Ah but the people contained within those fences is limited and they should know what to do with a loose horse, even if it is just to get out of the way.
Far safer to saddle up in the lorry, you've less chance of being hit by traffic, less chance of a horse being spooked by traffic and far greater control of the situation. Youve also got more control as they come off the lorry as they are in a bridle not a head collar, and you don't have that moment when you have to take the halter off to put the bridle on.
This article pretty much explains how I teach mine to tie up, I'm a bit less flowery about it but this is essentialy how I do it. Teach your horse toTie by Juli Realy
It is not a million miles away from what you do but what it does teach the horse is to yield to pressure, WITHOUT the risk of the horse breaking it's neck.
Stan could be trusted to the point that I didnt even have to tie him. Regularly at shows I just left him standing beside the lorry, reins over his neck, not tied up. He would stand there untill I came back for him. He was safe there because I put him there and he would stay there even when all hell let loose on the show field.
Yes I am confident that if they got loose I could catch him on the yard again, but I have never needed to do this. My bailer twine at home is old, rotten and frayed, it has seen too many winters outside, When it drops off due to the weight of the lead rope (record is just over 3 years!) they get replaced but otherwise I have never had need to.
I'm also very very aware that one knock, bump or scar on my horses will reduce thier value by thousands of ££. They are show horses, a scar or a splint could end thier competative career.