REVIEW 3 major objections 4 minor 40 references
An Exact System Optimum Assignment Model for Transit Demand Management
T0 review · 3 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read Exact system-optimum transit assignment shows congestion-reduction potential of 36.35%, more than double the approximate estimate of 17.39%.
desk verdict A useful linearization insight and a novel model combination, but the headline 'exact SO' result is really the optimum of a restricted no-denied-boarding model and the cost parameter table is internally inconsistent. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is the 'no denied boarding' simplification: inequality (8), which normally requires the number of boarding passengers to be no more than those waiting and is resolved by simulation, is replaced by equality (25), which states that every passenger waiting for a train boards it. With the denied-boarding variables set to zero, all remaining constraints are linear in the space-time flow variables (passenger counts grouped by origin-destination pair, route, and train), and the objective in Eq. (24) is a weighted linear sum of in-vehicle, waiting, and early/late costs. The result is an integer linear program that can be handed to a standard integer-linear solver; this is why the paper can claim global optimality rather than the local optimum produced by simulation-based approximate methods.
What would settle it
Optimize the same Hong Kong MTR instance with the denied-boarding variables restored (replace equality (25) by inequality (8)); if the resulting system cost is below $447,780, the no-denied-boarding constraint is binding and the paper's 'exact SO' is not the global optimum of the original problem.
Extended reading notes
Core claim
The paper's central claim, on its own terms, is that the system optimum for the schedule-based transit assignment problem can be computed exactly in realistic settings: hard train capacity, multiple interacting lines, and passenger flows that compete over both departure times and routes. It achieves this by assuming no denied boarding in the system optimum, so the boarding inequality becomes an equality, the denied-boarding variables drop out, and the model becomes an integer linear program with a linear cost objective. Solving that model for the Hong Kong MTR case study gives a minimum system cost of $447,780, which is 36.35% lower than the user-equilibrium solution, whereas an approximate SO solution suggested only a 17.39% reduction. The exact solution also identifies different origin-destination pairs as the best targets for shifting passengers, for example OD '3-13' with $22.05 of system cost reduction per passenger shifted.
Load-bearing premise
The system optimum is assumed to have no denied boarding: every passenger waiting for a train can board it, so the optimum is found under that extra constraint; if the true optimum would leave some passengers on the platform, the computed solution is the optimum of a restricted problem rather than the exact SO.
Editorial extensions
If this is right
- If the exact SO claim is correct, the Hong Kong MTR case study's congestion-reduction potential is 36.35%, more than double the 17.39% reported by approximate SO, so policies built on approximate optima understate what is achievable.
- The exact SO solution identifies which origin-destination pairs deserve the most attention: in the case study, OD '3-13' contributes the most to system cost and gives the largest per-passenger improvement ($22.05).
- Approximately 80% of passengers who need to change their departure time can be shifted within a 30-minute window, and about 94% within a 60-minute window, suggesting individualized incentives are practical.
- The improvement ratio (1 - SO/UE) grows from 36% at current demand to 53% at 165% demand, while capacity level matters less for the SO than for the UE, because the SO has no denied-boarding waits.
- The exact SO model remains solvable when extra routes and higher demand are added, with runtimes around 41-64 seconds on the test network.
Reading between the lines
- The no-denied-boarding constraint means the computed $447,780 is the optimum of a restricted problem; if the unconstrained system optimum would ever leave passengers on the platform, the true SO cost would be no higher, so the 36.35% figure may be conservative, but the computed plan would then not be directly implementable.
- Reintroducing the denied-boarding variables as integers would yield a harder but still exact model for the unconstrained SO; the paper does not attempt this, leaving the size of the gap from the constraint as an open question.
- As a routine check before incentive design, one could run the same UE-versus-exact-SO comparison on any network with timetable, capacity, and smart-card demand data; the model requires no simulation and no behavioral parameters beyond the cost weights.
