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· Registered
532 Posts
I found this online, I think it's right. I can't really remember from math or drivers ed class, but this seems to be right. The "20" is the "think time" in case you were wondering.

This is where I found it.

x*x / 20 + x = Overall stopping distance in feet


So say you're going...30..

30*30/20+30=75 ft.

Someone let me know if I am wrong.

· Subscribing Member
4,410 Posts
bpd19, you are asking a very broad question. You need much more info to come up with the answer. Here is an example

A vehicle traveling 50 miles per hours is traveling 73.5 feet per second (1.47 X M.P.H.)
Perception time is 1.5 seconds
We multiply 73.5 X 1.5 = 110 feet
D=111 feet
Total Distance is 110 feet + 111 ft = 221 Feet

Remember f is the drag factor. I used .75 for average travelled dry pavement, that can change dramatically based on the drag factor. You have to either do test skids or use a drag sled. Also you should computer for m=grade and super-elevation to get an accurate number.

Hope this helps, recon class!!

· Registered
4,123 Posts
Sorry for the late reply...just saw the topic!

Speed in miles-per-hour must be changed to velocity in feet-per-second. This is done by multiplying speed by 1.466 (a constant...divide velocity to get speed) thus:

s(1.466)=v..............or...........speed (in miles-per-hour) multiplied by 1.466 will give you the velocity in feet per second. conversely, velocity (in feet-per-second) divided by 1.466 will give you speed in miles-per hour.

1.5 seconds is an AVERAGE perception/reaction time. This is the time it takes a person to perceive the threat and react to it, i.e., see the vehicle, understand the threat and actually apply the brakes.

If you multiply the velocity by 1.5 seconds, it will give you a distance in feet:

X f/s (1.5 s) = Y feet. ( the units of time cancel out leaving only the unit of distance).

It has been my experience using a calibrated drag sled that the deceleration factor (f) is nearer to 0.80-0.88 in real life (dry pavement, not traffic polished or new). You need to factor in the grade and superelevation (crown) of the surface...easy to do if you pull your drag sled in the direction of the skid: those two variables are then factored in 'automatically'.

Keep in mind that 1.5 seconds for the perception/reaction time is an average and differs from individual based on age, driving ability and overall health.

Now, feel free to contact the troop duty officer in your area to get the assistance of your friendly neighborhood reconstruction specialist, at no cost to you or your department!

· Chapter 90 Enforcer
3,249 Posts
Speaking of all this stopping distance... Not too long ago I went to a roll over call on the pike... The operator amazingly survived and somehow was jammed between the back seat and drivers seat.. Car rolled at least 2 times before stopping 190 feet from where she struck the guard rail (thanks to LIDAR). When I pulled on scene I was looking for bodies in the road, I thought there was no way someone could have remained in the car after something like this.. Wish I had my Camera that day.

Just thought I'd share that...

· Registered
4,123 Posts
Weird things happen in collisions...well, they only seem weird to us, I guess. It depends on how the car rolled. If you could see a tape of the crash, you'd say "...oh, so that's why they weren't ejected..." .

Anyway, the science is called biokinetics, and I don't possess sufficient knowledge (for now!) to address the subject at length (or ad-nauseum for some :D ).

I am, however, ready, willing and able to bore you to death with lengthy-geeky discussions about most reconstruction topics! :shock:

· Retired Fed, Active Special
8,899 Posts
RPD931 @ 13 Sep 2004 13:50 said:
Yeah I noticed that many of the troopers know reconstruction stuff as good as I tie my shoes. It's like basic math for them.
Actually you'd be surprised how FEW accident reconstruction certified entities are in the commonwealth. Right J809!?
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