A minor mystery solved.

So they're laying a large oil pipeline through the utility easement behind the house. (We've known about it for over a year, so it's hardly a surprise. And they've done a reasonably good job of taking out no more trees than necessary.) Along with all the standard accoutrements of heavy construction, there's a fairly large pile of used car tires, the use of which was perplexing me. I just discovered what they're used for. The tires get laid down on the road so that the large triangle tracked bulldozer can cross the street without tearing up the road, or requiring a large trailering rig. ingenious! (there's also a few sheets of 1" plywood that go under the tires, no doubt intended to disperse the load even further.)