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  1. #1
    Speed Demon
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    Jul 2005
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    Default Vascar calculation?

    How do I do it? I see a road next to my window here at college (lots of cars go by) and I want to see how fast they're going.

    I see a light post and a speed limit sign. I can use that as the measured distance.

    What's a good way to measure the distance, other than using my car? When I measure the distance, what units should it be in? Feet?

    Do I start the stop watch when the FRONT of the car hits the light post and then stop the watch when the FRONT of the car hits the speed limit sign?

    I'll have a time (let's say 2.34 seconds) and then what do I do?

    Thanks.

  2. #2
    Old Timer
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    Default

    Speed is calculated in distance traveled divided by time elapsed (for cars, usually in miles per hour). If you have two reference points and you know the distance between them (for the sake of argument, let's say your objects are 300 feet apart), you'd do the following:

    1. Start the stopwatch when the front of the car reaches the first object. Stop the watch when the front of the car reaches the second object. Always start and stop when the same part of the car reaches the object (e.g. front of car). You could use the back of the car, antenna, or whatever, as long as you use the same reference point on the car for both starting and stopping the watch.

    2. The elapsed time is how many seconds it took to cross the indicated distance (300 feet in my example). If the car took 5 seconds, he went 300/5 = 60 feet per second.

    3. Convert feet/second to miles/hour as follows. 5280 feet in a mile, 3600 seconds in an hour, so divide 5280/3600 = 1.47. Divide feet/second by this number to get mph. 60 / 1.47 = 40.9 mph.

    4. To convert directly from time in seconds to mph, given the distance in feet is known, divide the distance by the 1.47 we came up with above. 300 / 1.47 = 204 (approx). Divide this number (204) by the number of seconds to get speed in mph (204 / 5 = 40.8 mph).

    It may seem a bit convoluted, but I'm a bit rusty in algebra (graduated HS 20 years ago, I don't do it very often)
    If I'm passing you on the right, YOU are in the wrong lane!

    If speed kills, how come I'm still alive?

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  3. #3
    Speed Demon
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    Default

    I wonder if there's any computer programs or websites that will do it.

  4. #4
    Old Timer
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    Default

    Here's a website: http://www.gazza.co.nz/distance.html

    The bottom-most calculator figures out speed based on distance/time, in various units (feet, meters, miles, seconds, etc.) It doesn't do furlongs per fortnight though.
    If I'm passing you on the right, YOU are in the wrong lane!

    If speed kills, how come I'm still alive?

    Active Countermeasures: V1 3.858, Escort Redline, Beltronics STi-R+, LI Dual 7.1x CPU/8.7 Heads (front)
    Other/Backup Countermeasures: V1 3.813 (loaned to friend), Beltronics Pro RX65 M4 6.3
    Vehicle: 2002 Audi A4 1.8T Quattro
    LEO Toys: Kustom Pro Laser II & III
    Encounters/Saves August 2011: Radar 3/1, Laser 0/0


  5. #5
    Yoda of Radar
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    NY
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    Default

    As stated just use two objects as a car passes by them....
    Make sure you start/stop it with the same part of the car...doesnt matter where as long as it is the same referance point on the car otherwise your time will be off.
    I found the best thing for giving the distance is: The little wheels attached to the pole that have a digital readout by the handle, and every 1/4, 1/2, or full revolution (depeding on how good it is) it equates to a certain distance. Think of it as: 1 revolution of the wheel it covers 'X' distance and the little device automatically calclates distance by:'X' times Revoltions to give you your foot/yard etc etc etc readout (they have all different sized ones). A lot of bio teachers have this for marking the distance between two species, or contruction/engineering courses will have this for obvious purposes.

    2 Problems:
    1) If the objects are at (different) angles compared to your POV your time will be off. So you cant have that.
    2) Using a small distance is a very bad thing to do...make it as long as you can. (because if you get trigger happy with the stopwatch and stop it 1/2 second sooner it will have a huge impact on your final numbers).
    (Most LEOS when using Air enforcement like to use 1/4mile markers, because most of the time [pending extremely high speeds] .25 or .50 seconds would not have a MAJOR impact on the readout.)

    Think about it if you are off by .5seconds in a 1/4 mile (1320ft) thats not huge.
    Now take that same .50 seconds and plug it into 1/18th of a mile (Approx 295ft). OR LESS and you have a some pretty big problems coming from .50seconds.

    (Because .50seconds will be a very small fraction of the total time in seconds it took a person to go a 1/4mile.....however it will be a much larger fraction of the time it took them to go 1/18th of a mile so it will skew the MPH calculation by much more.)

  6. #6
    Speed Demon
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    New York
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    Quote Originally Posted by kpatz
    Here's a website: http://www.gazza.co.nz/distance.html

    The bottom-most calculator figures out speed based on distance/time, in various units (feet, meters, miles, seconds, etc.) It doesn't do furlongs per fortnight though.
    Do I use statute mile per hour?

  7. #7
    Yoda of Radar
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    Could use anything you want. Speed limits on land are in statute miles

  8. #8
    Yoda of Radar
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    Easy post to anwser. First pick a place you want to start to measure as your start. Grab a stopwatch and drive the speedlimit constantly for .25 miles. see how long it takes on your stopwatch. Write down that time. Then at the .25 mile mark that spot some how, so you can see if .25 away (at the begining).

    You wont really know the exact speed of the vehicles going too slow or too fast but you will have an idea if they are going over or under the limit. In comparason to how long it took you to drive the .25 mile distance/ time.


    Hope this helped. This is the elchepo way!

    Stlouisx50
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