Work Conservation Comparison of BP, CABP and PWBP_with downstream incident
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WHAT IS IT?
This model shows you how the Position-weighted Backpressure (PWBP) differs from the original Backpressue (BP), and the Capacity-Aware Backpressure (CABP) in terms of work conservation in presence of an incident/accident on the downstream approach.
It has two intersections: the left intersection with two signal lights is the one that uses the BP-series controllers; the right intersection with only one signal light uses fixed timing controller.
The car-following model used in this model is a collision free car-following model for connected and automated vehicles from the literature.
HOW TO USE IT
Click on the Reset button to set up the network and vehicles.
Click on BP, CABP, or PWBP to start the simulation under different controllers. Note that when you switch between different controllers, you need to first click the previous controller to pause the simulation, and then click the new controller to continue the simulation.
You can move the model speed bar to adjust the simulation speed.
THINGS TO NOTICE
We can find that eventually the BP controller always gives green to the east-bound approach, even when the ingress of the east approach is fully occupied and hence no vehicles can actually pass the intersection during green. However, if the green is given to the north-bound traffic, vehicles will be able to pass the intersection. We call this phenomenon "loss of work conservation". The reason behind this is that BP only considers the vehicle number difference between incoming and outgoing approaches, and assumes that saturation flow is always guaranteed, which is reasonable in communication network, but obviously not the case in traffic network.
CABP can somehow relieve such situation, but it gives green to the north-bound traffic only when the south approach is fully occupied, which wastes a lot of time and loses some work conservation when the south approach is not fully occupied.
PWBP manages to resolve the basic issue: the way to calculate movement flow, along with movement weight. Hence work conservation is always guranteed in PWBP, no matter there is incident/accident or not.
HOW TO CITE
If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.
For the PWBP model:
- Li, Li, and Saif Eddin Jabari. "Position weighted backpressure intersection control for urban networks." Transportation Research Part B: Methodological 128 (2019): 435-461.
For the BP model:
- Wongpiromsarn, Tichakorn, et al. "Distributed traffic signal control for maximum network throughput." 15th international IEEE conference on intelligent transportation systems. IEEE, 2012.
For the CABP model:
- Gregoire, Jean, et al. "Capacity-aware backpressure traffic signal control." IEEE Transactions on Control of Network Systems 2.2 (2014): 164-173.
For the car following model:
- Li, Li, and Wanjing Ma. "A collision-free car-following model for connected automated vehicles." Proc. 96th Transportation Research Board Annual Meeting. 2017.
Please cite the NetLogo software as:
- Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
This work was funded in part by the C2SMART Center, a Tier 1 USDOT University Transportation Center, and in part by the New York University Abu Dhabi Research Enhancement Fund.
Comments and Questions
