Capacity Analysis of Traffic-Actuated Intersections Final Report (1996) / Chapter Skim
Currently Skimming:
APPENDIX C
APPENDIX C
Pages 79-110
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 79... ...
Non-actuated phases may be coordinated with neighboring signals on the same route, or they may function in an isolated mode without any influence Dom other signals. Non-actuated phases generally operate with fixed minimum green times which may be extended by reassigning unused green time from actuated phases with low demand, if such phases exist.
|
From page 80... ...
Actuated phases must be approximated for analysis purposes by their average green time, recognizing that the actual time may differ from cycle to cycle. For a given timing plan (i.e., constant or average green times)
|
From page 81... ...
It is common in capacity and level of seance analysis to use a "single-ring, sequential" representation of the phase plan in which a single phase is used to indicate the combination of aD movements that are proceeding at a given point in time. Modern tra~c-actuated controllers do not use this scheme.
|
From page 82... ...
It is necessary to know the average cycle length and effective green time for each lane group to be analyzed. The most desirable way to obtain these values is by field measurement, however, there are many cases when field measurement is not possible.
|
From page 83... ...
Examples of design parameters include: Tra~c-actuated controller settings (initial interval, allowable gap, maximum green time)
|
From page 84... ...
Collectively, these will be referred to as the "controller settings," because they must be physically set in the controller with switches, keypads or some other electrical means. The following settings will exert a significant influence on the operation of the intersection and must therefore be recognized by the analysis methodology: Maximum Initial Interval, MxI · Added Initial Per Actuation, AI · Minimum Allowable Gap, MnA · Gap Reduction Rate, OR Appendix C: Page 6
|
From page 85... ...
However, traffic-actuated controllers do not recognize specified cycle lengths. Instead, they determine, by a mechanical analogy, the required green time given the length of the previous red interval and the arrival rate.
|
From page 86... ...
Queue accumulation polygon illustrating two methods of green time computation Queue Service Time The queue service time, as, can be estimated as where qr r g = f qr, qg = red arrival rate (veh/sec) and green arrival rate, respectively (veh/sec)
|
From page 87... ...
Green Extension Time To estimate the extension time analytically for a particular phase, it is necessary to determine the expected waiting time for a gap of a specific length, given the average inter-vehicular headway, and some assumptions about the headway distribution. An analytical model for this purpose was descnbed by Ak,celik [1,2]
|
From page 88... ...
The logical starting point for the iterative process is the minimum times specified for each phase. If these times turn out to be adequate for all phases, the cycle length will simply be the sum of the minimum phase times for the critical phases.
|
From page 89... ...
The actuated controller settings are: Initial interval: Unit extension: Maximum green: Intergreen: 10 seconds 3 seconds 46 seconds 4 seconds The maximum phase time for each phase will be (46+4) = 50 seconds.
|
From page 90... ...
Apply Equation 3 to determine the expected waiting time: g = e Appendix C: Page 12 e A(eO+tO-~)
|
From page 91... ...
This indicates that the trial phase time was not adequate to satisfy rules under which the controller operates. It also suggests new trial green times of 25.49 seconds and a cycle length of 50.98 seconds for the next iteration.
|
From page 92... ...
Repeated iterations with this lower unit extension time would converge to a cycle length of 65.3 seconds. Minimum Phase Times The whole question of minimum phase time requires more attention.
|
From page 93... ...
an iterative computational structure that converges to a stable value for the average cycle length and green times; and (3) a procedure to account for minimum green times with low volumes.
|
From page 94... ...
Figure C-4b. Queue accumulation polygon for permitted left turns from a shared lane Appendix C: Page 16
|
From page 95... ...
Figure C-40. Queue accumulation polygon for permitted plus protected left turn phasing with an exclusive left turn lane Appendix C: Page 17
|
From page 96... ...
The actual length wait be the sum of the time required to service the queue that exists at the beginning of the phase plus the extension time. Points in the cycle at which the queue size is important to the computations are also identified as follows: Qr indicates the queue size at the end of the effective red Qq Indicates the queue size at the end of the interval gq Qp indicates the queue size at the end of the permitted green period Q'p Indicates the queue size at the end of the permitted green period, adjusted for sneakers Qga indicates the queue size at the beginning ofthe protected green (green arrow)
|
From page 97... ...
In the most common coordination scheme, a background cycle length is imposed. The actuated phases receive their allotment of green time In the usual manner, except that their maximum green times are controlled externally to ensure conformance to the specified cycle Appendix C: Page 19
|
From page 98... ...
