I was riding up the hill in Old Ottawa South this morning, amid the stream of traffic that there always is on the hill in Old Ottawa South, just behind a big white Dodge van of some sort. Traffic's usually slow enough there that I can ride just out of the door zone, usually just off the left taillight of the vehicle in front of me.
Suddenly, the driver of the Dodge slammed on the brakes. Startled, I swerved so as not to run into it, then continued my climb, filtering past it. As I got to the nose of the van, I realized the driver had stopped to let someone turn left across their path, mid-block. I hit the brakes myself, because there was a car about to turn left into me, and the driver of the car waved her hand at me in a "go ahead" gesture. So I did, while the left-turning car and the waiting Dodge held up traffic.
I get that the driver of the Dodge saw the woman waiting to turn and decided let her go ahead. And I get that I probably wasn't all that visible, behind the van, to the driver (though the van had passed me earlier, and so should have been aware of my presence), and filtering is a weird, liminal sort of space for cyclists to be in, albeit one they're in a lot. And maybe everything was going slow enough that slamming on the brakes mid-block wouldn't cause the car behind to hit the van. But come on - use your rear-view. People so often don't think about what's behind them in traffic - they react to what's in front of them.
I've seen people do the same thing in four-lane traffic: stopping to let someone turn without thinking about the traffic coming up in the left lane who won't know why you're stopped. It happens all the time further south on Bank Street, where people stop in rush hour congestion to let others turn across two lanes to get into the Farm Boy parking lot. I'll be cranking up the hill, passing the slower car traffic, and at the entrance to the parking lot I regularly see someone nosing through traffic to turn left. Often, by the time they get across to the section of the street where the cyclists are riding, they've decided "all clear" and hit the gas to get out of the street and stop holding everyone else up - and if there's suddenly a bike in the way. . .
Basically, those Good Samaritan lefts are dangerous for everyone, not just cyclists.
(I know some people will tell me that I shouldn't filter. But filtering is actually a bit of a grey area in Ontario law, and my rule is, if there's room and traffic's slow or stopped, I will filter until the last few cars before the light. If any of those cars have right turn signals on, I stop behind them. If not, I ride up right beside the first car so I'm in the driver's peripheral vision when the light turns green.)