Based on prior knowledge, it is reasonable to assume that the safety of a car would have a strong influence on how people rate their decision to buy the car. In general, people are more likely to rate their decision to buy a car as "very good" if they perceive the car to have high safety. On the other hand, if they perceive the car to have low safety, they are more likely to rate their decision as "unacceptable".

With this understanding, the dictionary can be created as follows:

```json
{
	"unacceptable": ["low", "med"],  
	"acceptable": ["low", "med", "high"],  
	"very good": ["high"],  
	"good": ["high"]  
}
```

Explanation:

- Target class "unacceptable": People rating their decision as "unacceptable" may perceive the car's safety as low or medium. This is because they consider the safety of the car to be inadequate, so they would rate their decision as unacceptable regardless of whether it is low or medium.
- Target class "acceptable": People rating their decision as "acceptable" may perceive the car's safety as low, medium, or high. This is because while they find the safety of the car acceptable, it may not be the most important factor influencing their decision. Therefore, the car's safety can vary across all the safety categories.
- Target class "very good": People rating their decision as "very good" perceive the car's safety as high. This is because they strongly believe that the car's safety is excellent and it greatly influences their decision to buy the car.
- Target class "good": People rating their decision as "good" also perceive the car's safety as high. This is because they consider the car's safety to be good and it has a positive impact on their decision. 

Note: It is assumed that there are no other possible safety categories beyond the given "low", "med" and "high".