Here's why:
* Darting eyes: This suggests quick, jerky movements, indicating a heightened state of alertness or fear.
* Road to rear-view mirror: This highlights the driver's constant scanning of their surroundings, both ahead and behind. This behavior is typical of someone who feels vulnerable or threatened, constantly checking for potential danger.
Therefore, the idiom signifies someone who is overly concerned about their surroundings and is constantly on edge. It can be used to describe:
* Someone driving cautiously due to bad weather or traffic conditions.
* A person feeling threatened or pursued.
* A driver who is easily distracted or lacks focus.
It's important to note that while this idiom often describes negative emotions, it can also be used in a more neutral sense to describe someone who is simply attentive and alert.