Jumping towards cars

Question
I have a 4 month old German Shepherd who jumps towards each car as we're walking on leash.  I thought it might have been that she was scared of them and she would get over it, but now I think she is excited.  
We have tried to ignore the behaviour thinking that if she didn't get anything positive out of it she would stop but that hasn't worked.  We've also tried disrupting the behaviour by qiving an quick pull on her leash just as she starts to go.  
She normally walks quite well when no cars are in sight.  Luckily we don't live in a high traffic area.
I don't know what else to try.  Any suggestions? Thanks!

Answer
That will be difficult to work out.  Be pro active.  Before the car gets close enough that she will jump, get her attention by talking to her.  Use a treat to get her attention if you need to.  Praise her if she lets the car go by.  

You might try a head collar too.  The leading brands are Promise, Haltie, and Gentle Leader. They have a strap going around the dogs nose looking something like a muzzle. They work by pulling the dogs head around. No other way gives you such great control with so little force. The prong collar is now a dangerous relic of value only for its macho looks. Do not consider using one without hands on instruction from somebody with plenty of experience with them.