I agree with Kevin, I would also like to add that tiptoeing around a horse that is already nervous can sometimes be more detrimentle than helpful. Here is an example: I have a friend with two horses of a similar age to mine, they are both absolutely terrified of traffic. They were bred and broken by my friend and have no reason to be afraid.
Neither of the horses that I have ever broken or schooled have ever been afraid of traffic. Similar situation and we live on the same road, within a couple of K's of each other.
The difference between us both is something that I noticed when I was riding with her. When a vehicle comes down the road she always stops her horse, gets as far off the road as she can and gets very 'reassuring'. Whereas I ignore the vehicle completely and keep my horse moving forward in the same line. As near as I can tell what is happening is that she has taught her horses that vehicles are a big deal and to be very wary. The older the horses have got the more wary they have become and now they just want to run away.
My point is that treating a horse with kid gloves can be as damaging as to much aggression. Most methods will work when applied, firmly, kindly and consistently. Sometimes we make mistakes deciding what is going to upset a horse.