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Sub-problem 1b: Maxwell Drive PM Peak Hour - With Conditions
Scenario P-1 is a good place to start our
discussion. It represents a legitimate option. The left turns are protected and the lanes are configured in
a logical way. (If there are no right- or left-turn lanes, the model assumes
the movements are shared with the through movement.) The model estimates a cycle
length of 171.4 seconds (for a condition where cycle length is the minimum
possible). That’s too long. If the number of northbound and southbound
through lanes is increased to 3, as in Scenario P-2, nothing
happens. That’s because the through movement is not the critical movement,
and so it does not determine the cycle length. If you reconfigure the north and
southbound approaches so there are two left-turn lanes and one
through-and-right lane, the cycle length drops to 104.5 seconds. This is
better, but it assumes it's possible to have simultaneous opposing dual lefts, which
isn’t allowed in New York State. If permissive lefts are assumed, the model drops the
cycle length to 18.5 seconds. That’s too short for most
practical situations. The model is reporting the shortest cycle
length that puts the intersection at capacity using the
equations used to set the signal timings. A more practical value, like 80
seconds, makes the model show that the intersection
is under capacity for that cycle length. At a cycle length of 20 seconds,
the model shows the intersection is near capacity and, with cycle lengths
of 25 seconds or
above, it shows the intersection operating under capacity.
Now we’ll do some sensitivity analyses, mostly
to illustrate trends. If we assume dual lefts northbound and southbound,
along with exclusive lanes for the throughs and rights, the cycle length
drops to 76.5 seconds (from the previous 104.5). If we add dual lefts and
three through lanes eastbound and westbound, the cycle length drops to 38.5
seconds.
If we take that same lane configuration and consider a more reasonable cycle
length, say 45 seconds, the model shows the intersection is near capacity.
If we go back to single left-turn lanes northbound and southbound, the model
calculates a cycle length of 53.5 seconds.
Discussion: |
Notice
that we only kept one dataset for all of the planning analysis runs. You can
generate the other results very easily. Simply change the lane configuration
in the dataset. You might explore other options, too. See if you think the
trends in the cycle lengths identified are reasonable given the
configuration options you explore.