It was a piece of work undetaken I believe by bookworm (or someone he was engaged with). I will see if I can get a copy.
If you think about it, it is very logical with a couple of assumptions
1 - seeing a motionless target is no easier off to the side (and maybe harder) than in front of you.
2 - cruising levels reduce the possible altitude separations to 2 or 4 per thousand feet, vs random altitudes which have at least 10 levels that would be a miss
Therefore cruising levels reduce significantly the odds of two aircraft approaching head to head at high relative speed, but due to compressing all aircraft into either 2 or 4 levels per thousand feet significantly increase the odds of you being at the level of the intersecting traffic.