‘because someone didn’t see them and pulled out in front of them.’ 

I’m going to question 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 tragic accidents. 

I want to discuss a phenomenon that is talked about, but I haven’t found an exact label used anywhere.  

But it will take me a little bit to get there, so please bear with me

Page 15 of Iowa’s Motorcycle Operator’s Manual           https://www.iowadot.gov/mvd/motorcycle/mcmanual.pdf 

talks about ‘Increasing Conspicuity’ of motorcycles, and this is kinda, sorta what I want to talk about here. 

'Traffic Parallax' is the term I want to offer. 

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. https://lco.global/spacebook/distance/parallax-and-distance-measurement/ 

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.  See: https://www.youtube.com/watch?v=sC7m3aJXzBg

3) The third kind is ‘graphics parallax’. This 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, seen oncoming traffic, and had no sensory reason to believe the vehicle was speeding, 

so I pulled out, believing there was time to safely pull into traffic. 

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 slower=moving person pulling out in front of a speeding vehicle, who calculates the on-coming vehicle is not speeding, 

when it actually is - what I call ‘traffic parallax’. Some intersections are built in such a way that promote traffic parallax. 

Flat roads seem more susceptible to parallax, because hills help reveal vehicle speeds. 

See the photo. Is road makes it difficult to spot a speeding vehicle.

And curvy roads help us better estimate the speed of oncoming vehicles than straight roads. 

 

I write this partly to protect bikers, 

but I also write this for car-motorists who are being blamed for the death of speeding bikers in traffic parallax. 

Perhaps motorcycles should have gadgets in their speedometers that record their speed upon impact.

 

If it shouldn’t be a big deal for a car to spend a little time waiting for a vehicle to pass that appears to be far away, 

then should it be a big deal for a motorcyclist (including our sons) to drive the speed limit, 

so stopped drivers can accurately estimate their oncoming speed? 

Traffic Parallax. Something to consider. 

Eric J. Rose