Run flat tyres are specially designed tyres which are able to keep working for a short while after they have been punctured. This can be a very valuable addition to any car, just think if your tyre got punctured in the middle of nowhere while it was raining and you were able to drive to the closest garage rather than change the tyre yourself. They also help reduce the dangers of a potential blow out.
Self-Supporting tyres are built with reinforced sidewalls, this means that if the tyre is punctured and the air escapes. The sidewalls are able to support the weight of the car rather than the air. Obviously you cannot drive on runflat tyres forever but you should be able to drive at 30mph for another 50 miles. That should give you the time to either drive to the nearest garage or even to drive yourself home. This range does depend on variables such as your driving speed, the load of your vehicle and the driving conditions.
The only real downside to runflat tyres is that most manufacturers do not recommend that they are repaired, this is because they may have been driven on with the puncture thereby compromising its strength and therefore putting you the driver in danger.
If you are thinking of fitting runflat tyres to your car you will need to consider several points.
You need to have a Tyre Pressure Monitoring System fitted, as you will not be able to tell when your tyre has been punctured you need to have a system which will alert you that you have suffered a puncture and remind you to get a new tyre fitted.
Your vehicle suspension may also need to be changed, as the tyres have a much stiffer construction.
Mixing conventional tyres with runflat tyres should be avoided, as their handling characteristics may differ. This is the same with mixing different brands of tyres on a car, you should try to avoid this and if you are not sure you should speak to your local Lookers tyre fitter.