The Cul-de-sac Formation On a Four-lane Highway (Two Lanes Going Each Way)

	In the cul-de-sac formation, you have two cars in two lanes. The one on the left is going only slightly faster than the one on the right. There's a huge line of cars behind the one on the left. There are no cars behind the one on the right.
	It's tempting to move into the empty right lane. Approaching the cul-de-sac formation from behind, it looks like you can drive faster that way and pass the big line of cars on the left. Maybe you think you can go up to the front, then cut back into the left lane and bypass everyone else. Suppose you succumb to the temptation and move into the right lane. For a short while you are indeed moving faster than everyone else.
	But then you end up behind the slow car at the front of the cul-de-sac. The cars on the left are going slower than you were, but faster than the car you are now behind - and (very slowly) they are passing you. Worse, few people want to let you back into the left lane. They're just as annoyed about the slow left passing as you are, and nobody likes a line-cutter. You're stuck there until the back of the line has passed - which might be farther back than if you'd stayed where you were.

Don't go into the right lane. Yes, it's slow going until the car at the front left has finished passing - at which point it will either move to the right lane and allow everyone else to pass, or it will eventually pull far enough ahead that everyone else can pass on the right. Until then, just be patient and stay where you are.

Cul-de-sacs also form on highways with more than two lanes each way. It's possible for surrounding lanes to be so full of traffic that all lane-changing in and out is blocked. But the principle is the same - look far enough ahead at the cars in front of you and check how fast they're going before moving into what looks like an empty stretch of lane.