When he starts to bark when a truck or semi comes along and you give him a toy or treat to distract him, you may be reinforcing the behavior. Try instead to reward him when cars and gradually larger vehicles go by and he doesn't bark..of course not treats or he would gain a lot of weight if he got one for each car he didn't bark at but the praise goes a long way. Hopefully this will help modify his behavior a bit for you. Good luck.