Turning Lanes
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​Dedicated left - and right-turn lanes may be used in situations where traffic volumes and speeds are relatively high and conflicts are likely to develop between through and turn traffic. Auxiliary lanes are an asset in promoting safety and improved traffic flow in such situations.
 
References
FHWA Benefits of Access Management Brochure (Turning Lanes)
Exclusive turning lanes for vehicles remove stopped vehicles from through traffic. Left-turn lanes at intersections substantially reduce rear-end crashes. A major synthesis of research on left-turn lanes demonstrated that exclusive turn lanes reduce crashes between 18 to 77 percent (50 percent average) and reduce rear-end collisions between 60 and 88 percent.
FHWA Benefits of Access Management Brochure (Turning Lanes)

Missouri DOT Turn Lane Guidance
Some major applications and considerations for the design of auxiliary lanes are as follows. Installing a right-turn acceleration lane. These lanes allow entering vehicles (those that have turned right from a driveway or minor public road onto the major route) to accelerate before entering the through-traffic flow. Acceleration lanes are to be considered on roadway segments, intersections and driveways with high traffic volumes where speed differential could result in unacceptable conflicts and/or delay.
Missouri DOT Turn Lane Guidance