A riddle: Why has the Toyota Prius enjoyed such success, with sales of more than 400,000 in the United States, when most other hybrid models struggle to find buyers?

One answer may be that buyers of the Prius want everyone to know they are driving a hybrid.

The Prius, after all, was built from the ground up as a hybrid, and is sold only as a hybrid. By contrast, the main way to tell that a Honda Civic, Ford Escape or Saturn Vue is a hybrid version is a small badge on the trunk or side panel.

The Prius has become, in a sense, the four-wheel equivalent of those popular rubber “issue bracelets” in yellow and other colors — it shows the world that its owner cares.

In fact, more than half of the Prius buyers surveyed this spring by CNW Marketing Research of Bandon, Ore., said the main reason they purchased their car was that “it makes a statement about me.”