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Sub-problem 3c - Page 3 of 4

ID# C103C03

Sub-problem 3c: Critical Movement Analysis

Why is there virtually no difference between estimated delay on the eastbound and westbound approaches to the intersection? The volume to capacity ratio of the intersection with a traffic signal installed is 0.35. Essentially, the intersection is operating well below capacity as you would expect because a signal was installed. The addition of heavy vehicles will reduce the saturation flow rate at the intersection. The presence of heavy vehicles may also have an effect other factors such as lost time (this will be discussed later on this page). Specifically for this intersection, the impact of 25% heavy vehicles on the traffic stream is to reduce the saturation flow rate by 20%, (the fHV for 25% trucks is 0.80). This is applied within the signalized intersection methodology and results in a new v/c ratio at the intersection of 0.43, which is approximately 20% higher than the existing scenario.

What is the effect of grade and heavy vehicles? Similar to the effect heavy vehicles have on the saturation flow rate, grades are treated by a separate factor fg.

How do changes in vehicle mix affect the intersections when the intersection operates near or at capacity? The HCM delay equation is more sensitive to changes in saturation flow rate when an intersection is near or at capacity. For instance, under post-football game traffic volumes, assuming the heavy vehicle percentage as a part of that analysis would yield the results described on the next page.

What effects do heavy vehicles have on the intersection beyond changes to saturation flow rate? Beyond design issues that must be considered for accommodating the heavy vehicles at an intersection, the presence of heavy vehicles may have an impact on the lost time, lane utilization, and arrival type at an intersection.  Heavy vehicles will have an effect on lost time at the intersection because of the acceleration characteristics of these vehicles. This will be exacerbated on a uphill grade. 

Heavy vehicles will also affect the arrival type for each approach to the intersection by increasing the amount of time that a platooned flow may arrive on a standing queue. Simulation models will explicitly model the vehicle characteristics and performance on the approach to consider the relationships between the factors.

We will now consider how to set up this problem.

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