Capacity and Level of Service at Unsignalized Intersections Final Report Volume 2 - All-Way-Stop-Controlled Intersections (1996) / Chapter Skim
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Chapter Three. Theory and Models
Chapter Three. Theory and Models
Pages 11-24
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From page 11... ...
Because of the interaction between the traffic streams on each approach, and because it is this interaction Hat governs the maximum flow rate on each approach, two capacity concepts must be considered: what is the capacity of a given lane or approach given He flow rates on the other intersection approaches. Here the question is: how much can the flow on the subject approach be increased, if the flows on He other approaches remain fixed.
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From page 12... ...
At the intersection of two one-way streets, the headways of vehicles departing from the subject approach fall into one of two cases. If there are no vehicles on any of He other approaches, subject approach vehicles can enter the intersection immediately after stopping However, if there are vehicles waiting on the conflicting approach, a vehicle from the subject approach cannot enter the intersection immediately after the previous subject vehicle but must wait for consensus writhe next conflicting vehicle.
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From page 13... ...
The headway of a subject approach vehicle is also dependent on its vehicle type and its horning maneuver. This description of intersection operations must be translated into computational models or procedures that can be used to calculate the service lime, capacity, and delay for given conditions of traffic flow rates and intersection geometry.
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From page 14... ...
...................................................................................................... ., , ., , , , ~ ~ ~ ~ ~ P Y Y Y Y Y Y P Y Y P Y Y Y Y P Y Y Y Y Y Y Y __ AWSC: Capacity Models Model 1 Model 2 Model 3 AWSC Delay Models Model 1 Model 2 Model 3 Notation Y = Yes, meets criterion N = No, does not meet criterion P = Partially meets criterion AWSC Intersection Capacity Models Cap acid Mode} ~ considered only two cases faced by the subject approach driver and is thus somewhat limited In its ability to deal with a broad range of traBic conditions.
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From page 15... ...
Configuradon-Formulabon 1 , Conflicting Approach Subject Approach The service time for a vehicle assumes one of two values, based onHebert's headway measurements: so is the service time if no vehicle is waiting on the conflicting approach and s2 is the service time if a vehicle is waiting on He conflicting approach. The mean service time for vehicles on an approach is the expected value of this bi-valued distnbution.
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From page 16... ...
This circularity is based on the interdependence ofthe traffic flow on all of the intersection approaches, and shows the need for iterative calculations to obtain stable estimates of departure headway and service time, and thus, capacity.
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From page 17... ...
~e degree of saturation on any one approach exceeds one. This flow rate is tiLe maximum possible flow or throughput on the subject approach under prevailing conditions.
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From page 18... ...
For concept one, increase the flow rate on the subject approach until the degree of saturation on any approach exceeds one, maintaining the flows constant on the other approaches. For concept two, increase the flow rates on each approach, maintaining the same proportion on each approach, until the degree of saturation on any approach exceeds one.
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From page 19... ...
In this case, these flow rates would be 3600/4.4, or 815 vph, for the northbound approach and 3600/4.6, or 777 vph, for the westbound approach K the volumes are increased to the values given as maximum flow rate, the mean headways are recalculated to 5.9 seconds on bow approaches! What has been computed is the mean value of We departure headway if there are 300 vph on the northbound approach and 200 vph on the westbound approach: 4.4 seconds on the northbound approach and 4.6 seconds on the westbound approach.
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From page 20... ...
Even Tough Were is seeming spare capacity on the westbound approach, increasing the flow on the westbound approach causes the northbound approach to consume more Can 3600 seconds and Bus Me volcanoes on its approach must be reduced. Suggested definition of capacityflow rate: theflow rate on each approach such that the degree of saturation of any of the intersection approaches exceeds one.
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From page 21... ...
for Multilane Sites For multi-lane sites, separate saturation headway values have been computed for Me number of vehicles faced by the subject vehicle for each of the degree of conflict cases. This requires a further extension of Me service time mode!
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From page 22... ...
12 he = ~P[C,J ho' f=1 where Ci represents each of Me twelve degree of conflict sub-cases and h,i is Me saturation headway for that case. The iterative procedure to compute the departure headways and capacities for each approach as a function of Me depar~eheadways on the other approaches is the same as described earlier.
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From page 23... ...
23 T"b~ 2D. Pm~li~ of D~me of ConOio1 C"~l~e ~SC Intersootions (1~ ~e ~pm~ ~ns)
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From page 24... ...
Figure 4 shows an example plot of the delay equation using the value of the northbound approach senice time (2.4 seconds) fiom Me previous example.
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