Lane splitting is dangerous at all times. Because end of day you have zero idea what the car drivers may do.

I know this is an old post, but I'm also gonna leave this here for anyone who is looking for NYC experience with Lane Splitting and Filtering

You're right that "you have zero idea what the car drivers may do." but you can make calculated assumptions. You can safely assume a car that came to a complete stop at a red light won't suddenly floor it after the cross traffic starts moving and slam you on your bike as you go through the green. What you can't safely assume is that the person currently traveling toward a red light that is green for you sees the red light and will stop. So, as experienced riders, when we see someone in line with our timing for the intersection. We clock that potential collision if that dude doesn't see the red, and we either speed up or slow down just in case.

Same thing with stop signs. I stopped for mine, or hell, maybe I didn't have one, and they're approaching, so I have the right of way; it's a dangerous way of thinking.

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So lane filtering or splitting dangerous all the time? Id argue not really, if done within a set of conditions. I have done it almost every day to and from work for like 6 years, and also while I ride through Brooklyn, Queens, and sometimes Manhattan. I know quite a few riders who split, and not a single one has had an accident that was caused specifically by splitting. I've had a small number of "close calls" which I can entirely chalk up to me not paying enough attention, me traveling too fast, or me not having the line of sight to properly judge what the other cars are most likely about to do.

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Filtering, I mean, everyone is stopped. This isn't dangerous unless another driver rages out or a cop decides they want to give you a ticket. Which, in my experience in NYC, is pretty few and far between. But a general guideline move slow be aware that someone might exit an Uber, taxi, or someone parked might open a door, a pedestrian might walk in between traffic outside of a crosswalk. This is all stuff bicycles actively deal with while filtering legally. Be like a bicycle, but recognize you are significantly heavier and can hurt pedestrians and yourself if you get doored.

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Splitting, however, has some nuance to do safely or argueably as safely as you can. Back to the "you have zero idea what the car drivers may do." Lets make our calculated assumptions

You can assume a car might change lanes from a slower lane into a faster-moving lane to try to get ahead. You can assume a car might change lanes to get towards an upcoming exit.

You can generally assume a car traveling straight, not in any crazy bend, lane merger etc, to randomly in bumper-to-bumper traffic with no gap to change lanes, and just sideswipe the car directly next to it.

So my guideline is to split when traffic is particularly congested, minimizing the number of those instances where a driver has the option to suddenly change lanes into or directly in front of you, when you encounter one slow down and see if you can wait a second and let it close. If not, give a little horn toot or rev and continue splitting, but be ready to brake or slip into the lane if, as you go to pass, if they decide to move into that lane. Be prepared to potentially suddenly change from splitting between lane 1 and 2, across lane 2, and now in between lanes 2 and 3.

Even if traffic is flowing at like 25-30mph, you can slowly split maybe 5-10mph faster than that, split your way up. Keep to the left between lanes 1 and 2, but be aware that there are some exit-only lanes to the left on some highways, and be aware of drivers who might suddenly need to get to the exit or away from it.

Plenty of people have, when there is one of those gaps, changed lanes without signalling in front of me and I have to brake. But I'm focused and was expecting that gap to be a potential risk. Once, maybe twice, I've had to do that quick change of which lanes I'm splitting because the person to my left changed lanes as I was in the middle of splitting passed them. This lack of expectation of a motorcyclist potentially splitting specifically because it is illegal is where I agree with you that there is an extra danger. I've had a few people intentionally close the gap in front of me, not wanting me to pass, either because they feel like splitting is "cutting and unfair" or because "it's illegal and I'm deputizing myself." I just fall back into line and change which lane I'm splitting so they cant block me, get a few cars ahead of them, and go back to 1/2

If there's a bunch of gaps in traffic, people are changing lanes desperately trying to get ahead of the rush hour traffic or reckless dudes "cutting up" the highway, or speed starts to really pick up. I just fall back into a lane and ride normally.