph = 58.8 ft/sec
50 mph = 78.5 ft/sec
60 mph = 88.2 ft/sec
If you don’t mind being off a bit in your calculations or would like to explain this concept to your students but don’t feel like multiplying by 1.47 every time you want to convert mph to fps, there is an easy way of doing all this. The first thing you do is round off 1.47 to 1.5. That’s a mathematically legal maneuver. Now to change MPH to FPS, multiply by 1.5 instead of 1.47. If you want to multiply by 1.5, you do it by adding. If you multiply 20 by 1.5, you take half of 20, which is 10, and add it to 20, which will give you 30. (Half of 20 = 10 and 20 + 10 is 30) so if you were going 20 mph, you would be moving at the rate of approximately 30 fps.
If your student is traveling:
40 MPH, half of 40 is 20, 40 + 20 = 60 they are moving 60 FPS
50 MPH, half of 50 is 25, 50 + 25 = 75 they are moving 75 FPS
60 MPH, half of 60 is 30, 60 + 30 = 90 they are moving 90 FPS
PRACTICAL USE OF FPS – Routine Patrol
You’re on a routine patrol, driving along at 40 mph (or 60 fps). Something causes you to look away from the road for three seconds. Simultaneously, another driver starts to cross an intersection 300 feet (the length of a football field) in front of you. Since your attention was diverted for three seconds and traveling at 60 fps, you drove a total of 180 feet (3 seconds x 60 fps) without looking where you were going. This puts you 120 feet (300 feet – 180 feet) from the intersection. At this point, you look forward again, see a car blocking the intersection and realize that if you don’t do something quickly, life is about to become terribly exciting.
You’re 120 feet in front of the conflicting traffic and closing on them at 60 fps. If you can get your foot on the brake in half of a second, that half of a second represents about 30 feet (half of 60 fps). So at the point of applying your brakes, you are about 90 feet (120 – 30) from the traffic, still doing 40 mph (60 fps). You have just applied your brakes. Can you stop in time?
At this point, avoiding a collision would depend more on luck than skill. The scenario above assumed that you instantaneously recognized the problem and went for the brake pedal. What if it took you a whopping one second to figure out what was going on and then reach for the brake pedal? Taking that second puts you 90 feet minus the one-second decision time of 60 feet, which places you approximately 30 feet in front of the problem going 60 feet a second. This is going to hurt.
Look at the following table:
One sec Two sec Three sec
30 mph 44.1 Ft 88.2 Ft 132.3 Ft
40 mph 58.8 Ft 117.6 Ft 176.4 Ft
50 mph 73.5 Ft 147 Ft 220.5 Ft
60 mph 88.2 Ft 176.4 Ft 264.6 Ft
PRACTICAL USE OF FPS – EVOC
Using FPS in a training situation can prove to be most valuable. Look at one of the staple exercises used in most EVOC programs, the Slalom course. If you have four cones separated by 60 feet, and the student is traveling 30 MPH, which is 45 FPS, the student has approximately 1.3 seconds to make a decision and get around each cone. If the student raises the speed to 35 MPH, which is 52.5 FPS, they now have approximately 1.1 seconds to make a decision and get around each cone, which is significantly more difficult. The difference in decision time between cones driving at 30 and then at 35 MPH is approximately .2 seconds. At 40 MPH, which is 60 FPS and 1 second of decision time between cones, it is challenging and not doable in most vehicles.
Training can lower the time to make a decision, and you can measure it. Let’s take the above-mentioned slalom courses. You have a student that hits cones at 30 mph, and you get the student to drive through at 35 mph without hitting cones. You have to decrease their time to decide by .2 seconds. Not much, you may say? Two-tenths (.2) of second at 35 mph is 10.5 feet. How many of you reading this article have avoided a bad accident by less than a few feet?
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