  When users lack specific knowledge of various system parameters, their
uncertainty may lead them to make undesirable deviations in their decision
making. To alleviate this, an informed system operator may elect to signal
information to uninformed users with the hope of persuading them to take more
preferable actions. In this work, we study public and truthful signalling
mechanisms in the context of Bayesian congestion games on parallel networks. We
provide bounds on the possible benefit a signalling policy can provide with and
without the concurrent use of monetary incentives. We find that though
revealing information can reduce system cost in some settings, it can also be
detrimental and cause worse performance than not signalling at all. However, by
utilizing both signalling and incentive mechanisms, the system operator can
guarantee that revealing information does not worsen performance while offering
similar opportunities for improvement. These findings emerge from the closed
form bounds we derive on the benefit a signalling policy can provide. We
provide a numerical example which illustrates the phenomenon that revealing
more information can degrade performance when incentives are not used and
improves performance when incentives are used.
