Congestion charging had become necessary because vehicle traffic was a major problem in the city centre despite high public transit ridership, the cost of which was estimated to be approximately GBP 4 billion. The aim of the charge was to reduce traffic and congestion and to raise revenue that would be re-invested in transport. The congestion charge has had several impacts, including a 30% decrease in congestion, a reduction in the total number of vehicles entering the zone per day, a 15% reduction in circulating traffic per weekday, and an 11% mode share shift away from cars and towards public transit, walking and cycling.