232.3 Two - Way Left - Turn Lanes
|Design Exception Information Form|
Traffic congestion often occurs along arterial streets through locations of strip commercial development, where there is left turn demand to approaches along both sides of the street. One of the key benefits of two-way left-turn lanes (TWLTLs) includes the removal of turning vehicles from the through traffic lanes, thereby reducing travel time and delay by 40 to 60 percent. Two-way left-turn lanes are far less controversial than raised medians, and can be an effective traffic management tool when used under the right circumstances; however, they do have limitations.
|Two-Way Left-Turn Lane|
Two-way left-turn lanes do not function well once certain traffic volumes or recommended driveway spacing limits have been exceeded. In areas of high traffic volumes (i.e. AADT greater than 28,000) raised medians are at least 25 percent safer than multilane undivided sections and 15 percent safer than TWLTLs. Two-way left-turn lanes should only be considered in places where commercial driveways make up a substantial portion of total driveways, overall driveway density is managed, and where the percentage of vehicles turning left at peak hour is at least 20 percent.
Two-way left-turn lane configurations should not be used in areas that are expected to remain rural in the foreseeable future or on roadways with posted speeds in excess of 45 mph (70 km/h). In no case will a two-way left-turn lane be provided on a route with more than two through-traffic lanes in each direction to create a “seven lane” facility.
Criteria for use and design of TWLTLs includes the following:
|Roadway/ Traffic Conditions||Two-Way Left-Turn Lanes (Five-Lane Facilities)||Two-Way Left-Turn Lanes (Three-Lane Facilities)|
|AADT||May be used where AADT in the design year is less than 28,000; otherwise use a raised median||May be used where AADT in the design year is less than 17,500|
|Driveway Spacing||May be used when driveway spacing is 12 or less per mile in each direction(Average spacing of 440 feet)||May be used when driveway spacing is 12 or less per mile in each direction(Average spacing of 440 feet)|
Where continuous two-way left turn lanes are considered, several elements of the design must be reviewed. Higher travel speeds combined with rolling terrain can create serious safety problems if a two-way left-turn lane is used. The number of movements made in the lane can become too large, resulting in an increase in accidents or near accidents. Consequently, the speed, traffic volume and topography must be analyzed prior to installation of this type of roadway.
In addition, the following elements must be included within the design of the TWLTL: · The median lane width is at least 14 ft. (4.2 m), · A minimum of 400 ft. (120 m) of stopping sight distance is provided within the limits of the continuous two-way left-turn lane.
Roads that may be candidates for TWLTLs (either as a 3 lane or 5 lane facility) must be evaluated to determine whether accidents are a significant factor, and if, alternatively, access management strategies could be implemented to reduce accidents. All core team members must be involved in the evaluation process, and alternatives weighed against the “Purpose and Need” of the proposed improvement. If a TWLTL is the best solution for existing conditions, supporting information addressing safety concerns must be included in the conceptual plan submittal.
Designing or implementing a TWLTL on a roadway that does not warrant its use requires a design exception. Lane markings for 3 lane and 5 lane roadways shall be in accordance with 620.2.3 Other Yellow Longitudinal Pavement Markings (MUTCD Section 3B.03).