Cost-benefit analyses (CBA) are often questioned on the grounds that results depend crucially on uncertain assumptions about the future, and on ethically or methodologically contestable trade-offs between different types of benefits. This paper explores the robustness of CBA rankings of transport investments with respect to two types of uncertainties: relative benefit valuations and scenario assumptions related to car ownership, car characteristics and driving costs. The impact of differentiating the value of time with respect to travel mode and purpose is also studied. The study concludes that CBA rankings are robust to all of the studied variations.