Massachusetts Cop Forum banner

Calling all nerds- MATH QUESTION related to speeding

2130 Views 12 Replies 8 Participants Last post by  mpd61
Can someone tell me the formula to use to equate speed in MPH to how many feet it will take in stopping distance? Thanks in advance
1 - 2 of 13 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!
See less See more
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:
1 - 2 of 13 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.