This paper studies the welfare and distributional effects of dynamically priced highway toll lanes. To quantify the equilibrium effects of tolling, we develop and estimate a model of driver demand, the road technology, and the pricing algorithm. The demand model features heterogeneous drivers who choose their departure time under imperfect information about travel times and prices and then, once the uncertainty is resolved, choose whether to take the priced (faster) or unpriced (slower) lanes. We estimate the model using data on toll transactions, historical traffic conditions, and driver characteristics for I-405 in Washington State. We find that tolling a subset of highway lanes increases welfare in aggregate, as well as for drivers in all income quartiles, especially when the newly priced lanes were previously carpool-only. A key component of these gains is the “option value” of tolling: even drivers who infrequently take the priced lanes benefit from having the option to pay for speed when traffic is worse than expected. Moreover, the largest gains accrue to drivers in the bottom income quartile, primarily due to the spatial distribution of lower- and higher-income drivers rather than preference heterogeneity. Lower-income drivers have longer I-405 commutes and—thanks to the design of the pricing algorithm—often face an advantageous tradeoff between time savings and price. Finally, we show how simple revisions to the pricing algorithm can increase aggregate welfare and help achieve redistributive goals.