This high-res photo illustrates the point of failure. Note that the point of connection is not the thick part of the arms, which are well over a quarter-inch wide - but a tiny strip at the top corner of each arm.

Here's the outer part of the pulley. When this sheared off, all engine accessories lost power. The biggest impact of this was that it became difficult to steer. Fortunately, I was able to make it off the freeway and pull to the roadside without incident.

This is the center of the pulley, where it fits around the crankshaft. It has six arms, each roughly 16 millimeters (0.625 inches) long by 1.5 millimeters (0.06 inches) wide at the point where the business end sheared off. It appears to be the thinnest, weakest part of the pulley, by far - a little bit less than one quarter of a square inch of area in total. The aluminum everywhere else on the pulley appears to be many times thicker.