  Traffic state prediction is necessary for many Intelligent Transportation
Systems applications. Recent developments of the topic have focused on
network-wide, multi-step prediction, where state of the art performance is
achieved via deep learning models, in particular, graph neural network-based
models. While the prediction accuracy of deep learning models is high, these
models' robustness has raised many safety concerns, given that imperceptible
perturbations added to input can substantially degrade the model performance.
In this work, we propose an adversarial attack framework by treating the
prediction model as a black-box, i.e., assuming no knowledge of the model
architecture, training data, and (hyper)parameters. However, we assume that the
adversary can oracle the prediction model with any input and obtain
corresponding output. Next, the adversary can train a substitute model using
input-output pairs and generate adversarial signals based on the substitute
model. To test the attack effectiveness, two state of the art, graph neural
network-based models (GCGRNN and DCRNN) are examined. As a result, the
adversary can degrade the target model's prediction accuracy up to $54\%$. In
comparison, two conventional statistical models (linear regression and
historical average) are also examined. While these two models do not produce
high prediction accuracy, they are either influenced negligibly (less than
$3\%$) or are immune to the adversary's attack.
