I would try patiently desensitizing. I do a lot of road riding, and one of the things I see with young horses that have never been off the ranch is that unfamiliar ground features always require desensitizing. I took our green broke 4 yr mare out for her first ride through a neighborhood a couple weeks ago and the only two things that really bothered her at all were 1) walking/stepping from grass to a concrete curb (I doubt she had ever walked on concrete) and 2) stepping on/over a crosswalk painted on the road (I know she had never seen a crosswalk)...both very light/white colored.
I have a friend that has a great horse they use for carriage rides for years. One day they went out and the yellow center lines in the road had been repainted (nice and bright), and the horse would not step on them at all.. even seasoned horses do not like 'change'.