DartCharge is the payment service for the Dartford Crossing, on the Thames between Kent, Essex and London. The charge exists to manage demand, not to pay for infrastructure: the crossing was built for 135,000 vehicles a day but now handles up to 160,000, and research suggests lifting the toll could raise traffic by 17%. The payment experience itself is therefore critical infrastructure. Across three research phases and 50+ interviews, we redesigned the web and native app to make paying simpler, alerts clearer, and account management calmer.