I think it is the herding instinct in BC's :D we have been lucky with Spice, she only barked at parked red cars in car parks? but she always chases my husband's works van when he is on sites,she even chews the bumper if he starts the engine before she is inside, so he won't drive off without her

but she doesn't do it on the normal walks with us or with our own car.
We always took her to busy roads, with heavy traffic when she was very young and just stood watching the traffic with her, making her sit patiently and praising her for being 'good', possibly that 'paid off' and she can be walked off lead around traffic now quite safely, although we don't tend to do this very much!
Also she will run on the path with us on pushbikes, even if we are on the road and not try to 'herd' us or run in front of us just keeps pace alongside quite happily, although this took some time before she got the idea that we were not in a race :D
Jacqueline :)