Visibility on a Motorcycle. Z-motion, X-motion, and why cars pull out in front of us.

Visibility on a Motorcycle

Anybody who has ridden a motorcycle for any appreciable amount of time knows the all-too-familiar feeling that we seem to be invisible on the road.

When the umpteenth person pulls across our path or pushes us out of our lane, it’s tempting (and quite natural) to get very annoyed at the distracted and inattentive driver.

In this post, I hope to provide an insight into the psychology of visibility, and offer some tips which can help.

Z-Motion

The human eye is most sensitive to movement that crosses its field of vision (right to left, or left to right). This is know as X-motion, and the eye is very good at detecting it.

On the other hand, movement directly towards the eye (Z-motion) is much harder to detect. In fact, the eye usually only detects such motion when the object in their field of vision presents some x-motion by taking up much more space in their view – and so presenting x-motion by virtue of the fact that it “grows” quickly in relation to the background.

Are you beginning to see the issue that us riders have to deal with?

Compared to a larger vehicle, we present little-to-no X-motion when approaching a vehicle. A larger vehicle, such as a car or truck “grows” in a person’s vision at a much faster—and linear—rate. It presents much more horizontal movement in relation to the background – even when coming straight towards the viewer.

Try this experiment when you are next at the side of a road: Watch a car coming towards you. You will see how it’s “growth” in your vision is quite linear as it comes towards you. It steadily grows and grows until it reaches the point where you are standing.
Now, watch a motorcycle or a scooter. You will find that its apparent size in your vision remains small, small, small, right up until it is very near to you, when it suddenly “grows” at an exponential rate.

“I’m so sorry! I just didnt’ see you!”

This is precisely why many drivers will pull out in front of us, then suddenly slam on their brakes at the last second when they finally see us.
Unlike a larger vehicle, as we approach a driver, say, at a junction, we present very little for the driver to see right up until the last moment when we “grow” exponentially in their vision.

So much for the theory. What can we do about it?

Well, it stands to reason that, having identified the root of the problem – Z-motion (or lack of X-motion), the way to deal with it is to do anything that presents more X-motion to the driver.
In a later post, I will be talking about the “SMIDSY” (sorry mate, I didn’t see you), and the “SAM” (the SMIDSY avoidance manoeuvre), but here I will restrict myself to a very simple and common-sense tactic related to your position in the road.

Take at look at the diagram below:

Visibility on a Motorcycle

The nearer you are to presenting a straight-on movement to the driver, the less X-motion you are presenting. The harder you will be to spot. By riding in the right of your lane, you are ensuring that you are riding straight towards the driver’s vision. You are presenting mostly undesirable Z-motion.

Now take a look at the second diagram. The rider has moved to the left of the lane.

Motorcycle Visibility

Just by virtue of the fact that the rider has moved to the left of the lane, he/she is presenting much more X-motion to the driver. There is a greater angle between the driver, and the rider. As the scooter continues towards the driver’s position, much more X-motion will be introduced to the driver’s vision.

Further, by the very act of shifting to the left of the lane, the rider presented X-motion while doing so:

Motorcycle Visibility

The other advantage to this is that you have also created a “buffer zone” between you and the car, should the driver pull out.
Once you have passed the hazard, you can smoothly resume your commanding position in the middle of your lane.

Passive Tactics

I see lots written about various tactics to increase your visibility to drivers – ranging from riding with high-beam on (seriously??), headlight modulators, to high-visibility clothing.

Of all these, my opinion is that the most useful is to wear bright “dayglo” clothing, such as the one below:

High visibility jacket

I would also highly recommend considering a bright helmet such as that shown below.

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You really can see drivers doing a “double-take” when you are wearing such clothing. It’s not exactly a fashion statement, but I’d rather be seen than fashionable.

So, next time you are out riding, give a little thought to the whole z-motion/x-motion thing. At the very least, it should help you to understand that the drivers who pull out in front of you are mostly not being vindictive or “out to get you”. They are simply displaying basic human physiology.

One More Point

How can you help to ensure you don’t pull out in front of a scooter or motorcycle when driving your car?
Do your best owl impression! Have you noticed how an owl moves it’s head from side-to-side when scanning it’s field of view? It is introducing X-motion.
As you’re waiting at a junction and looking for oncoming traffic, consider moving your head from side to side. This will introduce more x-motion into your field of vision, and you may just spot that rider hiding in the Z-zone.

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4 thoughts on “Visibility on a Motorcycle. Z-motion, X-motion, and why cars pull out in front of us.

  1. Having just survived a scooter crash where a driver pulled a dreaded left hand turn into my lane, I found this article useful. One thing I’ve gone over in reconstructing the event, and how to avoid the situation in the future, is the affect of riding at night (twilight in my case) where the driver could only gauge my approaching headlight as a way of measuring the distance between him and myself.

    Love the X/Z motion technique, but wonder if it is as affective at night, or will the “[it] remains small, small, small, right up until it is very near to you, when it suddenly “grows at an exponential rate” phenomena be applied here as well?

    My former bike was a Kymco Yager GT 200i sports scooter. It had a single headlight, fore and back disc brakes and adequate reflectors along the front/side and back. Since the accident I have moved up to the Kymco Downtown with dual headlights which are more powerful. I changed my helmet from a full face modular, to a full face modular with a LED light on the back with three settings. I have an armored riding jacket with a highly visible reflective strip running vertically on the front and back. Unfortunately, I wasn’t wearing it at the time of my crash. I was making a fast food dinner run to a place that was an exact mile (I’ve jogged the route many times) from my home in a residential neighborhood. Since then I’ve made armored night riding mandatory for myself.

    In my case the driver who turned in front of me was traveling down road in the opposite direction as I when he made his turn onto another residential street that was perpendicular to the road we were on. Since the crash I’ve begun to scan for what I call ‘points of intersection’ where I and another vehicle traveling in the opposing direction will pass one another and is it a point with a potential turning lane — driveway, street, parking lot, etc.

    My only worry with this idea is will it force me to fixate on theses points when I should be focused on the road — 12, 8, and 2 seconds ahead of me?

    -Wolf

    1. Hi Wolf. I’m glad you came out relatively unscathed from your crash. It sounds like you’re approaching this in the right way. I would think that X-motion/Z-motion comes into play at night as well, although, in your case, it sounds like the person would have turned no matter what. It’s interesting that your crash occurred at dusk. This is definitely the worst time. You don’t have the full effect of your lights yet, but contrast has dropped considerably, and colours have become muted. I always try to take extra care at dusk and dawn, for this reason.

      The fact that you are concerned about fixating on those points of intersection, shows again that you’re riding with a good outlook. If you haven’t yet, you may wish to read my post on Scanning and Hazard Fixation.

      Thanks for the comment, and be safe!

  2. Thanks , Alan. I’ll defiantly take a look. I recently returned to posting on the Kymcoforum site where I’m proud to have created a Road Craft section many years ago. Drop in if you get a chance. I’m sure our members could benefit from any of your suggestions.

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