I’m referring to the tendency of some people to follow a straight, unbranched path, with no deviations. For instance, waiting for a stoplight to change to green even at midnight even though there are no cars coming from a perpendicular direction.
The expression that I use is streng and eng. It is direct and straightforward, and even rhymes, so IMHO, it does the job. Is this, however, “standard” German, or is there a more idiomatic way to express this idea?
The word you are looking for is geradlinig.
Walter ist sehr geradlinig. Wenn er eine Sache anfängt, zieht er sie auch durch.
It depends a bit whether you want to express this trait as positive or rather as (partially) negative. I'm assuming a positive notion from your question. Then you might want to look along the lines of
streng und eng would probably not be understood or understood wrongly. That is not idiomatic in German and too close to engstirnig, which is "bigoted" and "narrow-minded". engstirnig and pedantisch could, however, be used for the more negative notions of this attribute.
I want to explain some idioms from the question and from other answers, to give you a chance to understand and compare them:
streng and eng
»and« is not a German word, but even »streng und eng« will not work in German. Nobody will understand what you mean. Maybe some English native speakers might understand it because they might translate it back into english, and then understand the English idiom.
»streng and/und eng« is like translating German idioms word by word into English, like in this example:
»My English is not the yellow from the egg, but it goes«. (The German idiom is: »Mein Englisch ist nicht das Gelbe vom Ei, aber es geht«, and a correct translation would be: »My English is not exactly brilliant, but it's ok.«)
The straight and narrow is "der Pfad der Tugend" in German, but this is kind of an archaic expression (as are many suggestions that have been made already). Think Shakespeare. It can also be translated as "die rechte Bahn", but this has more of a connotation of being normal and slipping into crime would be to deviate from the "rechte Bahn" (right road). Which is also an old fashioned expression not in common use anymore. (Since about a decade or two, I'd say.)
The straight and narrow, I think, expresses more of an inherent incapability to judge situations for what they are and instead follow rules blindly, like the traffic light example. It's a subtle way to mock someone for doing the right thing.
As such, I would suggest "Gutmensch". It describes someone that always does the right thing, even if it is to his own detriment and it is also not meant as praise, but to mock such a person.
Just to add one choice not yet mentiond
zielstrebig
is in my opinion quite close to the favourite geradlinig and would still fit when the person follows a moderately meandering path, but with the strong intention to follow it up to the end.
strikt (Adj.)
http://www.duden.de/rechtschreibung/strikt
http://dict.leo.org/german-english/strikt
.. someone who does not allow much space for interpretation of things and follows the rules.
I think the best fit is
aufrichtig
It describes the positive quality of being honest and good and also indicates that the person will not deviate from this path even if it would have advantages.
Another word which describes more the being strict to themselves part:
unbeugsam
describing a person which will not yield or change its behavior whatever comes. It could be described as negative trait for having rigid behavior, but mostly it has the positive connection that the person will not be dominated by anyone.
