After living of a farm for all of my child hood and teen age years and being around horses, I still to this day find 90% of horses too un predictable!
they are eaisly spooked by cars, dogs, motorbikes, noise basically most things, for that reason I strongly belive that they should be kept away from roads where things can spook them.
there are 1000's of events, fields, bridleways where they can be roam without needing road use! (yes I do realise acsess to bridle ways are from roads).

I know all horse riders will dis agree but if you truly look at the case you can not honestly say I'm 100% wrong!

If something is unstable in an envoroment, you remove it!
you wouldn't leave a baby in a room with a dog.. Just in case" smae thory applies in my eyes!