Sadly I don't think there is any way to train road safety into cats
As Mark says, they can be the most well behaved placid animals normally, but they could easily get distracted, or chased by another cat or something, which could startle them into the road.
I don't think there are any simple solutions to be honest, for anyone who wants their cat to have outside access, apart from maybe catproofing an outside area, but by the sounds of it, this would not work in your situation
You would have to hope that the lure of the open fields would outweight the noisy road, but unfortunately you just never know