Question:  My City has recently installed a new traffic signal.  There is a curve in the southbound approach that limits the visibility of the signal heads.  How do I know if I need a Supplemental head or not?    

 

Answer:  Chapter 4 of the California MUTCD has your guidance.  First determine your travel speed on the approach.  Then look up the minimum visibility requirement in Table 4D-1 of the CA MUTCD http://www.dot.ca.gov/hq/traffops/signtech/mutcdsupp/pdf/camutcd/CAMUTCD-Part4.pdf  .  Then go out and measure your specific location.  For your site: 

 

- The posted speed limit is 50 mph north of the signal and 45 mph south of the signal.  We used the car-following technique to determine travel speeds.  Out of the 10 cars we followed, one was traveling at 52 mph (SB), the rest all at less than 50 mph.  The slowest was 42 mph (NB).  We assume 50 mph for this dicsussion.

 

- Page 4D-19 of the California MUTCD set the guidelines: "The two signal faces required for each approach should be continuously visible to traffic approaching the traffic control signal, from a point at least the minimum sight distance indicated in Table 4D-1 in advance of and measured to the stop line. This range of continuous visibility should be provided unless precluded by a physical obstruction..."  Table 4D-1 calls for a minimum sight distance of 540 feet for 50 mph. 

 

- The southbound approach has a large radius curve to the right approaching the intersection.  There are trees on the right side of the road that block the view to the signal for drivers a long distance off.  There is an overhead flasher (Type 9) with a Signal Ahead warning sign about 500 feet in advance of the Stop Bar.  It is visible to southbound drivers a long distance ahead of the traffic signal.

 

- There are three signal displays for southbound drivers.  A Mast Arm head, a far-side-right pole mounted head, and a near-side-right pole mounted head.  All have 12" circular lenses. 

 

- The Mast Arm head is visible to southbound drivers up to 715 feet in advance of the Stop bar.  715 feet also happens to be the minimum sight distance required for a travel speed of 60 mph.

 

- The far-side-right head is fully visible to southbound drivers up to 570 feet in advance of the Stop bar.  There is a brief segment of road between 550' and 490' in advance of the stop bar where the view of this head is blocked by the near-side-right head.

 

- The near-side-right head is fully visible to southbound drivers up to 570 feet in advance of the Stop bar. 

 

CONCLUSION:  Since all traffic signal heads are 12" lenses, one traffic signal head is visible to drivers from 715 feet in advance of the intersection and at least 2 are visible from 570 feet in advance, a supplemental head does not appear to be needed.