i have a 12 week old puppy usually on the way back from a walk while waiting to cross the busy main road out side my house she will  lunge at the cars, the strange thing is she dosnt do it on the way out.
any advice on how to stop this would be great the last thing i want is for it to become a big problem.

Can I ask what breed?  If a BC this is quite a usual trait but does need to be nipped in the bud fairly quickly.  We had this and got over it by asking for a nice sit and getting attention on me by using either a treat or a ball.  Teaching a 'watch me' command is very good.

arin is a gsd, we have been working at the look at me comand but as yet we are not 100%

My GSD (adopted at 18m) has a terrible problem with traffic so nipping it in the bud is well-advised - others better qualified will be on to give advice as I know nothing about puppies.

The reason for the differential behaviour on different parts of the journey may be the direction of the traffic.  I always used to wonsder why Zack was better on the way to the park than on the way back when common sense suggested the opposite should be the case (going home  nicely knackered after a good run about.)

I finally realised (doh!) he found walking in the same direction as the traffic flow much more difficult, probably because the cars are 'coming at' him from behind and making him even more insecure.  Just an idea.

Good luck - it's probably just a phase.

I was wondering which side your puppy walks on. It might be that you have your body between the pup and the traffic on the going out walk but not on the return journey - just a thought :)

she dosnt do it when walking up the road she does it as we are waiting to cross she also does it to cars coming both ways (the road is very busy so we have to stop on an island. we are facing the road she is sitting on my left.

Perhaps she is a bit overwhelmed with the traffic passing so close to her nose. You could try stopping further back from the edge of the pavement so there is more space between her and the cars as they pass. Is it possible to avoid having to cross that road for a bit so you don't have to wait in the island where there obviously isn't much room to give her some distance from the cars?

I would find somewhere that you can just sit with her so she can see the traffic but is not that close to it and watch the world go by. Once she is happy with that you could begin to increase your proximity to the traffic so she gets used to it in her own time and isn't having to confront it right in her face until she is happy with the situation.