- A testable prediction of the paper is that if passengers followed the SO schedule, platform queues would disappear; an experiment or agent-based simulation with the SO plan could check whether the predicted load redistribution actually removes denied boarding.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an exact system optimum (SO) model for the schedule-based transit assignment problem (STAP) with hard train capacity, multi-line networks, and simultaneous departure-time and route choice. The model is formulated as an integer linear program under a no-denied-boarding assumption and solved with Gurobi via MiniZinc. In a case study on part of the Hong Kong MTR network (5 lines, 12 OD pairs, 52,717 passengers), the authors report an exact-SO cost of $447,780, which is 36.35% lower than the user equilibrium (UE) cost and more than double the 17.39% reduction estimated by their approximate SO (Approx. SO) method. The paper also presents sensitivity analyses for capacity and demand levels and policy insights on passenger shifting and stop-skipping.
Significance. If the reported numbers were reliable, the claim that exact SO reveals a much larger congestion-reduction potential than approximate methods would be of practical value for transit demand management. The paper also demonstrates a fast integer-linear formulation and provides a detailed formal appendix and algorithm description. However, the current formulation solves a restricted, no-denied-boarding problem rather than the original STAP, and the cost parameter table is inconsistent with the model equations, so the central quantitative claims are not reproducible as stated. The contribution may be salvageable if reframed as a policy-constrained SO benchmark, but the present version overstates what it computes.
major comments (3)
- [Section IV-B, Eq. (25)] The simplification f^k_ne(s,l_t)(t,r) = g^k_s(t,r) and the setting db_k_s(t)=0 are extra constraints, not consequences of system optimality. The original model in Eqs. (1)-(20) includes denied boarding as a feasible state with its own cost in Eq. (17), so the feasible set of the SO problem is strictly restricted. When cumulative demand on a leg exceeds cumulative train capacity over the modeling horizon, the no-denied-boarding model is infeasible even though the original problem has a finite optimum. Therefore the reported $447,780 and the 36.35% improvement in Section VI-B are the optimum of a constrained problem, and they are an upper bound on the true SO cost of the original STAP, not the exact SO potential. The abstract and conclusion consequently overstate the comparison with Approx. SO.
- [Section VI-A, Table V vs. Eqs. (13), (15), (17)] Table V labels α as the waiting cost per unit time ($18/h) and β as the in-vehicle cost per unit time ($6/h), but Eqs. (13) and (15) define α as the in-vehicle cost and β as the waiting cost, and Eq. (17) uses β for denied-boarding waiting cost. The text says waiting time is weighted as three times in-vehicle time, which matches the table's values but not the equation labels. This internal inconsistency means the reported system costs, the percentages 17.39% and 36.35%, and the policy comparisons are not reproducible from the stated parameterization and must be corrected.
- [Section V-B / Section VI-B, model comparison] The comparison between Approx. SO and exact SO is asymmetric: the Approx. SO model (Section V-B) is evaluated with the simulation that allows denied boarding, whereas the exact SO model prohibits it. Thus the difference between 17.39% and 36.35% reflects a difference in problem definition as well as in solution quality. If the no-denied-boarding variant is the intended contribution, the paper should clearly frame it as a policy-constrained SO benchmark and not as the exact SO of the original STAP; alternatively, the exact SO should be solved with the denied-boarding variables included.
minor comments (4)
- [Section I, Introduction] There is a duplicated word in the sentence "This UE approach is useful for for approximating a system state" (second column, end of first paragraph).
- [Section III-C, Eq. (8)] The sentence "However the actual number that board the train - and the number who fail to board it - is predicted by a simulation rather than a mathematical constraint" is misleading because Appendix A provides formal constraints with a simulation variable; please clarify that simulation supplies the value of the boarding variable in the UE/Approx. SO models.
