HEAVY RAIN Your visibility may be greatly reduced in heavy rain. Consider using headlights, and maybe fog lights in bad conditions, but remember to turn the fog lights off once you can see 100m again.
Braking distances can double from the norm, so give yourself more room to stop, and slow down, as your stopping distance will reduce greatly the slower you go.
Even when not slowing or steering, you may lose control of the car if you drive into standing water. Aquaplaning is caused by driving too fast into water. So fast that the tread on the tyres does not have time to disperse the water under the tyres. This will mean that the tyre will not be able to reach the road surface, where the grip is achieved. Effectively, the car starts skimming across the surface of the water, with no steering or speed control. The best way to minimise this risk is to drive slower, giving the car time to sink through the water, to reach the road surface. The deeper the tyre tread, and the slower you travel, the more water the tyre can disperse, and therefore stay in contact with the road.
If you need to drive through a flooded road, there are a number of things to consider: How deep is the water? Can you vehicle travel through that depth of water without flooding the exhaust, air intake, or electrics? Can you see where the road is, and where the edges are? You do not want to hit kerbs, or drive off the tarmac onto soft verges etc.
How fast is the water moving? You really do not want to drive through a flood and be forced to one side or another.
How likely are you to meet someone travelling the other way? Is there room to pass each other? Will the bow wave they are creating affect your vehicle? Will yours affect theirs?
Make sure you know where the road goes, and how deep the water is. You should aim to travel though the shallowest part of the flood, which is normally in the centre of the road, at the highest part of the camber, but remember that any on-coming traffic will also have the same plan.
Test your brakes as soon as you are clear of the flood, to ensure they are working efficiently.