Peter and Karen Darnell pose for a portrait with their three-year-old Toyota Prius at their home in Westford, Mass. Credit...Erik Jacobs for The New York Times 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