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Subproblem 4a - Page 2 of 4

ID# C304A02

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 must recognize that the eastbound through traffic is the only opposing movement in the first part of the two stage movement. The analysis that follows considers the first stage in evaluating the capacity of the northbound Krome Avenue left-turn movement. It should be noted that this is a simplification and may not consider the effect of vehicles in the median, which can prevent northbound left-turn traffic from even initiating the first-stage movement. 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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