よう in 巡回中の警官が車を止めるよう合図した。

Hi guys, this is an example sentence from goo辞書: 巡回中の警官が車を止めるよう合図した。They translate it as "A patrolling policeman signaled (to) me to stop my car. I don't really understand how よう works here. I guess because I have encountered ように and ような a lot, I would expect ような合図. because it's a sentence (車を止まる) modifying a noun (合図).

Many thanks for your help.

Edit: I found this example as well on goo辞書: 彼らに黙るように合図した。which is translated as "He made a sign to them [motioned them] to stop talking". I think this ように indicates the purpose of the gesture and on second thought this is probably what I would expect in the first sentence as well (車を止めるように合図).

よう is used for embedded order, request, advice, etc., similar to ～かどうか for embedded yes-no question.
e.g.
警官が「車を止めるなさい」と言った。
警官が車を止めるよう（に）言った。

医者は「野菜をもっと食べたほうがいいですよ」と勧めた。
医者は野菜をもっと食べるよう（に）勧めた。

彼は「道を譲ってよ」と要求した。
彼は道を譲るよう（に）要求した。

cf.
彼女は「彼は来たの」と聞いた。
彼女は彼が来たかどうか聞いた。

In your example, it's not what the policeman really said, but his gesture works as an order, so よう（に） is used.

EDIT:
I changed my interpretation. It would be more appropriate to interpret that よう（に） shows the purpose of the action. My initial interpreattion "the contents of the action" is also possible, though. に is optional, anyway.

Thanks Toritoribe-san, I understand now. I just have one question: in your example 警官が「車を止めるなさい」と言った。why isn't it 止めなさい ( with なさいattached to the -masu stem) ? I'm not familiar with this form.

I don't like to speak for others (especially not someone as esteemed as Toritoribe-san ), but I'm pretty comfortable speaking for him in this case and saying that 止めるなさい is a simple typo for the correct 止めなさい.

Ooops! My bad! Yes, that's just a typo of 止めなさい. Sorry!

I thought it could be a typo but Japanese is full of surpises so I wanted to make sure! Thank you both!