Whistle boards/trestles

I work on a Friends crew that helps maintain the historic MOW signage on the C&T, and I'm hoping somebody can direct me to any standards or rules, if there were any, on where the DRGW placed or didn't place whistle boards at trestles. The documentation that we have from 1919 indicates there were whistle boards at some trestles (Cascade, Lobato, Chama river) but not at smaller ones nearer Antonito. We've considered practical reasons for why there'd be whistle boards at some trestles like limited (or ample) line-of-sight visibility and size, but it'd be helpful to know how the railroad made its decision, especially if that was put in writing. Thanks.
Paul