It is not implied whether drivers on the busiest highways are more or less likely to become distracted than drivers on other highways, so this is useless without more information.

This adds more detail to the phenomenon without explaining it. It does not address the busiest highways, or explain why they have fewer fatal accidents than other types of highways.

It is not implied whether commuters are more or less likely than other drivers to be in a fatal accident, so this is useless without more information.

This explains why the busiest highways have the lowest rate of fatal traffic accidents. Because of their congestion, these highways are less likely to carry fast-moving vehicles, and so are less likely to be the site of fatal accidents.

This does not imply that the busiest highways carry fewer trucks than other highways. It allows for the possibility that they often or usually carry a higher proportion of large trucks than other highways.