SharQ may even be underestimating the danger of this situation: in my experience, not only the drunk or numb cause accidents like this.
I have been in three serious car accidents, all three in the Netherlands, where driving is on the right; two of them happened in this situation. In both, the sun was high in the sky, with hardly a cloud in sight; perfect visibility.
In the first accident, my mother was driving on a very busy provincial road. We needed to turn left, so she put out the indicator, slowed down, came to a stop at the crossing, and waited for oncoming traffic to pass. Three or four cars had already queued up behind her when she turned. Then she saw a car overtaking the queue behind her at great speed; a crash was unavoidable, but she managed to steer back just enough to make the cars collide side by side, minimizing the impact. Both cars darted off the road, but quickly came to a standstill, and everybody got out unharmed, after which the other driver started venting at my mother.
But there was no way my mother could possibly have expected the other car. According to the bystanders, its driver's attempt to overtake was nothing short of suicidal. So what was the other driver shouting at my mother for? How could she have thought that overtaking in this situation was safe? There is only one possible explanation: she hadn't been thinking at all. She was on the main road from A to B; she didn't anticipate that someone might go somewhere else. In particular, it didn't hit her that someone might actually turn into traffic - until it was too late.
In the second accident, I was behind the wheel myself. Another provincial road, not far from the previous one. I needed to turn left in the middle of a straight stretch of road, at least half a mile long. The car I saw behind me was at least 10 seconds away. I put my indicator out, slowed down, stopped. I could have made the turn before the oncoming car, but as Sharq mentions, its speed can be hard to estimate on this type of road where the speed limit is little more than a paper rule. So I waited. I didn't feel at ease, of course, but I waited. The oncoming car passed. I started to make the turn - then the car behind me crashed into us. Hard. It had hardly slowed down.
If another car had been coming from the other side, I wouldn't be sitting here. My wheels had already turned. Read SharQ's wise words. Lying in my bed that week, all I could think of was the direction my wheels were pointing.
But wait a minute. Didn't I tell you that I put my indicator out, slowed down, and came to a standstill, hundreds of meters (yards if you wish) ahead of the car behind me, with nothing but a perfectly clear, perfectly straight stretch of road between us? Yes, that was the exact situation. So how could the driver possibly not have noticed that I was no longer moving at the same speed, that I was in fact standing still on that road, that he was approaching me, that he might have to do something, like, use his brakes? Again, there is only one possible explanation: it didn't occur to him. It was a crystal bright Saturday morning, he wasn't tired, he wasn't drunk, his vision wasn't impaired, no other traffic was interfering. Yet the possibility that I might be slowing down and stopping to turn left didn't enter his brain at all - again, not until it was far too late to do avert a collision.
So heed SharQ's advice, boys and girls. What is more: if you can avoid it, don't turn into traffic at all. Calvin said it best: Be careful, or be roadkill. Don't sit your sorry ass on that road even for a second. If you have to leave a main road and cross the oncoming traffic, do as I now prefer: turn in the other direction, then make a full turn and cross. Drivers on the main road will happily continue from A to B and from B to A; have mercy on them, and don't put yourself in the way. Your car will be happier. They will be happier. You might even save a life here and there.