Final answer:
When approaching a narrow bridge or underpass with no shoulder, you should reduce your speed, maintain space between vehicles, and stay attentive to safely navigate the tighter area.
Step-by-step explanation:
When you approach a narrow bridge or underpass where each lane becomes narrower and there is no shoulder, it is important to exercise caution. You should reduce your speed and be prepared for the reduced space. Ensure you have adequate space between your vehicle and others, and focus on the path ahead. Visibility may also be limited, so be prepared to react to any unexpected obstacles or changes in traffic flow. It is critical to remain attentive and drive defensively through these tighter spaces to maintain safety for yourself and others on the road.