chasing cars?
 

Kylie walks pretty nicely on the leash, but at least 1x on each walk (and never more than 3x), she will lunge after a car that passes us. Usually this happens with a car that approaches from behind, but sometimes with a car that approaches from in front.

She doesn't do this with every car. We generally have 5-10 cars pass us while we are walking. I have no idea why she chooses particular cars to lunge after. It's pretty frightening when she does it though.

Any ideas about how to train her NOT to do this?

This will be hard to break but can be done with persistence on your part. I think it has a lot to do with the herding instinct.. Nala is now eight months and we are finally making progress with this.. When she was younger and I saw a car coming. I would grab the scruff of both sides of her head with the collar. She than couldn't turn her head easily at all. Straddle your dog If you have to. Don't say anything. This is one of those times silence is better. Don't scold or praise, say nothing. She will start to pick up that this isn't much fun your pup gets older and has a pretty good grasp of sit/stay and leave it, that is what we do now. When I hear a car coming we sit / stay. Than leave it. Say it only once. She is like I Sid only doing pretty well at leaving it now, but I can still occasionally see that look in her eyes "let me at it, just one".
It also helped to walk her on the sidewalks of really busy streets where it became sensory overload. She virtually ignored them in this situation and that's when I praise and give her a treat.

i would train in heavy traffic areas and train often.

Abbis 3.5 and she goes after cars. Its like night and day though. We can go down a city street with tons of cars and she wont make a move but every now and then she goes after 1 or 2 (usually slower moving)