This is accomplished by increasing the maximum green times for the coordinated phases. The recommended procedure is to increase the length of the coordinated phases by one half ofthe difference between the specified and computed cycle lengths on each iteration.
|
From page 99... ...
The length ofthe variable initial interval shown in Figure C-6 is equal to the product of the number of vehicle arrivals on red and clearance intervals and the specified time interval for each vehicle actuation. This feature increases the minimum assured green so it wail be long enough to serve the actual number of vehicles waiting for the green between the detector and the stop line.
|
From page 100... ...
The specified time for gap reduction is called time to reduce. Thus, the gap reduction rate is equal to the difference between the passage time and minimum gap settings divided by the setting of time to reduce.
|
From page 101... ...
plus variable initial interval (added initial) subjected to the constraint of specified maximum initial settings where the specified minimum green here is equal to the minimum initial interval Am)
|
From page 102... ...
is the assured green time that will be displayed. If, at the end of the initial interval, the length of the queue from the stop line does not reach the upstream detector, the final queue service time should be equal to the minimum value of the initial interval and computed queue service time.
|
From page 103... ...
Because of its importance to traffic-actuated control, it is essential that the proposed analytical model recognize this phenomenon. A set of curves was developed to illustrate the effect of the free queue parameter on the estimated phase time as a function of the approach volume.
|
From page 104... ...
Through-car equivalents, ELI, for permitted left turns in a shared lane with one free queue 0 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 I 05 1.05 1.05 1.05 1.05 1.05 Appendix C: Page 26 0.1 1.05 1.10 1.29 1.56 1.90 2.31 2.78 3.30 3.87 4.49 5 14 5.82 6.54 7.29 8.06 8.85 0.2 1.05 1.20 1.56 2.05 2.64 3.31 l 4.05 4.84 .
|
From page 105... ...
O 1 05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 0.1 0.2 05 1.05 1.1 ~ 1.2 .2 14 13 16 1.4 1.8 1.5 2.0 1.6 2.2 .7 2.4 .8 2.6 1.9 2.8 20 3.0 21 3.2 2.2 3.4 2.3 3.6 2.4 3.8 2.5 4.0 Th ~ 0.30.40.50.6 title 1.05 1.05 1.05 1.051.051.05 1.05 1.05 13 14 15 1617i8 i 9 2.0 1.6 1.8 2.0 2.22.42.6 2.8 3.0 1.9 2.2 ~ 2.5 2 83 13 4 3.7 4 0 2.2 1 2.6 1 3.o 1 3.41 3-81 42 1 46 1 5-0 2.5 3.0 3.5 4.04.55.0 5.5 6.0 2.8 3.4 40 465258 64 7.0 3.1 3.8 4.5 5.25.96.6 7.3 8.0 3.4 4.2 5.0 5.86.67.4 8.2 9.0 3.7 4.6 5 5 6 47 38.2 9.1 10.0 4.0 5.0 6.0 7.08.09.0 10.0 1 1.0 4.3 5.4 6.5 7.68.79.8 10.9 12.0 4.6 5.8 7.0 8.29.410.6 11.8 13.0 4.9 6.2 7.5 8.810.11 1.4 12.7 14.0 5.2 6.6 8.0 9.410.812.2 13.6 15.0 55 70 85 10.011.5130 14.5 160 When the new through-car equivalent of a lefc-turn vehicle is computed, the permitted saturation flow of asharedlane curing the period of unsaturated opposing flow can tee computed as follows, if a free queue exists: s 1 PL (ELI (new)
|
From page 106... ...
The leD turning vehicle in the free queue will not block the through vehicle in the shared lane, so the free green will increase. The proposed method to estimate the new free green for the free queue setting is describe as follows.
|
From page 107... ...
p L Assuming the free queue is n vehicles, (O |
From page 108... ...
Based on the proposed method, the phase time estimation with free queues is shown in Figure C-8. In this figure, the x axis represents the approach volume, y axis shows free queue values, and the vertical axis is the estimated phase times by the proposed method.
|
From page 109... ...
Effect of the free queue on phase times for the example problem For the same free queue value, the phase times will increase due to the increasing blocking effect by left turns In the shared lane. On the other hand, if the value of the Dee queue increases, the blocking effect is reduced because left tarrying vehicles are able to wait in the free queue before turning left.
|
From page 110... ...
Ak~celik R Estimation of Green Times and Cycle Time for Vehicle-Actuated Signals. Paper No.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.