Question:  I have two new signal systems about 500’ apart and speed limit is 30mph.  I worried about there would be visibility conflict between these two signals.  So, in what situation the louvered signal head is required?

 Answer:  The issue with closely spaced intersections is that the driver wating at the stop bar of the first one might see a green at the second one and start driving, even when the closer signal display is still red.  The Caltrans traffic manual had a guideline to restrict the visibility of the signal displays at the second intersection to avoid this occurrence.  It can be found in page 9-03.18 in http://www.dot.ca.gov/hq/traffops/signtech/signdel/tmrevisions/TM-03-1.pdf.  It recommends restriction of the display with Programmed Visibility heads, although louvers also work quite well, if the traffic signals are 300 feet or less apart.  Since your location has a longer separation you will have to use your judgment.  You might consider louvers on the green displays only.