Scientists in Italy recently published a study where twelve standardbred trotters were transported for 200km in three different positions: forwards, backwards, and sideways (in relation to direction of travel). Their aim was to assess the behaviour and physiological condition of the horses before and after travel, in an effort to ascertain the best direction of travel for horses.
In Australia, a majority of horses are transported facing forwards (either in a straight load or angle load float [45° angle]). There is a small amount of transport done with the horse completely sideways (i.e. 90°), and even less where the horse will be facing backwards.