Modeling Impedance Effects of Left Turners from Major Streets with Shared Short Lanes at Two-Way Stop-Controlled Intersections

At two-way stop-controlled intersections without exclusive turning lanes on major streets, all three movements (right turn, through, and left turn) must share a lane. The Highway Capacity Manual (HCM) provides a special formula to deal with this lane configuration. However, this formula cannot consider short lanes such as left-turn pockets on major streets. This paper presents a new model that accounts for shared short lanes and thus delivers more realistic results for estimating the impedance effect of left turners. The new model can also deal with the effect of the so-called back-of-queue in reality. The presented theoretical concept can be extended to include the consequences of a short left-turn pocket on major streets and the corresponding capacities of the major streets with shared short lanes. Equations are derived for calculating the influence of the length of the left-turn pocket on the capacity of minor-street movements and on the probability of blockage to the through movement resulting from queued left-turn vehicles. Comprehensive simulation studies are conducted to confirm the derivation, and a general formula that considers both the short lane and back-of-queue is recommended for further applications. This formula will be incorporated into the upcoming edition of HCM.