There is a chain of thought that horses travel better if facing backwards hence lots of horseboxs are rear facing. Professional shipping companies however think this is an old wives tail and use boxes that face forward. Would be interesting to see some actual studies.
I think horses actually seem to travel well in the bigger lorries when they are standing "herringbone" - on a slight diagonal - and they do like to look out. Most lorries have windows and so to trailers.
I've not known them to travel badly as such - they balance themselves quite well but some are better than others (some really stomp around as you are travelling).
They fly very well. It is quite rare for horses not to travel well by plane with really only the take off and landing being a problem. In fact if horses do get "shipping fever" it is often due to delays on the tarmac between flight legs. I also find they seem to ship better going West. You rarely get horses getting a fever going to USA but going to Asia - same time in the air - you seem to get maybe 1 in 25 that gets a temperature.
Was driving along behind an enclosed horsebox t'other day - you know the sort, looks like a proper lorry, not the ones you attach to a towbar. And I got to musing that the horses inside probably couldn't see out, as there didn't seem to be many windows. And then to thinking if they can't see where they're going, do horses get travelsick?
Well, do they??
<Disclaimer: I might have been doing a rather long drive all on my ownsome, and lacking in other things to think about >