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Comment by nonrandomstring | Hacker News Reader

My take is that this is about parallax [1], although I haven't read the paper and so that's an assumption.

Creatures with forward facing stereoscopic vision (like us) can judge the speed and distance of an object if we can determine its size and see something of the background. In the dark, some information is missing when we rely on attached lights.

Most people know the approximate size of a car, and so with two lights its easier to guess the distance (and crucially speed as the distance between the lights seems to change).

I've noticed it's harder to cross a road when bikes are approaching than cars, because if you're looking along the axis of travel then its hard to gauge their speed.

Attaching more than one light to a bike allows us to judge the speed better. It seems that it's irrelevant whether the spacing is horizontal or vertical for our visual systems to make this judgement.

[1] https://en.wikipedia.org/wiki/Parallax

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This research, & further your comment, makes me wonder:

Customized cars often project vanity mood lighting down towards the pavement. (I believe the legality of such nonstandard lighting varies across jurisdictions.)

Perhaps motorcycles could be required to project a large light-splash down, creating a more-visible 'light puddle' below them. It could even be something with very-sharp boundaries, & a standard oval size, so that the shape more strongly communicates distance/angle/speed.

That's really quite a good idea.

I've seem bicycles in London projecting a green bike emblem into the road about 5 meters ahead using a laser.

Two somewhat higher intensity spots on the road ahead would work as you indicate, At least in ideal road conditions.

I’ve never seen this in Australia. But my main gripe with motor/push bikes is a) People riding without any lights. b) People riding without sufficient lightly.

Re. b)If a bike comes up really close to the rear of your vehicle, and the rider has no reflective gear, the light can be obscured by your own vehicle.

Re. a) I’m not sure if this is some Aussie thing. Especially with push bikes. The number of people I’ve seen on the road at night with no lighting and zero reflective gear.

So as a relatively recent migrant to Aus, I find that odd because AFAICT the regulations here are more strict than back in the UK. I.E. you must wear a helmet, you must have lights after dark, your bike must have a certain configuration of reflectors (two in the wheel, back/front of each pedals, front and rear) etc.

From what I understand the reflector stuff is routinely ignored by 'the community' because it's outdated, rules out use of clipless setups etc.

But the lights ... no idea. It's really really bad idea to cycle on the road at night with no lights. I wouldn't want to risk it, and it's scary as a driver when all of a sudden there's this dark figure out of nowhere.

I suspect this would harm the motorcyclist's night-vision in a way that a car driver is not affected due to the light reflecting off the ground and right back at them. Whereas the body of the car protects the driver from the glare.

Rider here. It'd be out of my field of view when riding. A friend had such a system on his Harley and the visibility was helpful.

Bicycle rider with a somewhat weak top-of-helmet mounted light (maybe 100 or 200 lumens): I have a tough time telling if it's on while riding if it's somewhat bright. Zero impact to view.

Can really help with retro-reflective signage, since I can point a light directly at the sign. But definitely not a blinding return.

I use a 300 lumen one, and it's the same, reflection is just not a problem at any reasonable distance (like over a metre)

A good idea that needs more compliance/regulation work ahead of standardising.

One issue is: - Propensity of people to use "lasers" for this, see above link. - Bikes with and without motors, lean to the side. - People don't like lasers being flashed in their eyes.

But even motorbike foglights, turned almost vertically down at the road, would achieve something like this.

I just moved to the Netherlands where bikes are aplenty and saw one of these two nights ago. It was not so dramatic as the examples you linked but still very visible. I think this could be a good option for motorcycles.

Motorcycles also suffer a depth perception problem in the daytime:

https://www.youtube.com/watch?v=doSDfIo61r0

Yes, you're onto something. Ryan F9 has an amusing video that dives into the "pigeon issue" of mis-judging approach speed of oncoming traffic that results in one of the most common motorcycle accidents. https://www.youtube.com/watch?v=doSDfIo61r0

That has long been my practical intuition. To put add an additional issue, at a glance, a motorcycle light at night just looks like a car that's far away. That's definitely not going to work out well.

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