How can I convert the result of Trips::model()->findAll()
to an array?
Don't used CHtml::listData for this. It has to be used for other purposes. There is an index property of CDbCriteria which is suitable for you requirement.
//1st option
Trips::model()->findAll(array('index'=>'trip_id'));
//2nd option
$c = new CDbCriteria();
$c->index = 'trip_id';
Trips::model()->findAll($c);