Sub-problem 3c - Page 3 of 4
Sub-problem 3c: Critical Movement
Why is there virtually no difference
between estimated delay on the eastbound and westbound approaches to 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
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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