Forms API #states with multiple radios
In misc/states.js, it looks for the closest form-item, form-submit, or form-wrapper. Here's the line:

$(e.target).closest('.form-item, .form-submit, .form-wrapper').toggle(e.value);


This does not play well with the way radios are themed, and states.js does not find anything to toggle. Wrapping the itinerary_name radios in a container and applying #states to the container item is what I would call the quickest workaround.

$form['itinerary_radios'] = array(
  '#type' =&gt; 'container',
  '#attributes' =&gt; array(),
  'radios' =&gt; array(
    '#type' =&gt; 'radios',
    '#options' =&gt; array(
      'new' =&gt; t('Create a new itinerary'),
      'existing' =&gt; t('Use an existing itinerary'),
    ),
  ),
  '#states' =&gt; $trip_states
);