One of my Scotties was like this, she was terrified of traffic, any loud noises, even if you clapped your hands she cringed. We tried all sorts to,desensitise her, but to no avail. We just avoided pavement walks where possible, and we live a quiet life anyway, but it did cause a few problems. I'm afraid she never grew out of it, so I'm sorry I can't offer any advice. She didn't get any worse though, so maybe it's worth getting your dog vet checked in case there may be some underlying cause.
Lesley and Dylan