Traffic Parallax

This article is not really for middle-graders, but for their uncles that like to drive motorcycles real fast. 

So many times we hear of a wreck, where, 'someone didn’t see the motorcycle and pulled out in front of it.’ 

I’m going to challenge this explanation in certain cases. I have a son who loves motorcycles. 
So, I am not against motorcycles or those that ride them, but there may be another explanation for these accidents. 

I want to discuss a phenomenon that is talked about on Page 15 of Iowa’s Motorcycle Operator’s Manual: 
talks about ‘Increasing Conspicuity’ of motorcycles, and this is kinda, sorta what I want to talk about here. 

I submit that the motorcycles' small size is not always the problem, but may be what I call 'Traffic Parallax'. 
Where does the term ‘parallax’ come from? 

The process of parallax was first used in 189 BC by Greek astronomer Hipparchus. 
‘Parallax’ has three previous uses that I know of: 

1) In measuring the distance between heavenly bodies: “Parallax is the apparent displacement of an object 
    because of a change in the observer's point of view". The video below describes how this effect can be observed
    in an everyday situation, as well as how it is seen and used for finding distances to stars. 
2) Analog Parallax: The distortion that occurs when reading an analog instrument from an angle different than 
     dead-center.     Analog measuring instruments are prone to parallax, due to human error. 
3) 'Graphic Parallax' is when graphics in computer displays are manipulated to give 2-D images a 3-D effect. 
     This is done by making the objects on the screen move at different speeds from each other. 

Now, for ‘traffic parallax’. 

Many times, I have stopped at a stop sign, saw the oncoming traffic. 
Judging by my sense of traffic flow and the posted speed limit,
I had no sensory reason to believe the oncoming vehicle was speeding. 
I pulled out, believing there was time to safely pull into traffic, at the posted speed. 
Occasionally, the oncoming vehicle WAS speeding, and the problem became immediately clear. 

While I have never been in a wreck because of this, there have been a few close calls in my 50+ years of driving. 

So, the person pulling out in front of a speeding vehicle, who calculates the on-coming vehicle is not speeding, 
is caught by ‘traffic parallax’. 

Some roads lie in such a way that promotes traffic parallax. Flat roads seem more susceptible to parallax, 
because hills help reveal vehicle speeds. 
And curvy roads help us better estimate the speed of oncoming vehicles than straight roads. 
In the photo above, the other vehicle's speed would be easier to judge in the upper part of the photo, eh?

I write this partly to protect bikers, 
but I also write this for car-motorists who are inaccurately blamed for accidents, due to traffic parallax. 

Traffic Parallax. Something to consider. 

Eric J. Rose 
photo: Bruce Fingerhood, via

PS: States with motorcycle helmet laws see 1/3 fewer head-injury deaths for motorcyclists. 
       So, bikers who wear hemlets will se 1/3 fewer head-injury deaths than non-helmeted riders, hmm?

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