The way those worked here was simply that it was up to any overtaking driver to make sure that the middle lane was clear far enough head to complete the maneuver and pull back to the left in safety.
Both could use the lane, but neither had priority, if you see what I mean.
If you had drivers coming from opposite directions, each deciding to pull out to pass without signaling first, then there was going to be a big problem.
Most nearly fatal accident we had as a family back in the early 1970's. Freckleton marsh, south of Blackpool. Three lane road with common centre passing lane. Dad pulls out to overtake and at the same time, in the distance, oncoming car does the same - both cars pull back. Thinking one's letting the other go Dad has another go - the other driver obviously thought the same - it was a bit like meeting someone on the pavement and both stepping side to side trying to pass each other.
Situation ended up with four cars on a three lane road with the centre pair passing at 140 mph! There were no wing mirrors on cars in those days and the passing gap was so small that three pairs of door handles got smashed! My mother's palms were bleeding from clenching her fists. Not something to be repeated and I'm glad these roads have at last been phased out.
South Africa has them with an important modification - one direction has a dashed and single white line rather than just a dashed one which reinforces that the other direction has priority. I have still though seen what was probably a fatal head-on in this situation. Not good.
