FairSCOSCA: Fairness At Arterial Signals -- Just Around The Corner

TL;DR

FairSCOSCA targets fairness in arterial traffic signal control by extending SCOOTS/SCATS with two practical features: incorporating cumulative waiting times into green-phase optimization and enabling early termination of underutilized green phases. Through a calibrated SUMO microsimulation of Esslingen's arterial network, FairSCOSCA demonstrably improves multidimensional fairness (Egalitarian, Rawlsian, Utilitarian, Harsanyian) with only modest efficiency trade-offs compared to Fixed-Cycle, Max-Pressure, and baseline SCOOTS/SCATS. The method remains simple and deployment-friendly with open-source code on GitHub. These results bridge fairness theory and real-world signal-control practice, supporting broader acceptance and equitable urban traffic management.

Abstract

Traffic signal control at intersections, especially in arterial networks, is a key lever for mitigating the growing issue of traffic congestion in cities. Despite the widespread deployment of SCOOTS and SCATS, which prioritize efficiency, fairness has remained largely absent from their design logic, often resulting in unfair outcomes for certain road users, such as excessive waiting times. Fairness however, is a major driver of public acceptance for implementation of new controll systems. Therefore, this work proposes FairSCOSCA, a fairness-enhancing extension to these systems, featuring two novel yet practical design adaptations grounded in multiple normative fairness definitions: (1) green phase optimization incorporating cumulative waiting times, and (2) early termination of underutilized green phases. Those extensions ensure fairer distributions of green times. Evaluated in a calibrated microsimulation case study of the arterial network in Esslingen am Neckar (Germany), FairSCOSCA demonstrates substantial improvements across multiple fairness dimensions (Egalitarian, Rawlsian, Utilitarian, and Harsanyian) without sacrificing traffic efficiency. Compared against Fixed-Cycle, Max-Pressure, and standard SCOOTS/SCATS controllers, FairSCOSCA significantly reduces excessive waiting times, delay inequality and horizontal discrimination between arterial and feeder roads. This work contributes to the growing literature on equitable traffic control by bridging the gap between fairness theory and the practical enhancement of globally deployed signal systems. Open source implementation available on GitHub.

Figures (4)

• Figure 1: Signalized Intersection Management with SCOOTS/SCATS: SCOOTS/SCATS consist of three optimizers, that dynamically adjust cycle length, green phases, and offsets of traffic lights across an arterial road. The two proposed design features FairSCOSCA_1 and FairSCOSCA_2 are dedicated to improving the green phase optimization and to enable early termination of unused, running green phases. Doing so, the proposed methods can achieve higher levels of fairness without affecting efficiency too much. This visual illustrates the minimal yet impactful modifications needed to enhance fairness in a real-world-deployable manner.
• Figure 2: Case Study "Schorndorfer Strasse": The arterial network "Schorndorfer Strasse" with five signalized intersections from the city of Esslingen am Neckar in Germany serves as case study to demonstrate the potential of the proposed FairSCOSCA traffic light controller. The traffic microsimulation model is calibrated based on real-world demand, covers 22 traffic lights, 29 loop detector sensors, 26 bus stops, and 11 bus lines besides car, motorcycle, and truck traffic.
• Figure 3: Efficiency Analysis: Visualization of Macroscopic Fundamental Diagram (MFD). The two figures show flow and average vehicle speed for varying vehicle density across the simulation. The Fixed-Cycle controller obtains the lowest network capacity, Max-Pressure gains improvements. The proposed design features FairSCOSCA_1 and FairSCOSCA_2 obtain even further efficiency gains, similar to those of SCOOTS/SCATS (SCOSCA) (best). The MFD was calculated for every 5 minutes across all simulation runs (with different seeds), and the density is measured as the number of vehicles in the case study network.
• Figure 4: Equity Analysis: The diagrams of the first row show the distribution of delays for different controllers. Contrary to Fixed-Cycle and Max-Pressure, SCOOTS/SCATS (SCOSCA) achieves lower average delays and a higher concentration of those delays (more equal distribution). The proposed design features achieve similar results with slight deviations. The diagrams of the second two rows show the delay and normalized distribution for vehicles starting their trip from arterial or feeder roads for the different versions of SCOSCA. Normalized delays refer to delay per travelled distance. The number below "arteria" and "feeder" describe the median of the delay. While vehicles from the arterial roads have lower delays for SCOSCA (discrimination of feeders), the proposed design features enable to improve the situation of feeders without impairing those vehicles from the arteria.