What must you reckon with here?

In the scenario you've presented, when approaching an underpass, it's important to anticipate potential driving hazards.

Firstly, the vehicle in front may change lanes, which can happen if there's an adjustment needed for the upcoming road conditions or obstacles. This requires constant vigilance and readiness to react.

Secondly, there is often denser traffic beyond an underpass, especially if the traffic flow increases as the road opens up again. This can lead to sudden stops or slower speeds, so it’s crucial to maintain a safe following distance.

Lastly, if you're in the right-hand lane, you must be aware that it may come to an end, which could force you to merge into another lane unexpectedly. Failure to recognize this can lead to potential accidents.

In summary, being aware of all these factors is essential for safe driving according to German traffic laws, enabling you to make informed decisions as you navigate through varying traffic conditions.