There are multiple ways to attract the attention of the monkey brain and there are pros & cons to each.
1. Relative movement. Our eyes were meant to follow targets moving against a background and then after recognizing what the target is our brains are pretty good at solving for size and speed. Crossing from one side to another in front of somebody is hard to miss. Coming head-on gives virtually no relative movement, so a lot of fire trucks in our area have lights which physically move in a big circle on the fronts of the trucks, in addition to all the flashing lights. The statistics show they have less accidents. The SMIDSY weave can be a form of relative movement. Unfortunately, it can also be insufficient in a crowded environment to stick out. It also can confuse drivers while taking some of the rider's attention when he should be otherwise preparing or focused on his environment and what he's doing. Pros and cons.
2. Contrast of standing out against a background. This is turning on the high beam and it works where the background doesn't already have numerous high contrast targets. For example, a headlight on a motorcycle with a simple green leafy background. It is much less successful when in the city with all the lights and perhaps some sun reflection. The other thing it does not do well is give distance information to the observer, which is what Eric was talking about.
3. Conspicuity angle is essentially the width or height of the target. Basically, you notice a bigger target because it is wider across your field of vision. A lot of Harleys have this going for them, because they mount a couple of larger lights (also adding contrast) far apart, making them look bigger. Same with bikes which have a pair of lights to the left and right of the front wheel, creating a large triangle with the headlight. The other benefit to this is that changes in the angle are easier to perceive, which provides range info to the observer when you are coming head-on. The downside is the physical space on the bike for where to mount the lights.
4. Recognition. Again, we all have monkey brains, meaning that we are programmed to recognize and avoid the face of a lion waiting for us. There was a Japanese study some years ago, in which motorcycle "faces" were judged to be animal-like or not and observers were much better at spotting the fronts of motorcycles which looked like faces. I'm not about to paint the face of Mufassa the Lion King on the front of my bike, it's just an interesting bit of trivia.
Obviously, none of these are 100%, we all need to stay on our toes, ride as if we aren't seen, and play all the other tricks, yada, yada, yada. But there ARE numerous times when we are putting our lives in the hands of strangers. When I was younger, a guy coming the other way suddenly decided to turn left in front of me, which luckily only put me in the hospital for a night. We constantly pass traffic which can pull in front of us from a side street when we have no reaction time. All we can do is try to reduce our risks as best as possible.
