Observations and Patterns

These are some of the things that I've noticed, and assumptions that I hope may help lead to me, or someone, figuring it out.

• Usually (but not always) placed at or near street intersections
• Always in asphalt (not concrete)
• So far, they seem to be embedded in new, or relatively good asphalt, which leads me to believe that they are possibly put in place at the time the street is re-paved. Have yet to see any in old, ratty, deteriorating pavement.
• So far they have mainly been in groups of 1, 2 (in a "T") or 3 domino formations
• Haven't found them outside of the St. Louis city limits, yet.
• They couldn't have all been put there at the same time, because those streets weren't all paved at the same time - they've been placed over a period of time - Most likely within the last 5-10 years at the most.
• Always just one set of dominoes to an intersection, with the exception of Arsenal and Kingshighway, which has 3 sets, and Hampton and Bancroft with 2 sets.
• The two sets in Forest Park seem to mark significant landmarks - The Grand Basin/Art Hill, and The Highlands Golf Course. 
• Potentially put in by placing the on the ground and covering them with tar paper, getting pressed into the pavement by passing cars.
• The end is near, and these are possibly harbingers of the apocalypse.