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