Another factor in situations like this is a vehicle's speed. I would have said "speed limit" but that seems more irrelevant for a lot of today's drivers....... There are certainly a lot of factors to quickly process when making the decision for a situation like this. I have a similar turn where I now exit off the road on the right hand side at the start of the turn lane. I look for a clear opening in traffic and then dart across both lanes over to the turn lane when I can do it safely. It's amazing how fast vehicles can come up on you when they are traveling 45-55 mph.