Saint Bernard aggresion

Question
Dear Chris,
Lucy is our Saint Bernard. She is 1.5 yrs and is the biggest sweetest baby, ever. She is fantastic with our 2 %26 5 yr olds, surprisingly careful with everyone and lays down for anyone who walks into our yard (so they can pet her belly). However, whenever another dog is being walked down our street she will run through her electric fence and attack the dog. She has been socialized with numerous other dogs, unfamiliar dogs as well, they can play without a problem in their territory or neutral territory. I should point out that bunnies and cats are a favorite to chase but she does not run through the fence for them, only dogs. This fence breaching is very recent and we have retrained her on it but she continues to break out and attack. We don't want to build a fence since the electric one worked for fall and winter but I don't want to get into any legal matters either. Everyone loves her but because of the dog attacks even our best friends are weary of her. Is there some way we can teach her to leave animals alone? To stay in her territory? I'm pretty familiar with dominant signs and she only exhibits them when there are smaller dogs around. Let's face it, just about every dog is smaller compared to a Saint B. Please help us. We love her so much and she has changed our family since we rescued her in August of 2009. It would break our hearts if we ever had to get rid of her. Thank you.
Sincerely,  Koli

Answer
I'm not sure I'll be able to give you much help, but here are my thoughts.

Once a dog has figured out that it can get through the electric fence, it's very hard to contain them.  The reason they break through is because they are after something, as with Lucy.  It's either to chase a car, another dog, a rabbit, whatever.  Their adrenaline is racing and they're willing to withstand the shock in order to get to the object.  Once the chase is over, the dog isn't willing to go back through the shock to get back into its yard.

In most cases, and my first recommendation, is that an actual physical fence is the best containment, but of course must be tall enough that the dog cannot jump or climb over.  

If that is not an option, perhaps a trolley run would work:
http://www.unchainyourdog.org/Trolley.htm

You are correct in being concerned about legal matters - an animal that is seen as a threat by neighbors can be a liability.  As a responsible dog owner, you owe it to Lucy to find a way to keep her safe and to make your neighbors feel safe as well.