A two-way left turn lane may be considered a special type of “intersection” design, since its purpose is to provide a separate lane for traffic in both opposing lanes to slow down and turn out of the traffic stream in front of opposing traffic. Rather than concentrate the left turners at a single crossroad intersection, the two-way left turn lane spreads out the turning movements over a continuous stretch of roadway. Mid-block left turns are often a serious problem in urban and suburban areas. They can be a safety problem due to angle accidents with opposing traffic as well as rear-end accidents with traffic in the same direction. Midblock left turns also restrict capacity. Two-way left turn lanes (TWLTLs) have proven to be a safe and cost-effective solution to this problem. TWLTLs should be considered whenever actual or potential mid-block conflicts occur. This is particularly true when accident data indicate a history of mid-block left turn-related accidents. Closely spaced driveways, strip commercial development, and multiple-unit residential land use along the corridor are other indicators of the possible need for a TWLTL. Some guidelines that may be used to justify the use of TWLTLs are listed below:
• 10,000 to 20,000 vehicles per day for four-lane highways
• 5000 to 12,000 vehicles per day for two-lane highways
• 70 mid-block turns per 1000 ft (305 m) during peak hour
• Left turn peak hour volume 20 percent or more of total volume
• Minimum reasonable length of 1000 ft (305 m) or two blocks
Widths for TWLTLs are preferably the same as through lane widths. Lane widths may be reduced to as little as 10 ft (3.0 m) in restricted areas. Care should be taken not to make a TWLTL wider than 14 ft (4.3 m), since this may encourage shared side-by-side use of the lane.
