When few cars are using a road, speeds are high, but the light volume means few cars get through. Add more cars and eventually speeds start to slow, but the increase in volume means that throughput rises. When a road is just crowded enough so that speeds are around 45 m.p.h., the most cars are pumped through the system.
But add even more cars and trouble starts. Speeds break down, taking throughput down with them. When roads are severely congested, you get a paradoxical situation: the more cars you jam in at one end, the fewer come out the other end.
If this is true, then the argument that congestion charging is bad because it ‘deprives’ poor people of a chance to use the road is weakened (note that it’s a particularly bad argument in Singapore’s context, where the poor simply cannot afford cars, and much of the middle class uses public transport). Under certain conditions, charging might actually allow more cars to use the road than before.