Sub-problem 4a: Northbound Left-Turn Capacity

Step 2: Results

What volume-related factors affect the northbound left-turn movement capacity? The basic relationship between movement capacity is defined by the conflicting flow rate and the driver characteristics (critical gap and follow-up time). Using what are widely considered default values for critical gap and follow up time as described in HCM Exhibit 17-5, one can graphically represent the relationship as shown below (which is similar to HCM Exhibit 17-7). This exhibit shows how the capacity for the stopped movement decreases as the conflicting volume (flow ratio on the x axis) increases. At very high levels of conflicting traffic, the capacity for the stopped movement becomes effectively zero because the availability of acceptable gaps is eliminated.

What geometry-related factors affect the northbound left-turn movement capacity? How could we take into account the separation of the roadways? A review of the aerial shows that the median space provides a potential refuge for vehicles that use two-stage gap acceptance. The right-turning traffic is removed from the intersection. Thus, consideration of the northbound left-turn movement becomes the eastbound through traffic as the only opposing movement in consideration of the first part of the two stage movement. The analysis that follows considers the first stage in consideration of the capacity of the northbound Krome Avenue left-turn movement. It should be noted that this is a simplification and may not consider the operation of vehicles in the median blocking northbound left-turn traffic from initiating this first stage of the two-stage gap acceptance. In this example, it is clear that the primary conflict under these traffic conditions is the eastbound through movement which is significantly higher than the westbound through movement.

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