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.