- [Appendix A, Eq. (A.3)] The condition (i=1 ∧ i=tct_l+1) in Eq. (A.3) is never satisfied, so the equation incorrectly states that the denied-boarding variable is never forced to zero for the first train or the last train-plus-one; it should use a logical OR (i=1 ∨ i=tct_l+1), consistent with Eqs. (5) and (6).
- [Table I] The last two rows of Table I are difficult to read because the checkmarks are not aligned with the column headers "UE", "Approx. SO", and "Exact SO"; please reformat the table for clarity.
Circularity Check
No significant circularity: the 36.35% figure is the computed optimum of an explicitly restricted MILP, not a refit of the input data; the main concerns are modeling restrictions and an internal label swap, not circular derivation.
full rationale
The derivation chain is self-contained. The exact SO model is formulated as an integer-linear program (Section V-C) using the objective in Eq. (24) and constraints Eqs. (1), (4)-(7), (9)-(20), (25)-(27). The cost coefficients (Table V) are taken from external sources [36] and [37]; no parameter is fitted to reproduce the UE, Approx. SO, or exact SO system costs, and the claimed 36.35% reduction is an output of the solver, not an input. Self-citations ([27], [28], [29], [30]) support simulation and algorithm components but are not load-bearing for the central claim: the exact SO optimum is produced by Gurobi on a MiniZinc model, an independent computational check. The one passage that could be mistaken for circularity is Section IV-B, where the paper asserts "in a system optimal solution, therefore, it is required that all passengers waiting for a train will be able to board it" and replaces Eq. (8) by Eq. (25). This is a modeling restriction, not a circular derivation: it constrains the feasible region of the original STAP, so the reported optimum is the exact optimum of the restricted no-denied-boarding model rather than of the full STAP with denied boarding. That is a validity or scope limitation, not a reduction of the result to its inputs. The comparison with Approx. SO is also asymmetric because Approx. SO permits denied boarding while the exact model forbids it, so the gap partly reflects different feasible regions; again, this is a modeling asymmetry rather than a circularity. I additionally note Table V swaps the labels of α and β relative to Table II and Eqs. (13)/(15); that is an internal-consistency error affecting cost interpretation, but it is not circularity.
Assumptions & free parameters
assumptions (3)
- domain assumption In-vehicle travel times are constant, schedules are fixed, and train capacity is fixed.
- ad hoc to paper No passenger is denied boarding in the system optimum (db_k_s(t) = 0 for all k, t, s).
- standard math The integer-linear model can be solved to proven optimality with Gurobi.
Cite this review
Pith. "Pith review of An Exact System Optimum Assignment Model for Transit Demand Management." pith.science (2026). https://pith.science/paper/JIO7YV6C
@misc{pith2026250522241,
author = {Pith},
title = {Pith review of: An Exact System Optimum Assignment Model for Transit Demand Management},
year = {2026},
howpublished = {\url{https://pith.science/paper/JIO7YV6C}},
note = {Machine review of arXiv:2505.22241}
}
read the original abstract
Mass transit systems are experiencing increasing congestion in many cities. The schedule-based transit assignment problem (STAP) involves a joint choice model for departure times and routes, defining a space-time path in which passengers decide when to depart and which route to take. User equilibrium (UE) models for the STAP indicates the current congestion cost, while a system optimum (SO) models can provide insights for congestion relief directions. However, current STAP methods rely on approximate SO (Approx. SO) models, which underestimate the potential for congestion reduction in the system. The few studies in STAP that compute exact SO solutions ignore realistic constraints such as hard capacity, multi-line networks, or spatial-temporal competing demand flows. The paper proposes an exact SO method for the STAP that overcomes these limitations. We apply our approach to a case study involving part of the Hong Kong Mass Transit Railway network, which includes 5 lines, 12 interacting origin-destination pairs and 52,717 passengers. Computing an Approx. SO solution for this system indicates a modest potential for congestion reduction measures, with a cost reduction of 17.39% from the UE solution. Our exact SO solution is 36.35% lower than the UE solution, which is more than double the potential for congestion reduction. We then show how the exact SO solution can be used to identify opportunities for congestion reduction: (i) which origin-destination pairs have the most potential to reduce congestion; (ii) how many passengers can be reasonably shifted; (iii) future system potential with increasing demand and expanding network capacity.