globals [comfortable-acceleration max-deceleration Tdeta Tdc Tde Vp Pp Vf Pf Lp A1obj_lower A1obj_upper A2obj_lower A2obj_upper A3obj_lower A3obj_upper Aobj_lower Aobj_upper cor-unit tick-unit W-volume S-volume vehicle-size-in-Netlogo d L Source-Queue-W Source-Queue-S W-weight S-weight W-phase S-phase min-Green Wmu Wx-in Wx-out Smu Sx-in Sx-out Wspillbacks Sspillbacks Wsubjectcar Ssubjectcar W-almost-spillback S-almost-spillback W-flow S-flow WCars-out-approach SCars-out-approach W1-in W1-out S1-in S1-out W2-in W2-out max-xcor max-ycor W1-light-xcor W2-light-xcor S1-light-ycor Mid-light-xcor] ;;cars in different approaches breed [Wcars Wcar] breed [Scars Scar] breed [Wlight1s Wlight1] breed [Wlight2s Wlight2] breed [Slight1s Slight1] breed [Midlights Midlight] turtles-own [acceleration pre-speed speed next-speed next-dis next-dis-in-netlogo min-speed max-speed pre-position PV distance-of-PV PL spillback-same-direction spillback-other-direction block] ;; PWBP how to correctly evaluate the real discharging rate based on downstream density? Current algorithm tend to switch very frequently. to setup clear-all set max-xcor 350 set max-ycor 140 ask patches [setup-road] set Tdeta 0.1 ;;communication interval set Tdc 0.1 ;;communication delay = 0.1 s set Tde 0.1 ;;execution delay = 0.1 s set vehicle-size-in-Netlogo 5 ;; a vehicle occupies 5 patches in Netlogo is set cor-unit 5 ;;suppose 1 cor = 5 m set L vehicle-size-in-Netlogo * cor-unit ;;vehicle length in real world set d 3 ;; the safety stop distance set Lp L + d ;; vehicle space headway when stop set tick-unit 0.1 ;;suppose 1 tick = 0.1 s ;;hence 1 cor/tick = 5 m/ 0.1 s = 50 m/s, 0.1 cor/tick = 5 m/s, and 0.01 cor/tick2 = 5 m/s2 set comfortable-acceleration 2 ;;comfortable acceleration is 2 m/s2 set max-deceleration -6 ;;max-deceleration is -6 m/s2 set W-volume 3600 ;;volume of west approach set S-volume 200 ;;volume of south approach set min-Green 10 ;;minimum green is 10 s set Source-Queue-W 0 ;;initialize the source queue at west approach set Source-Queue-S 0 ;;initialize the source queue at south approach set W-weight 0 ;;initialize the weight of west approach set S-weight 0 ;;initialize the weight of south approach set W-phase 0 ;;initialize the state of west phase set S-phase 0 ;;initialize the state of south phase set-default-shape turtles "new-car" setup-lights reset-ticks end to setup-road ;; patch procedure set W1-in 190 set W1-out 195 set S1-in 70 set S1-out 75 set W2-in 270 set W2-out 275 if (pycor > S1-out) or (pycor < S1-in) [set pcolor gray] if (pycor <= S1-out) and (pycor >= S1-in) [set pcolor white] if (pxcor <= W1-out) and (pxcor >= W1-in) [set pcolor white] if (pxcor <= W2-out) and (pxcor >= W2-in) [set pcolor white] end to setup-lights create-Wlight1s 1[ set shape "cylinder" set color red set W1-light-xcor (W1-in - 3) set xcor W1-light-xcor set ycor (S1-in + S1-out) / 2 set heading 90 set size 5] create-Wlight2s 1[ set shape "cylinder" set color red set W2-light-xcor (W2-in - 3) set xcor W2-light-xcor set ycor (S1-in + S1-out) / 2 set heading 90 set size 5] create-Slight1s 1[ set shape "cylinder" set color green set xcor (W1-in + W1-out) / 2 set S1-light-ycor (S1-in - 3) set ycor S1-light-ycor set heading 0 set size 5] create-Midlights 1[ set shape "triangle 2" set color yellow set Mid-light-xcor (W1-out + 35) set xcor Mid-light-xcor set ycor (S1-in + S1-out) / 2 set heading 90 set size 8] end to go-PWBP operate-lights-PWBP generate-vehicles car-run-CFFM tick end to go-BP operate-lights-BP generate-vehicles car-run-CFFM tick end to go-CABP operate-lights-CABP generate-vehicles car-run-CFFM tick end to generate-vehicles ;; sprout new cars in western approach