"no arrests have been made"
I'm not sure what you're expecting here but unless there is clear evidence at the scene of criminality or an indication that a potential suspect is a significant flight risk then there is no justification for police to arrest a drive at the scene of a collision.
No arrest isn't an indication that the police think the driver is innocent but that they want to investigate further before they determine what charges, if any, they wish to pursue.
A default practice of arresting drivers is unnecessary and opens the police force up to being sued for unlawful arrest.
The police can still make an arrest later on when they have sufficient evidence.