You're right, good thing you weren't next to it.

Looks like an old radial, pretty much worn down. Based on some sign near the edge of the blown area I'd say the tire had what's called a tread separation. Before the tire blew, you might have been able to look at the tread area and see a "wiggle" in the tread pattern. Or just a slight "bubble" in the tread face. Both are signs of tread separation which happens between the layers of tread, and frequently involves a broken belt. In the mid '70's it was real common on domestic, steel radials, as the manufacturers were "learning" how to make them. The Firestone F71 was notorious (they seem to have a problem with negative experience don't they, but they weren't alone).

I once had a "good" experience with tread separation. I was in the market for a car (like that's unusual!). It was '77, and I took a used '76 280z for a test drive. The front end shimmied a bit. I suspected the tires, looked at them, and sure enough, separation sign. Got back to the dentist that was selling it, and asked about the shimmy. He had that look like he wished I hadn't noticed. Given the car had "unknown problems" it became an excellent negotiating point (for me) and I drove away a happy owner. Four new tires, which I was able to buy wholesale at the time, and I had a nearly new car for a cheap price.