if ticks mod int((3600 / W-volume) / tick-unit) = 0 ;;volume is the traffic demand for each lane [set Source-Queue-W (Source-Queue-W + 1)] let near-origin-Wcars turtles with [xcor < 8] if Source-Queue-W > 0 and (count near-origin-Wcars) = 0 [set Source-Queue-W (Source-Queue-W - 1) ask patch 0 round((S1-in + S1-out) / 2) [sprout-Wcars 1 [ set color blue set size vehicle-size-in-Netlogo set ycor (S1-in + S1-out) / 2 set heading 90 set speed 5 set pre-speed speed set pre-position xcor * cor-unit set max-speed 20 set min-speed 0]]] ;; sprout new cars in southern approach if ticks mod int((3600 / S-volume) / tick-unit) = 0 [set Source-Queue-S (Source-Queue-S + 1)] let near-origin-Scars turtles with [ycor < 8] if Source-Queue-S > 0 and (count near-origin-Scars) = 0 [set Source-Queue-S (Source-Queue-S - 1) ask patch round((W1-in + W1-out) / 2) 0 [sprout-Scars 1 [ set color blue set size vehicle-size-in-Netlogo set xcor (W1-in + W1-out) / 2 set heading 0 set speed 5 set pre-speed speed set pre-position ycor * cor-unit set max-speed 20 set min-speed 0]]] end to car-run-CFFM ask Wcars [ ifelse xcor >= world-width - 1 [die] [ifelse any-PV? [follow-PV-CFFM] [speed-up-CFFM]]] ask Wcars [ set pre-speed speed set pre-position xcor * cor-unit fd next-dis-in-netlogo set speed next-speed] ask Scars [ ifelse ycor >= world-height - 1 [die] [ifelse any-PV? [follow-PV-CFFM] [speed-up-CFFM]]] ask Scars [ set pre-speed speed set pre-position ycor * cor-unit fd next-dis-in-netlogo set speed next-speed] end to-report any-PV? let myhd heading let front-cars turtles with [(((distance myself) > 0 and color = blue) or ((distance myself) > 5 and color = red) or ((distance myself) > 5 and color = yellow and block = 1) or ((distance myself) > 5 and color = green and (spillback-same-direction = 1 or spillback-other-direction = 1))) and (towards myself) != myhd and heading = myhd] let front-lights turtles with [(((distance myself) > 5 and color = red) or ((distance myself) > 5 and color = yellow and block = 1) or((distance myself) > 5 and color = green and (spillback-same-direction = 1 or spillback-other-direction = 1))) and (distance myself) <= 15 and (towards myself) != myhd and heading = myhd] set PV (min-one-of front-cars [distance myself]) set PL (min-one-of front-lights [distance myself]) ifelse PV != nobody [set distance-of-PV distance PV ifelse distance-of-PV <= 15 [if PL != nobody [if PL != PV [set PV PL ]] report true] [report false]] [report false] end to follow-PV-CFFM set Vp [pre-speed] of PV set Pp [pre-position] of PV ;;calculate the distance travelled in this cycle ifelse speed + acceleration * Tdeta < min-speed [let t-deceleration (min-speed - speed) / acceleration set next-dis speed * t-deceleration + 0.5 * acceleration * t-deceleration * t-deceleration + min-speed * (Tdeta - t-deceleration)] [ifelse speed + acceleration * Tdeta > max-speed [let t-acceleration (max-speed - speed) / acceleration set next-dis speed * t-acceleration + 0.5 * acceleration * t-acceleration * t-acceleration + max-speed * (Tdeta - t-acceleration)] [set next-dis speed * Tdeta + 0.5 * acceleration * Tdeta * Tdeta]] set next-dis-in-netlogo next-dis / cor-unit ;;update the speed at the beginning of next cycle ifelse speed + acceleration * Tdeta < min-speed ;; to avoid speed < min-speed [set next-speed min-speed] [ifelse speed + acceleration * Tdeta > max-speed ;; to avoid speed > max-speed [set next-speed max-speed] [set next-speed speed + acceleration * Tdeta]] set Vf next-speed ifelse heading = 90 [set Pf xcor * cor-unit + next-dis] [set Pf ycor * cor-unit + next-dis] ;;calculate the acceleration during next cycle let Vfmax max-speed let apmax- max-deceleration let afmax- max-deceleration let afmax+ comfortable-acceleration let A1 Vf * Vf / -2 let B1 Pp + Vp * Vp / (2 * apmax-) - Pf - Lp let A2 Tdeta * Tdeta / (-2 * afmax-) let B2 Tdeta * Tdeta / 2 - Vf * Tdeta / afmax- let C2 Pf + Lp + Vf * Tdeta - Pp + Vp * Vp / (2 * apmax-) - Vf * Vf / (2 * afmax-) let A3 (Vfmax - Vf) * (Vfmax - Vf) / -2 let B3 Vfmax * Vfmax / (2 * afmax-) - Vp * Vp / (2 * apmax-) - Vfmax * Tdeta + Pp - Pf - Lp ifelse -1 * (Vf / Tdeta) > afmax- ;;calculate the solution of segment 1 [ifelse B1 > 0 [ifelse A1 / B1 >= -1 * (Vf / Tdeta) [set A1obj_lower afmax- set A1obj_upper -1 * (Vf / Tdeta)] [ifelse A1 / B1 >= afmax- [set A1obj_lower afmax- set A1obj_upper A1 / B1] [set A1obj_lower 10 set A1obj_upper -10]]] [ifelse B1 < 0 [set A1obj_lower 10 set A1obj_upper -10] [ifelse Vf = 0 [set A1obj_lower afmax- set A1obj_upper -1 * (Vf / Tdeta)] [set A1obj_lower 10 set A1obj_upper -10]]]] [set A1obj_lower 10 set A1obj_upper -10] ifelse B2 * B2 - 4 * A2 * C2 >= 0 ;;calculate the solution of segment 2 [ifelse (B2 + sqrt (B2 * B2 - 4 * A2 * C2)) / (-2 * A2) >= (Vfmax - Vf) / Tdeta or (sqrt (B2 * B2 - 4 * A2 * C2) - B2) / (2 * A2) < (-1 * Vf) / Tdeta [set A2obj_lower 10 set A2obj_upper -10] [ifelse max (list ((B2 + sqrt (B2 * B2 - 4 * A2 * C2)) / (-2 * A2)) ((-1 * Vf) / Tdeta) (afmax-)) <= min (list ((sqrt (B2 * B2 - 4 * A2 * C2) - B2) / (2 * A2)) ((Vfmax - Vf) / Tdeta) (afmax+)) [set A2obj_lower max (list ((B2 + sqrt (B2 * B2 - 4 * A2 * C2)) / (-2 * A2)) ((-1 * Vf) / Tdeta) (afmax-)) set A2obj_upper min (list ((sqrt (B2 * B2 - 4 * A2 * C2) - B2) / (2 * A2)) ((Vfmax - Vf) / Tdeta) (afmax+))] [set A2obj_lower 10 set A2obj_upper -10]]] [set A2obj_lower 10 set A2obj_upper -10] ifelse (Vfmax - Vf) / Tdeta < afmax+ ;;calculate the solution of segment 3 [ifelse B3 < 0 [ifelse A3 / B3 > (Vfmax - Vf) / Tdeta [set A3obj_lower (Vfmax - Vf) / Tdeta set A3obj_upper min (list (A3 / B3) (afmax+))] [set A3obj_lower 10 set A3obj_upper -10]] [set A3obj_lower (Vfmax - Vf) / Tdeta set A3obj_upper afmax+]] [set A3obj_lower 10 set A3obj_upper -10] ifelse A3obj_lower <= A3obj_upper ;; obtain Aobj [set Aobj_upper A3obj_upper ifelse A2obj_lower <= A2obj_upper [ifelse A1obj_lower <= A1obj_upper [set Aobj_lower A1obj_lower] [set Aobj_lower A2obj_lower]] [set Aobj_lower A3obj_lower]] [ifelse A2obj_lower <= A2obj_upper [set Aobj_upper A2obj_upper ifelse A1obj_lower <= A1obj_upper [set Aobj_lower A1obj_lower] [set Aobj_lower A2obj_lower]] [ifelse A1obj_lower <= A1obj_upper [set Aobj_lower A1obj_lower set Aobj_upper A1obj_upper] [set Aobj_lower 10 set Aobj_upper -10]]] ifelse Aobj_lower <= Aobj_upper [set acceleration min (list (afmax+) (Aobj_upper))] [set acceleration afmax-] end to speed-up-CFFM ifelse speed + acceleration * Tdeta < min-speed [let t-deceleration (min-speed - speed) / acceleration set next-dis speed * t-deceleration + 0.5 * acceleration * t-deceleration * t-deceleration + min-speed * (Tdeta - t-deceleration)] [ifelse speed + acceleration * Tdeta > max-speed [let t-acceleration (max-speed - speed) / acceleration set next-dis speed * t-acceleration + 0.5 * acceleration * t-acceleration * t-acceleration + max-speed * (Tdeta - t-acceleration)] [set next-dis speed * Tdeta + 0.5 * acceleration * Tdeta * Tdeta]] set next-dis-in-netlogo next-dis / 5 ;;update the speed at the beginning of next cycle ifelse speed + acceleration * Tdeta < min-speed ;; to avoid speed < min-speed [set next-speed min-speed] [ifelse speed + acceleration * Tdeta > max-speed ;; to avoid speed > max-speed [set next-speed max-speed] [set next-speed speed + acceleration * Tdeta]] set acceleration comfortable-acceleration end to operate-lights-PWBP operate-other-lights capture-spillback-other-direction capture-spillback-same-direction if ticks mod (min-Green / tick-unit) = 0 [calculate-weight-PWBP operate-main-lights] end to operate-lights-BP operate-other-lights capture-spillback-other-direction capture-spillback-same-direction if ticks mod (min-Green / tick-unit) = 0 [calculate-weight-BP operate-main-lights] end to operate-lights-CABP operate-other-lights capture-spillback-other-direction capture-spillback-same-direction if ticks mod (min-Green / tick-unit) = 0 [calculate-weight-CABP operate-main-lights] end to operate-other-lights ifelse ticks mod (90 / tick-unit) <= (80 / tick-unit) ;; the cycle for this light is 90, with 80 green [ask Wlight2s[set color green]] [ask Wlight2s[set color red]] ifelse ticks mod (9 / tick-unit) <= (4 / tick-unit) ;; the cycle for this light is 9, with 4 green [ask Midlights[set block 0]] [ask Midlights[set block 1]] end to operate-main-lights ifelse W-phase = 1 [ask Wlight1s[set color green]] [ask Wlight1s[set color red]] ifelse S-phase = 1 [ask Slight1s[set color green]] [ask Slight1s[set color red]] end to capture-spillback-other-direction set Wspillbacks Wcars with [xcor >= W1-light-xcor and xcor <= (W1-out + 2)] ifelse any? Wspillbacks [ask Slight1s[set spillback-other-direction 1]] [ask Slight1s[set spillback-other-direction 0]] set Sspillbacks Scars with [ycor >= S1-light-ycor and ycor <= (S1-out + 2)] ifelse any? Sspillbacks [ask Wlight1s[set spillback-other-direction 1]] [ask Wlight1s[set spillback-other-direction 0]] end to capture-spillback-same-direction set Wsubjectcar min-one-of (Wcars with [xcor >= (W1-light-xcor - 15) and xcor <= W1-light-xcor]) [xcor] if Wsubjectcar != nobody [let xtemp [xcor] of Wsubjectcar set W-almost-spillback Wcars with [xcor > xtemp and xcor < Mid-light-xcor] ifelse count W-almost-spillback >= round((Mid-light-xcor - W1-out) / 5.6) ;;(W2-light-xcor - W1-out) / 5.6 calculates the jam vehicle number on the approach, floor is for conservation [ask Wlight1s[set spillback-same-direction 1]] [ask Wlight1s[set spillback-same-direction 0]]] set Ssubjectcar min-one-of (Scars with [ycor >= (S1-light-ycor - 15) and ycor <= S1-light-ycor]) [ycor] if Ssubjectcar != nobody [let ytemp [ycor] of Ssubjectcar set S-almost-spillback Scars with [ycor > ytemp and ycor < max-ycor] ifelse count S-almost-spillback >= (round((max-ycor - S1-out) / 5.6) - 1) [ask Slight1s[set spillback-same-direction 1]] [ask Slight1s[set spillback-same-direction 0]]] end to calculate-weight-PWBP let W-weight-in (sum [xcor] of Wcars with [xcor >= 0 and xcor <= W1-in]) / (W1-in - 0) let W-weight-out (W2-in * (count Wcars with [xcor >= W1-out and xcor <= W2-in]) - (sum [xcor] of Wcars with [xcor >= W1-out and xcor <= W2-in])) / (W2-in - W1-out) set W-weight (W-weight-in - W-weight-out) let S-weight-in (sum [ycor] of Scars with [ycor >= 0 and ycor <= S1-in]) / (S1-in - 0) let S-weight-out (max-ycor * (count Scars with [ycor >= S1-out and ycor <= max-ycor]) - (sum [ycor] of Scars with [ycor >= S1-out and ycor <= max-ycor])) / (max-ycor - S1-out) set S-weight (S-weight-in - S-weight-out) ifelse W-phase = 1 [set Wmu floor(min-Green / 1.7)] ;;according to observation, the start-up lost time is 4.5s, and the