Figures
Figures from the paper (6 more)
Reference graph
Works this paper leans on
-
[1]
C. Ma, C. Wang, and X. Xu, ”A multi-objective robust optimization model for customized bus routes,” IEEE Trans. Intell. Transp. Syst., vol. 22, no. 4, pp. 2359–2370, Sep. 2020
work page 2020
-
[2]
R. Yin, Z. Liu, and N. Zheng, ”A simulation-based model for continuous network design problem using Bayesian optimization,” IEEE Trans. Intell. Transp. Syst., vol. 23, no. 11, pp. 20352–20367, May 2022
work page 2022
-
[3]
Q. An, X. Fu, D. Huang, Q. Cheng, and Z. Liu, ”Analysis of adding-runs strategy for peak-hour regular bus services,” Transportation Research Part E: Logistics and Transportation Review, vol. 143, p. 102100, Nov. 2020
work page 2020
-
[4]
D. Rey, V . V . Dixit, J. L. Ygnace, and S. T. Waller, ”An endogenous lottery-based incentive mechanism to promote off-peak usage in con- gested transit systems,” Transp. Policy, vol. 46, pp. 46–55, Feb. 2016
work page 2016
-
[5]
X. S. Lu, X. Zhang, J. Long, and Y . Li, ”Managing rail transit peak-hour congestion with step fare schemes,” Transportmetrica A, Transp. Sci., vol. 16, no. 3, pp. 1490–1511, Jan. 2020
work page 2020
-
[6]
L. L. Cheung and A. S. Shalaby, ”System optimal re-routing transit assignment heuristic: A theoretical framework and large-scale case study,” Int. J. Transp. Sci. Technol., vol. 6, no. 4, pp. 287–300, Dec. 2017
work page 2017
-
[7]
J. Sun, J. Wu, F. Xiao, Y . Tian, and X. Xu, ”Managing bottleneck congestion with incentives,” Transp. Res. B, Methodol., vol. 134, pp. 143–166, Apr. 2020
work page 2020
-
[8]
P. Zhang and S. Qian, ”Path-based system optimal dynamic traffic assignment: A subgradient approach,” Transp. Res. B, Methodol., vol. 134, pp. 41–63, Apr. 2020
work page 2020
Show all 40 references
-
[9]
Zhu and S
F. Zhu and S. V . Ukkusuri, ”Efficient and fair system states in dy- namic transportation networks,” Transp. Res. B, Methodol., vol. 104, pp. 272–289, Oct. 2017
2017
-
[10]
Hamdouch, M
Y . Hamdouch, M. Florian, D. W. Hearn, and S. Lawphongpanich, ”Congestion pricing for multi-modal transportation systems,” Transp. Res. B, Methodol., vol. 41, no. 3, pp. 275–291, Mar. 2007
2007
-
[11]
A. H. Chow, ”Dynamic system optimal traffic assignment–a state- dependent control theoretic approach,” Transportmetrica, vol. 5, no. 2, pp. 85–106, May 2009. JOURNAL OF LATEX CLASS FILES, VOL. 14, NO. 8, OCTOBER 2024 13
2009
-
[12]
J. T. van Essen and G. H. Correia, ”Exact formulation and comparison between the user optimum and system optimum solution for routing privately owned automated vehicles,” IEEE Trans. Intell. Transp. Syst., vol. 20, no. 12, pp. 4567–4578, Jan. 2019
2019
-
[13]
A. H. Chow, ”Properties of system optimal traffic assignment with de- parture time choice and its solution method,” Transp. Res. B, Methodol., vol. 43, no. 3, pp. 325–344, Mar. 2009
2009
-
[14]
Salonen and T
M. Salonen and T. Toivonen, ”Modelling travel time in urban networks: Comparable measures for private car and public transport,” J. Transport Geogr., vol. 31, pp. 143–153, Jul. 2013
2013
-
[15]
Hamdouch, H
Y . Hamdouch, H. W. Ho, A. Sumalee, and G. Wang, ”Schedule-based transit assignment model with vehicle capacity and seat availability,” Transp. Res. B, Methodol., vol. 45, no. 10, pp. 1805–1830, Dec. 2011
2011
-
[16]
Gentile, M
G. Gentile, M. Florian, Y . Hamdouch, O. Cats, A. Nuzzolo, and M. Friedrich, ”The theory of transit assignment: basic modelling frame- works,” in Modelling Public Transport Passenger Flows in the Era of Intelligent Transport Systems: COST Action TU1004 (TransITS), Cham, Switze...
