The New Zealand Railways Magazine, Volume 2, Issue 7 (November 1, 1927)
The principal considerations are location and visibility.
Location means correct interval to allow of accommodation and train control; a situation where the best view is obtainable both by driver and signalman; a correct placing behind fouling points of conflicting routes; and protection for watering engines, etc.
Visibility means correct aspect in the case of bracket or gantry signals in relation to both height and spacing, to indicate divergent routes. The height of signals and the placing of shunting signals on posts where advisable also comes under this heading.