saturated headway is 1.7s. [set Wmu 1 + floor((min-Green - 4.5) / 1.7)] set Wx-in count Wcars with [xcor <= W1-in and xcor >= (W1-in - Wmu * 6.6 - 10)] ;;6.6 = 5.6 + 1, a vehicle takes up 5.6 cor when saturated, 1 cor is considered as a buffer in case some vehicles could catch up, 10 is the distance from the first vehicle to the stop line set WCars-out-approach (Wcars with [xcor >= W1-out and xcor <= (W1-out + 5.6 * (min list Wmu Wx-in))]) ifelse any? WCars-out-approach [let Wmean-speed mean [speed] of WCars-out-approach let Wmean-density (count WCars-out-approach) / (5.6 * (min list Wmu Wx-in) * cor-unit) set Wx-out Wmean-speed * Wmean-density * min-Green] [set Wx-out min list Wmu Wx-in] set W-flow min (list Wmu Wx-in Wx-out) ifelse S-phase = 1 [set Smu floor(min-Green / 1.7)] [set Smu 1 + floor((min-Green - 4.5) / 1.7)] set Sx-in count Scars with [ycor <= S1-in and ycor >= (S1-in - Smu * 6.6 - 10)] set SCars-out-approach (Scars with [ycor >= S1-out and ycor <= (S1-out + 5.6 * (min list Smu Sx-in))]) ifelse any? SCars-out-approach [let Smean-speed mean [speed] of SCars-out-approach let Smean-density (count SCars-out-approach) / (5.6 * (min list Smu Sx-in) * cor-unit) set Sx-out Smean-speed * Smean-density * min-Green] [set Sx-out min list Smu Sx-in] set S-flow min (list Smu Sx-in Sx-out) ifelse W-weight * W-flow > S-weight * S-flow [set W-phase 1 set S-phase 0] [ifelse W-weight * W-flow < S-weight * S-flow [set W-phase 0 set S-phase 1] [let x random 2 ifelse x = 0 ;;breaking ties arbitrarily [set W-phase 1 set S-phase 0] [set W-phase 0 set S-phase 1]]] end to calculate-weight-BP let W-weight-in (count Wcars with [xcor >= 0 and xcor <= W1-in]) let W-weight-out (count Wcars with [xcor >= W1-out and xcor <= W2-in]) set W-weight max list (W-weight-in - W-weight-out) 0 let S-weight-in (count Scars with [ycor >= 0 and ycor <= S1-in]) let S-weight-out (count Scars with [ycor >= S1-out and ycor <= max-ycor]) set S-weight max list (S-weight-in - S-weight-out) 0 ifelse W-weight > S-weight [set W-phase 1 set S-phase 0] [ifelse W-weight < S-weight [set W-phase 0 set S-phase 1] [let x random 2 ifelse x = 0 ;;breaking ties arbitrarily [set W-phase 1 set S-phase 0] [set W-phase 0 set S-phase 1]]] end to calculate-weight-CABP let WCa round((W1-light-xcor - 0) / 5.6) ;; the maximum vehicles when in jam density let WCb round((W2-light-xcor - W1-out) / 5.6) let Cinf 500 let m 4 let WQa (count Wcars with [xcor <= W1-in]) let W-weight-in min list 1 (WQa / Cinf + (2 - WCa / Cinf) * (WQa / WCa) ^ m) / (1 + (WQa / WCa) ^ (m - 1)) let WQb (count Wcars with [xcor >= W1-out and xcor <= W2-in]) let W-weight-out min list 1 (WQb / Cinf + (2 - WCb / Cinf) * (WQb / WCb) ^ m) / (1 + (WQb / WCb) ^ (m - 1)) set W-weight max list (W-weight-in - W-weight-out) 0 let SCa round((S1-light-ycor - 0) / 5.6) let SCb round((max-ycor - S1-out) / 5.6) let SQa (count Scars with [ycor >= 0 and ycor <= S1-in]) let S-weight-in min list 1 (SQa / Cinf + (2 - SCa / Cinf) * (SQa / SCa) ^ m) / (1 + (SQa / SCa) ^ (m - 1)) let SQb (count Scars with [ycor >= S1-out and ycor <= max-ycor]) let S-weight-out min list 1 (SQb / Cinf + (2 - SCb / Cinf) * (SQb / SCb) ^ m) / (1 + (SQb / SCb) ^ (m - 1)) set S-weight max list (S-weight-in - S-weight-out) 0 ifelse W-weight > S-weight [set W-phase 1 set S-phase 0] [ifelse W-weight < S-weight [set W-phase 0 set S-phase 1] [let x random 2 ifelse x = 0 ;;breaking ties arbitrarily [set W-phase 1 set S-phase 0] [set W-phase 0 set S-phase 1]]] end
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