I'm bumping this 10-year-old thread because of left-turner crashes in the news recently. To BARF OGs the advice here is familiar stuff. But I would guess that a fair number of y'all haven't seen it.
Here's the TL;DR for approaching a left-turn threat:
Identify it. Vehicle waiting to turn left either oncoming or from a driveway or cross street on the right.
Slow down if necessary. David Hough says "slow down by 10" but that could be either more or less than necessary (see post #1).
SEE and BE SEEN. Adjust position to clear the sightline between you and the threat (see posts #23, #24, and #26)
Do the weave to break motion camouflage (see posts #8 and #83).
Move laterally away from the threat (see posts #53 and #86).
Cover the brake, if you aren't already.
Watch the front wheel. It's easier to spot initial rotation than initial movement of the car itself.
I also recommend visiting the thread Hidden Hazards, which is devoted to situations where sightline obstructions contributed to a crash.
Two factors that make the left-turner a much more serious problem are high speed and lateral proximity. The faster you're going, and the closer your path is to the vehicle that wants to cross it, the greater the danger. Space cushion and reduced speed are your very best friends when a left-turner threatens.
The northbound thru lane was stopped, so the motorcyclist used the right-turn lane to pass. But a southbound pickup waiting to turn left couldn't see him--he was hidden behind the stopped traffic after all. The two collided in the intersection.
In a post called Vanishing Act in a different 1Rider thread, I listed some ways that speed can make a motorcycle invisible to other drivers. Top of the list:
Out-riding your sight distance
At high speed a motorcycle becomes a danger that a driver must reckon with when it is still far down the road. In the worst case, the motorcycle is on a collision course even though it is out of sight, beyond an intervening rise or bend. The driver checks carefully, sees clear roadway, and pulls out. But as he does, a motorcycle crests the hill and collides with the car. Driver inattention has nothing to do with that kind of crash.