You are driving on a road that has several marked lanes going in one direction. When should the alternate merging procedure be employed?

You answered correctly. The alternate merging procedure (Reißverschlussverfahren, or “zipper merge”) is required in two situations: when a lane comes to an end and when a lane is blocked due to an obstacle and continuous traffic flow is not possible. In both cases, vehicles from the ending or blocked lane should merge alternately with vehicles in the adjacent lane at the point of restriction. This helps prevent long queues and keeps traffic flowing smoothly. It is not mandatory when a merging lane ends on the motorway; in that case, drivers entering must yield to through traffic. Always use the alternate merging procedure only when a lane actually ends or is blocked.