Run flat tyres are those specially designed to maintain the car's mobility if the tyre has a puncture. This is mainly achieved by the tyres having much thicker sidewalls which will continue to hold the weight of the car even when the air pressure inside drops. This safety feature allows the driver to continue driving until it is safer and more convenient to stop and get the tyre repaired.
The pressure is monitored in a variety of ways depending upon the vehicle manufacturer. Many systems have the sensor on the valve inside the tyre so any damage to this will necessitate a replacement valve, sometimes at considerable cost.
When a tyre is punctured, or the pressure drops below a predetermined level, the driver is alerted by a tyre pressure monitoring system in the car. The vehicle handbook will give precise guidance but generally the car should not be driven above 50mph or for more than 50 to 100 miles before the tyre must be repaired or replaced. The further it is driven, the less likely it is to be repaired as the increased weight on the tyre sides could damage it inside.
Once a run flat tyre suffers a puncture, you can't keep driving on it forever – usually you will be able to drive at c. 30 mph for another 50 miles – plenty of time to get to your nearest garage. The exact range depends on variables like your driving speed, load of your vehicle and driving conditions.
You should not assume that you can continue to drive on run flat tyres without causing damage the whole point is that the tyre will get you to your nearest garage. This is an example of a tyre we changed recently where the customer was under the wrong impression that he could continue to drive on the tyre:-