Multiple collie owner here - the best dogs ever!
Car chasing is very common and luckily pretty easy to fix
Collies, as we know, react to movement so the car rushing past sends them into herd mode. But luckily for us the car also makes a noise so we can work on changing behaviour when the dog hears the car coming
rather than launch into herd mode.
Get a clicker the best treats ever, chicken hotdog, cheese find out what your dog loves maybe do not feed before the training session.
Location makes is important so if you could go to a park or field that you can be set back from the road but still see and hear the cars.
First (just me I am a dog trainer and no you are not rewarding bad behaviour!) as soon as you hear a car coming click and feed the dog just pop the treats on the floor or hand to the dog, when the car has passsed stop treating. Walk around chat etc , then the minute you hear the next car click and treat until the car has passed.
Over time you can move nearer to the traffic and go to areas where the traffic is more constant but do not rush to this.A new location will make it harder so you will have to treat more and maybe be further away from the traffic again.
So what is happening in the collie brain is hear the noise of a car get yummy treats, noise stops no more food.
Eventually (and quite quickly) the dog will turn to you when they hear the noise of the a car, you are rewarding them and there is no need to chase.
I would do this for short sessions every day for a week, see how the dog progresses. If you are not able to click and treat try to avoid the cars until the new behaviour of looking to you is established.
If you want to pm your area in Wales I can put you in touch with a trainer who is doing zoom.They will be able to iron out any issues or hiccups you may come across.