When do police officers call for back up?

[True2marie]

On TV, I only see officers call for back up if they're about to confront a violent criminal. Yet, in real life, I have seen a couple of cop cars on the freeway seemingly dealing with someone who's less than dangerous. So, when do police officers call for back up?

 

[downsouth]

Most likely when there is imminent danger or they realize they cannot handle a person on their own.

 

[bala]

They call for back up if they
1.don't have enough arms to tackle the situation,like when a mob is protesting and there are only a couple of cops on scene
2.Dangerous criminals who can confront 3-4 cops with ease.
Basically,anything they think they cannot do it on their own..they call for back up.

 

[Ricardo187]

Whenever they see that there are many people in the car, or a possible violent/strong driver they couldn't handle if he goes mad, or whenever it's a criminal or there's any other risk. They never know when one will act violent or try to evade.

 

[Gelsemium]

Most likely when there is imminent danger or they realize they cannot handle a person on their own.

That is how I think too, if they see there is imminent danger they call back up, but most likely there are protocols, it's not a decision made on that moment right?