Driving safely in the rain
When driving in the rain water on the road surface activates the oil, rubber deposits and other substances that can act as lubricants on the road surface - this makes it more difficult for the tyres to grip and so it takes longer for the car to stop. Although water on the road can have this effect at any time it is often worse when there is a slight shower after a long dry spell.
The best thing that you can do to counteract the effects of the rain is to slow down. When you reduce your speed your tyres will work better at moving the water that in their path and you will have more time to respond in the event of a problem. The condition of your tyres is critical in wet weather - less tread you have, the less grip you will get. In addition to driving more slowly you should aim to drive as smoothly as possible; smooth driving is always important but especially so in the wet.
Stopping distances much greater in wet weather - depending on the road surface it can take up to three times longer to stop - with this in mind you're going to need more space and time to react. On the open road the best way to achieve this is to (at least) double your normal separation distance. This is especially important when you're following behind large vehicles because a large tyres will throw up lots of spray making it even more difficult to see.
In order to steer and brake your tyres need to be in contact with the road - in wet weather it's possible for your tyres to lose their grip completely and for the car to effectively ‘water ski’ across the road surface. This is more likely to happen in a rear wheel drive cars (cars with the engine connected to the rear wheels) than in front wheel drive cars.
What happens when the car starts aquaplane is that the front tyres become so swamped with water that they can no longer clear it through their tread grooves any more; when this happens the caution of water builds up underneath the tyre and it is no longer able to grip the road surface leaving you with no steering and no braking from the front wheels - the first that many drivers know about this is when they try to brake or steer and nothing happens!
In a rear wheel drive car aquaplaning is difficult to recognise, there will be some reduction in road noise but this could be drowned out by the sound of heavy rain and the windscreen wipers, your first clue might be that the steering is unresponsive, and that could be too late - this is why it's so important to slow down in wet weather.
In front wheel drive car things are more noticeable. The engine will rev harder as the front wheel start to spin (in the same way that wheel spin on ice) - often when this happens it will be instinctive to ease off the gas.
The fix for aquaplaning in any car is to take your foot off the gas and wait for the car to slow down regain its grip, this should happen fairly quickly.
What you can see
In wet weather your visibility will be reduced, this is another reason why it's important to slow down. Use your windscreen wipers and screen demisters is to keep your car windows clear.
Sometimes the rain will be so heavy that even if you take sensible precautions with your wipers demisters and clean windows you still want be able to see the car ahead of you - if this is the case the safest thing to do is to stop until the rain storm passes.
You will notice in wet weather that it's much easier to see other vehicles when they have their lights on, this is why you should switch your lights on when visibility becomes restricted in heavy rain - especially on fast-moving roads where there is a lot of spray. When driving at night in very heavy rain consider keeping your headlamps dipped, this is because the glare as the raindrops reflect the full beam light back to you can make it more difficult to see them if you simply had dipped headlights.
You might be able to see puddles on the road, but you can't see what's underneath them. Is there a big pothole that could damage your vehicle? Could there be a man-hole cover missing? Is there broken glass lurking beneath the puddle in the gutter? You might not always be able to avoid puddles but drive through them with care and slow down just in case something hidden from your view.
Another good reason for slowing down when there are puddles is related to aquaplaning. When your tyres hit a puddle they will be slowed down as they try to push their way through the water - this can pull your car quite violently in the direction of the puddle leading to a loss of control if you're going too fast.
You will also see pedestrians when you're driving in the rain, but will they see you? Often pedestrians will have hoods pulled up, will be carrying umbrellas and if they're walking into the wind and the rain is blowing in their faces they will have their eyes partially closed. It will also be more difficult for them to hear you. Although I said you will see pedestrians when you are driving in the rain, this might not be the case on dark wet nights when pedestrians can become almost invisible, especially if they are wearing dark clothing so take extra care on road where you might expect to see pedestrians.
Pedestrians on the only other road users who might not see you, cyclists are also particularly vulnerable, they will often have their heads down and could be blown into your path by the wind; cyclists will also want to avoid puddles so be alert to the fact that they might suddenly swerve out.
In addition to seeing pedestrians and cyclists you also need to be careful that you don't splash them where there is a lot of surface water or puddles - driving without consideration in this way could lead to prosecution and points on your licence.
Planning your Journey
Before starting a journey check the weather forecast, if rain is forecast or if it's already raining allow yourself more time. There will be more traffic congestion in wet weather and you may need to drive quite a lot slower especially if it's a long journey that involves motorways.
Driving when you're tired is never a good idea but it's a particularly bad idea in wet weather, especially after dark, because the more demanding conditions will require extra concentration.
Sitting in a warm car on a wet day is probably a much better experience than trudging through puddles or sitting on the bus in damp clothes - but remember, to get home safely in the rain you need to take care.