My problem with it is that most people on motorcycles today aren't very quick on the takeoff.
That would mean that everyone stuck behind them has to wait even longer than usual.
The rear end excuse, doesn't really fly as most rear ends happen when you're at the front of the line, and the person behind expects that you've already moved without them having to look. A friend has been hit twice this way, waiting to make a right on a red, in Toronto. The other aspect, is that motorcycles are multiple times more likely to run into another vehicle than vice versa. Letting poorly coordinated bikers try and thread the needle, would result in many collisions, and unnecessary stoppage of traffic. Look at how often it happens with bicycles and they are traveling much slower than other traffic. If you're "all in" for taking up less space, then the bus is for you.
Don't the States that allow it have a "you break it, you buy it" policy regarding splitting/filtering? i.e. You're always at fault?
Almost nothing you say here lines up with my own experience. Pretty much every motorcycle or scooter I see takes off from lights far faster than almost every car, especially as the odds of being distracted by a phone are a lot lower. On top of that, if you're filtering as they do in most of the world, you won't actually be directly in front of a car when the light turns green because you'll still be on the dashed line at the light. It's only after taking off (much faster than the cars) that you settle into a lane.
As for being rear-ended, the risk I'm thinking of mostly comes from distracted drivers that 'look through' a motorcycle, not registering the bike and only seeing the car ahead. Data shows this does happen, as the human brain is wired to recognize shapes we're already looking for and can be blind to things we're not (recall that
viral video of people playing basketball with the guy in the gorilla suit that walks through and usually gets missed). By taking yourself out of the path of cars, you eliminate that risk.
On bikes being ridden by poorly coordinated riders riding into cars, I can't speak to that. I've never seen it personally, but I guess it exists? I haven't seen it happen with bicycles, either. I have seen bicycles get hit by cars, but that's usually a SMIDSY scenario as described above. I guess the risk is higher with big bikes like baggers that have less room to spare, but I would think they would be more selective about filtering.
Ultimately, pretty much the whole world except parts of the US and Canada allow it with lots of benefits and very few drawbacks. Interestingly, Western Australia began allowing it in 2021, and their two-year review showed mo meaningful change in crash statistics, either protecting riders from being rear-ended or from new side-swipes. Granted, Western Aus is a pretty small sample size (mostly Perth, I'd imagine), but I suspect we'd see similar results here:
https://www.wa.gov.au/system/files/2024-01/motorcycle-lane-filtering-report-summary.pdf
If the worst case is nothing changes except motorcycles and scooters get where they're going quicker, then really the only reason not to allow it is reason 'b' in my post above. But reason 'b' is pretty powerful, especially in me-first Canada, so I suspect not much will change to allow it.
