In domestic abuse situations if there has been a physical altercation usually there is a determination of who the aggressor is. S/he then is arrested and brought to jail under the domestic abuse laws.
In regards to a regular fight, there are usually always two sides of the story. Even witnesses don't see the same thing everytime. For me, if two are involved in a fight and there is enough probable cause to arrest both since we don't know who the aggressor is, arrest both. That happens more often than not. However, there are certain times when some drunk beats the crap out of someone for no reason and obviously he gets the bracelets.
A more senior officer always told me that if anyone gets out of their vehicle to confront another over a traffic incident in a threatening manner to arrest for disorderly conduct. I tend to go with that philosophy and if any more charges can be brought up, go ahead and do it.
Remember, there are a couple ways of arresting people. Everyone knows the way shown on television with the whole handcuffs, then reading the rights (which rarely happens right away), putting in the back of the squad car, and then going to jail. However, probably a more common occurrence is sending down warrant recommendation charges to the district attorneys office for a subject. Many times it is impossible to locate the suspect so the case can be cleared by arrest by sending down charges and waiting for suspect to be picked up on the warrant. So in your road rage case, if someone could ID the two by some chance then you could send down charges.
Every situation is different. That is why common sense goes a long way in this job.
K9
"The difference between the mile and the marathon is the difference between burning your fingers with a match and being slowly roasted over hot coals."
-Hal Higdon-
"You have to forget your last marathon before you try another. Your mind can't know what's coming." -Frank Shorter-