Left Turn Lanes. Probably the singl

Left Turn Lanes. Probably the single item having the most influence on intersection
operation is the treatment of left-turning vehicles. Left turn lanes are generally desirable
at most intersections. However, cost and space requirements do not permit their inclusion
in all situations. Intersection capacity analysis procedures should be used to
determine the number and use of all lanes. Left turn lanes are generally required under
two conditions: (1) when left turn design volumes exceed 20 percent of total directional
approach design volumes, and (2) when left turn design volumes exceed 100 vehicles
per hour in peak periods.
Opposing left turn lanes should be aligned opposite each other because of sight
distance limitations. They are developed in several ways depending on the available
width between opposing through lanes. Figure 2.36a shows the development required
when additional width must be generated. The additional width is normally accomplished
by widening on both sides. However, it could be done all on one side or the other. In
Fig. 2.36b, the median width is sufficient to permit the development of the left turn
lane. Figure 2.37 shows the condition where an offset left turn lane is required to
obtain adequate sight distance in wide medians.
In developing turn lanes, several types of tapers may be involved as shown in
Fig. 2.36: