Speed Perception (…again, Yours vs. Mine)

We’ve been talking quite a little bit about speed this past week which begged a further question.  Why is it when I ask someone if they knew how fast they were going they are frequently slower than what I confirmed their speed as with my lidar?  Am I lying?  Are they?  The answer to the first question is easy: No.  The answer to the second may surprise you: Probably not.

What?!?  MC, have you relocated your faith in the human race?

Oh, no, dear friends.  However, I am well aware that not everyone I stop is lying for the sake of lying.  It’s their perception that is off, not their moral compass.Here’s a typical scenario:

MC: Do you know how fast you were going?

Violator: 55

MC: Unfortunately (stifles chuckle), I got you at 68.

Violator: That’s impossible!  When I saw you, I looked down and I was doing 55!

Did you catch it?  There are a couple of key things hidden in the sentence above:

1. “When I saw you…” The problem with this is that by the time you’ve seen me (more often than not), I’ve already confirmed your speed, put my lidar away, and am simply waiting for you to pass me so’s I can come and make your acquaintance.

photo © 2008 Nathan | more info (via: Wylio)2. “…I looked down…” I mentioned the super-secret-squirrel math previously, but let’s take a quick gander at your speed as it relates to feet per second (fps).  68 mph is equal to 99.73 fps.  I typically lock someone in anywhere from 200′ to 1300′ away from where I may be sitting.  Let’s take the average and say I locked in the speed at 750′ away.  The odds of you seeing me from that far away are minimal.  The average time it takes to perceive and react to something while driving is 1.5 seconds.  Let’s assume you see me at 600′.  By the time you see me and react by either lifting your foot off the accelerator and/or stepping on the brake, you will have traveled 149.60′.  About now, you’ll look down at your speed.  It may very well have bled off to the 55 mph.


Consequently, I don’t think every time someone tells me their perception of how fast they were going they are lying; however, their perception is (again, more often than not) askew.

Please note: I reserve the right to delete comments that are offensive or off-topic. Snark is encouraged. Being a prat is not.

5 thoughts on “Speed Perception (…again, Yours vs. Mine)

  1. I have suspected as much. Now you have confirmed it. So basically by the time I notice you the damage is done? So are you saying I should obey speed laws even when I am not aware that there is a cop present? That’s crazy talk! 🙂

  2. I have to admit, even though I generally try to go the speed limit (none of my cars have cruise control so sometimes I drift high on the highway before noticing) every time I see an officer on the side of the road, my heart races a little, I let off the gas, and I look down then I think “wait, I wasn’t speeding anyways” and accelerate back to the speed I was traveling. Where I tend to get in trouble is that in my wife’s car, the speedometer is about about 10% fast (when the speedometer is reading 60, actual speed is about 55), but my truck is about 10% slow (when the speedometer reads 55, actual speed is 60) If I’ve been driving the car for a period of time then switch back to the truck, I’ll realize that I’ve been driving with the speedo on 60, when I should’ve been driving with it on 50.

  3. I’ll be honest. This is why I keep a person in front of me well in my sight and at approximately the same speed so I can see if they hit the brakes for no apparent reason, and if they don’t… they get bagged instead of me 🙂

    (I keep my head on a swivel and both eyes open while driving, due to the general idiocy of drivers around here I have to basically assume everyone is unaware of my presence on the road or even trying to hit me)

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