About One-way streets, narrow single carriageways, separated carriageways and service roads: which statement is WRONG?
A sign usually indicates which carriageway to follow; if there are no signs, you must keep right and therefore follow the right-hand carriageway.
The prohibition applies to reversing as a driving manoeuvre along the road and to U-turns; backing into a parking space is explicitly still allowed.
Watch out for a sharp edge and/or a lower-lying verge on narrow single carriageways outside the built-up area; if your right-hand wheels leave the road, you quickly lose control of your steering.
A service road (ventweg) next to the main carriageway is simply an extra lane you can use for fast, through traffic.
Explanation
trap: a quiet road mistaken for a fast road. Book p.66 - the main road is intended for fast and through traffic, the ventweg for slow and local traffic (sign C5 at the entrance)