2016
-
[17]
J. Xie, S. Zhan, S. C. Wong, and S. M. Lo, ”A schedule-based model for passenger-oriented train planning with operating cost and capacity constraints,” IEEE Trans. Intell. Transp. Syst., vol. 23, no. 11, pp. 21314–21328, May 2022
2022
-
[18]
B. Mo, H. N. Koutsopoulos, Z. J. Shen, and J. Zhao, ”Robust path recommendations during public transit disruptions under demand uncer- tainty,” Transp. Res. B, Methodol., vol. 169, pp. 82–107, Mar. 2023
2023
-
[19]
Yang and Q
H. Yang and Q. Meng, ”Departure time, route choice and congestion toll in a queuing network with elastic demand,” Transp. Res. B, Methodol., vol. 32, no. 4, pp. 247–260, May 1998
1998
-
[20]
Y . Tang, Y . Jiang, H. Yang, and O. A. Nielsen, ”Modeling and optimizing a fare incentive strategy to manage queuing and crowding in mass transit systems,” Transp. Res. B, Methodol., vol. 138, pp. 247–267, Aug. 2020
2020
-
[21]
Q. Tian, P. Liu, G. P. Ong, and H. J. Huang, ”Morning commuting pattern and crowding pricing in a many-to-one public transit system with heterogeneous commuters,” Transp. Res. E, Logist. Transp. Rev., vol. 145, p. 102182, Jan. 2021
2021
-
[22]
T. Y . Ma and J. P. Lebacque, ”Dynamic system optimal routing in multimodal transit network,” Transp. Res. Rec., vol. 2351, no. 1, pp. 76–84, Jan. 2013
2013
-
[23]
Z. Y . Sun, Y . Li, W. C. Qu, and T. Muhammad, ”Unified framework for optimal routing choice under guidance information,” Complexity, vol. 2018, p. 4801489, Aug. 2018
2018
-
[24]
Araldo, S
A. Araldo, S. Gao, R. Seshadri, C. L. Azevedo, H. Ghafourian, Y . Sui, S. Ayaz, D. Sukhin, and M. Ben-Akiva, ”System-level optimization of multi- modal transportation networks for energy efficiency using personalized incentives: formulation, implementation, and performance,” T...
