While I can't help you OP and for that I'm sorry as I know the dilemmas that come with road riding, I will say what it is like in NZ, as I'm not sure where you are from.
In NZ, a horse is classed as a legal mode of transport and motorists do not have to give us horse riders any more respect than they do to that of an actual car, they are legally allowed to pass us at 100km/h and don't even have to give horses a wide berth or anything. Even out here in the countryside I have had people driving full horse trucks (which would indicate they know about horses) come absolutely flying up behind me on a very green horse not giving a toss what-so-ever. Honestly, when the road is concerned in this country, it's full of idiots, even the equestrian community when driving on the roads don't seem to give a ****.
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms