As fast as you like?

The road sign depicts the maximum speed for vehicles on this stretch of road. The road though is "closed" ordinary cars will struggle, taller vehicles with good ground clearance can get through today, the maximum depth of water was around 1ft 3in, 38cm, but has reached 7ft 6in (2.3 metres). 

The flooding is deliberate, using this area to store excess water before it can be conducted to the river and then to the sea, avoiding flooding areas of habitation..