If you have been reading on the site. I am sure you have heard of the look at that game. I would play that while someone else was driving the car. Make a command word to get his attention so you can do that while you are driving alone. You could also put a crate in the car and end it all together. If he cant see them he cant bark at them.
My terrier Tippy was doing some major barking at one point and time. I used a water bottle for a short period and it worked wonders for her. I wont use one in the house and dont particularly think they are the best thing for training. But she absolutely hates water and I was driving. Very easy for me to put my hand up and behind me. Now on the rare occasion she does this I can just hold my drink up and say "ahh" and it seems to work.
My terrier Tippy was doing some major barking at one point and time. I used a water bottle for a short period and it worked wonders for her. I wont use one in the house and dont particularly think they are the best thing for training. But she absolutely hates water and I was driving. Very easy for me to put my hand up and behind me. Now on the rare occasion she does this I can just hold my drink up and say "ahh" and it seems to work.