When traffic is dense, you brake and accelerate more frequently. This jerky driving style pushes up fuel consumption unnecessarily.
Plan and combine your journeys to make them cost-effective. Wherever possible, avoid rush hours and traffic jams. Optimise your route so that you can drive smoothly, limit acceleration and braking and reduce mileage.
If you get stuck in a traffic jam, turn your engine off temporarily when safe to do so. This always saves energy.