The problem with leaving a gap is that, as soon as it's a few car-lengths long, people in the adjacent lane(s) notice that there's a gap and quickly cut into your lane to fill it, because it moves them a car-length or two forward. In order to do this, they have to cut in front of you, speed up quickly, and brake quickly. And you're left in the same position that you were in before you made the gap -- plus one extra car.

I try to do this all the time and it's frustrating because it's completely impossible. As soon as a vacuum exists, drivers rush to fill it.