class MapPage_Controller extends Page_Controller {
...

function doForm($data, $form) {
...
$url = 'http://maps.google.com/maps?f=d&hl=de&saddr=' . $source . '&daddr=' . $destination;
Director::redirect($url);
}
}

I want to redirect into a new window (like target="_blank"). Is there a way to do this?

Not without javascript, and that introduces an accessibility issue. To achieve what you're looking for, you need to have PHP draw a script tag and open a new window with the desired location.

Now i find a way to do this...

class MapPage_Controller extends Page_Controller {
...
function Form() {
$fields = new FieldSet(
...
);
$actions = new FieldSet(
new FormAction('doForm', 'Show')
);
$form = new Form($this, 'Form', $fields, $actions);
$form->setTarget('_blank');
return $form;
}
function doForm($data, $form) {
...
$url = 'http://maps.google.com/maps?f=d&hl=de&saddr=' . $source . '&daddr=' . $destination;
Director::redirect($url);
}
}