  Major cities worldwide experience problems with the performance of their road
transportation systems. The continuous increase in traffic demand presents a
substantial challenge to the optimal operation of urban road networks and the
efficiency of traffic control strategies. Although robust and resilient
transportation systems have been extensively researched over the past decades,
their performance under an ever-growing traffic demand can still be
questionable. The operation of transportation systems is widely believed to
display fragile property, i.e., the loss in performance increases exponentially
with the linearly increasing magnitude of disruptions, which undermines their
continuous operation. The risk engineering community is now embracing the novel
concept of (anti-)fragility, which enables systems to learn from historical
disruptions and exhibit improved performance as disruption levels reach
unprecedented magnitudes. In this study, we demonstrate the fragile nature of
road transportation systems when faced with either demand or supply
disruptions. First, we conducted a rigorous mathematical analysis to
theoretically establish the fragile nature of the systems. Subsequently, by
taking into account real-world stochasticity, we implemented a numerical
simulation with realistic network data to bridge the gap between the
theoretical proof and the real-world operations, to study the impact of
uncertainty on the fragile property of the systems. This work aims to help
researchers better comprehend the necessity to explicitly consider antifragile
design toward the application of future traffic control strategies, coping with
constantly growing traffic demand and subsequent traffic accidents.
