Question:  In a recent signal design, I needed the mast arm signal raised a bit higher for clearance and suggested connecting the MAS mount between the yellow and green indications.  A reviewer told me this is not a good idea, and I’m hoping for a more detailed reason why not. 

Answer:  Caltrans Standard Plan ES-4D (http://www.dot.ca.gov/hq/esc/oe/project_plans/highway_plans/stdplans_US-customary-units_06/viewable_pdf/rspes-04d.pdf) shows the MAS mounting but doesn’t really explain how the mount attaches to the mast arm tenon.  Photo 1 shows a close up of the mounting.  The 3/8” bolt goes all the way through the mount.  Note that on the right side the mount has a slot instead of a circular hole, which allows the signal head to be rotated to vertical. 

 


The three set screws are used to lock the mount into position after the signal head is aligned in a good vertical position.  Photo 2 shows a mast arm tenon.  Note it is pre-drilled with holes for the 3/8” bolt.

 

The set screws are not a permanent attachment.  With regular maintenance, they will keep the head safely up on the pole, but with constant wind loading they can loosen over time.  They rely on gravity to help keep the signal head vertical, so MAS mounts are usually attached between the red and amber indications so most of the weight of the head is below the mount and helps keep the head vertical.  If a head were attached between the amber and green sections, gravity would be trying to flip the head upside down.  The 3/8” bolt would not allow this to happen, but the slot in the MAS mount would allow the head to tilt.