Philip N. Brown

The Robustness of Marginal-Cost Taxes in Affine Congestion Games

The network routing literature contains many results showing that tolls can be used to improve the efficiency of network traffic routing. These results typically require toll-designers to have an exact characterization of the network and user population. We relax this strict informational dependence and present a simple setting in which scaled marginal-cost tolls can be guaranteed to provide significant efficiency improvements over the un-tolled case, even if the toll-sensitivities of the users are unknown.

Social coordination in unknown price-sensitive populations

In this paper, we investigate the relationship between uncertainty and a designers ability to influence social behavior. Pigovian taxes are a common approach to social coordination. However, guaranteeing efficient behavior typically requires that the system designer has complete knowledge of the user populations sensitivity to taxation. In this paper, we explore the effect of relaxing this requirement in the context of congestion games with affine costs. Focusing on the class of scaled Pigovian taxes, we derive the optimal tolling scheme that minimizes the worst-case efficiency loss under uncertainty in user sensitivity. Furthermore, we derive explicit bounds which highlight how the level of uncertainty in sensitivity degrades performance.

Avoiding perverse incentives in affine congestion games

In engineered systems whose performance depends on user behavior, it is often desirable to influence behavior in an effort to achieve performance objectives. However, doing so naively can have unintended consequences; in the worst cases, a poorly-designed behavior-influencing mechanism can create a perverse incentive which encourages adverse user behavior. For example, in transportation networks, marginal-cost tolls have been studied as a means to incentivize low-congestion network routing, but have typically been analyzed under the assumption that all network users value their time equally. If this assumption is relaxed, marginal-cost tolls can create perverse incentives which increase network congestion above un-tolled levels. In this paper, we prove that if some network users are unresponsive to tolls, any taxation mechanism that does not depend on network structure can create perverse incentives. Thus, to systematically avoid perverse incentives, a taxation mechanism must be network-aware to some extent. On the other hand, we show that a small amount of additional information can mitigate this negative result; for example, we show that it is relatively easy to avoid perverse incentives on affine-cost parallel-path networks, and we fully characterize the taxation mechanisms that minimize congestion for worst-case user populations on such networks.

Studies on mechanisms for robust social influence

An important goal in engineering design is to ensure that infrastructure systems are utilized efficiently by the public. One example is a transportation network which exhibits congestion effects due to self-interested routing choices by drivers. We study the use of road tolls to positively influence routing choices, focusing on deriving pricing mechanisms which are robust to mischaracterizations of network topology and driver price-sensitivity. We show that robust taxation is possible, but only if road taxes are allowed to vary with traffic loads.