2019
-
[25]
Yang and Y
H. Yang and Y . Tang, ”Managing rail transit peak-hour congestion with a fare-reward scheme,” Transp. Res. B, Methodol., vol. 110, pp. 122–136, Apr. 2018
2018
-
[26]
Khani, ”An online shortest path algorithm for reliable routing in schedule-based transit networks considering transfer failure probability,” Transp
A. Khani, ”An online shortest path algorithm for reliable routing in schedule-based transit networks considering transfer failure probability,” Transp. Res. B, Methodol., vol. 126, pp. 549–564, Aug. 2019
2019
-
[27]
X. Zhou, H. S. Mahmassani, and K. Zhang, ”Dynamic micro-assignment modeling approach for integrated multimodal urban corridor manage- ment,” Transp. Res. C, Emerg. Technol., vol. 16, no. 2, pp. 167–186, Apr. 2008
2008
-
[28]
Ma and H
Z. Ma and H. N. Koutsopoulos, ”Optimal design of promotion-based demand management strategies in urban rail systems,” Transp. Res. C, Emerg. Technol., vol. 109, pp. 155–173, Dec. 2019
2019
-
[29]
B. Mo, Z. Ma, H. N. Koutsopoulos, and J. Zhao, ”Capacity-constrained network performance model for urban rail systems,” Transp. Res. Rec., vol. 2674, no. 5, pp. 59–69, May 2020
2020
-
[30]
C. C. Lu, H. S. Mahmassani, and X. Zhou, ”Equivalent gap function- based reformulation and solution algorithm for the dynamic user equilib- rium problem,” Transp. Res. B, Methodol., vol. 43, no. 3, pp. 345–364, Mar. 2009
2009
-
[31]
Verbas, H
¨O. Verbas, H. S. Mahmassani, and M. F. Hyland, ”Gap-based transit assignment algorithm with vehicle capacity constraints: Simulation-based implementation and large-scale application,” Transp. Res. B, Methodol., vol. 93, pp. 1–16, 2016
2016
-
[32]
L. Pan, E. O. Waygood, and Z. Patterson, ”Would you wait? Bus choice behavior analysis considering various incentives,” Transp. Res. Rec., vol. 2676, no. 7, pp. 117–127, Jul. 2022
2022
-
[33]
M ¨uller, M
D. M ¨uller, M. G. M ¨uller, D. Kress, and E. Pesch, ”An algorithm selection approach for the flexible job shop scheduling problem: Choosing constraint programming solvers through machine learning,” Eur. J. Oper. Res., vol. 302, no. 3, pp. 874–891, Nov. 2022
2022
-
[34]
Nethercote, P
N. Nethercote, P. J. Stuckey, R. Becket, S. Brand, G. J. Duck, and G. Tack, ”MiniZinc: Towards a standard CP modelling language,” in Proc. Int. Conf. Principles Practice Constraint Programming, Berlin, Germany: Springer Berlin Heidelberg, 2007, pp. 529–543
2007
-
[35]
Gurobi Optimization, LLC, Gurobi Optimizer Reference Manual, 2024
2024
-
[36]
De Palma, M
A. De Palma, M. Kilani, and S. Proost, ”Discomfort in mass transit and its implication for scheduling and pricing,” Transp. Res. B, Methodol., vol. 71, pp. 1–8, Jan. 2015
2015
-
[37]
Balcombe, R
R. Balcombe, R. Mackett, N. Paulley, J. Preston, J. Shires, H. Titheridge, M. Wardman, and P. White, ”The demand for public transport: a practical guide,” TRL Report, TRL593, Transport Research Laboratory, Woking- ham, UK, 2004
2004
-
[38]
Y . Wang, B. De Schutter, T. J. van den Boom, B. Ning, and T. Tang, ”Efficient bilevel approach for urban rail transit operation with stop- skipping,” IEEE Trans. Intell. Transp. Syst., vol. 15, no. 6, pp. 2658–2670, Oct. 2014
2014
-
[39]
Freyss, R
M. Freyss, R. Giesen, and J. C. Mu ˜noz, ”Continuous approximation for skip-stop operation in rail transit,” Procedia Soc. Behav. Sci., vol. 80, pp. 186–210, Jun. 2013
2013
-
[40]
Zhang, M
L. Zhang, M. Zhang, W. Yang, and D. Dong, ”Golden ratio genetic algorithm based approach for modelling and analysis of the capacity expansion of urban road traffic network,” Comput. Intell. Neurosci., vol. 2015, no. 1, p. 512715, 2015. Xia Zhoureceived the B.S. degree in Engin...
2015
Reviewed August 7, 2026 · model on record in the stance chip above.
Discussion (0). Sign in to comment.