To understand why an **excessive number of passengers** reduces the rear field of vision of the vehicle, it is important to consider how **rearview mirrors** work. Rearview mirrors are essential for seeing what is happening behind and to the sides of the vehicle. In a passenger car, for example, a central interior rearview mirror and two exterior mirrors (one on each side) are used to maximize visibility.

When there are too many passengers in the vehicle, especially if they are seated in the rear seats, they can obstruct the view through the interior rearview mirror. This limits the driver's ability to clearly see what is happening behind the vehicle, which can be dangerous as it prevents detecting vehicles or situations at the rear.

On the other hand, **transparent sunshades** do not significantly affect visibility if used correctly, as they are designed to block the sun without obstructing the view. Additionally, a **poorly ventilated cabin** does not have a direct impact on the rear field of vision, although it can affect the driver's comfort.

In summary, an excess of passengers can block the view through the interior rearview mirror, reducing the rear field of vision and increasing the risk of not detecting hazards.