For all the criticism and new legal bans, texting by drivers just keeps increasing, especially among younger motorists. About half of American drivers between 21 and 24 say they’ve thumbed messages or emailed from the driver’s seat. And what’s more, many drivers don’t think it’s dangerous when they do it -- only when others do. A national survey, the first government study of its kind on distracted driving, and other data released Thursday by the National Highway Traffic Safety Administration underscore the difficulty authorities face in discouraging texting and cellphone talking while driving.