The Federal Highway Administration (FHWA) recommends a five-section signal head tied to the adjacent through lanes for left-turn control. Many states have adopted the five-section cluster or "doghouse" signal located between the through and turning lanes, which provides a green arrow for the protected phase and a circular green to indicate the permissive phase.

Transportation professionals have chosen protected/ permitted left-turn (PPLT) phasing for many reasons, including minimizing delay, improving progression, and reducing fuel consumption and air pollution. An agency may use PPLT phasing to increase the operational efficiency of an intersection. PPLT phasing can also reduce delay for left-turning vehicles under low-to-moderate traffic volume conditions. Protected/permitted left-turn phasing is an especially effective remedy for reducing left-turning vehicle delay...
National Cooperative Highway Research Program

I’m not a traffic engineer - but I did do some homework to create this website in order to backup my opinions with facts.
I found data supporting my point of view is overwhelming.

How do traffic engineers determine when to use a left turn signal, or, especially a “permitted only left”- and how does our intersection compare?
  1. High traffic volume.
    The FixThisLight intersection traffic count is low compared to other
    intersections on the same major road that ALL have     protected/permitted left turn signals.  In fact, according to Hamilton County traffic counts - this intersection volume has actually DECREASED over the past few years! (SEE CHART BELOW)
     
  2. High accident rate related to left turns
    Again - the data just doesn't support this at this intersection.
    In the past 4 years, there have been a total of only 5 non-DUI accidents and no injuries related to left-turns. (Source: Hamilton County Engineering Department Data)
     
  3. Limited sight distance.
    Our intersection has wonderful sight distance     of over 1000 level & straight feet in both directions (See photo at top of page)
     
  4. Unusual geometry of lanes or excessive width of opposing lanes.
    Our intersection has two measly lanes to cross while turning left. It’s a nice “T” intersection at beautiful right angles.
     
  5. Posted speed limit of opposing traffic exceeds ___MPH.
    Some states that post their left-turn rules use 45MPH as the guide.  Our intersection is in a leisurely 45MPH speed limit zone. It used to be higher but I guess when they widened the road they felt the need to offset the new convenience.

Source:  Hamilton County Data Services

‘Left-turn arrows result in longer cycle lengths, which in turn have a detrimental effect by increasing stops and delays.’
A Metropolitan traffic Engineering Divisision

“...the "protected only" phasing of the left turn signal may not be the best solution at this particular location...(refering to Hixson Pike @ Dallas Hollow Road)” 
Tennessee Department of Transportation Official