My younger mare is safe around the "normal" vehicles-cars, trucks, jeeps, vans, trucks pulling trailers, dump trucks etc, but I have yet to meet a motorcycle or a transport on her. I think she may be alright with the transport, but motorcycles...honestly, I'm not looking forwards to it
As for my older mare...well, she lived down around the city before I got her, so she is absolutely, 188% traffic safe. I have met all manner of vehicles, and the worst I've ever gotten out of her was a hollowed back and a small flinch as two motorcyclists roared by (they didn't stop or slow down..) One time, I was about a mile from home on a dirt road and heard a transport truck coming up behind us hauling hay. Well, I was way more concerned than the horse was, and I got her slightly off the road and sat there until it went by (we would've had to do it anyways-with the truck passing, there would be no room for us to stay on the road). It went by with all the dust and engines roaring, and the horse literally didn't even flick an ear. There is something really, really amazing about having a horse so totally traffic safe.