Excessive speed is a factor in the causation and severity of many road accidents. Speed restrictions are intended to promote safe use of the road network by keeping traffic speeds below the maximum that is appropriate for a given traffic environment, thereby protecting vehicle occupants and other road users. Greater adherence to speed limits would avert many accidents and reduce the severity of those that occur.
Excessive speeding is sometimes unintentional. Drivers who are tired or otherwise distracted may allow their speeds to drift above the maximum allowed for that road. Others may inadvertently miss a traffic sign alerting them to a change in the speed limit, such as when entering a built-up area. Speed alert or Intelligent Speed Assistance (ISA) systems help drivers to keep their speeds within the recommended limits.
Some systems display the current limit so that the driver is always aware of the maximum speed allowed on that road. The speed limit may, for example, be determined by software which analyses images from a camera and recognises traffic signs. Alternatively, satellite navigation is becoming increasingly accurate and could be used to provide information to the driver. However, this relies on the most up to date digital maps being available at all times. Systems may or may not issue a warning to the driver when the speed limit is being exceeded and current systems are voluntary: they can be switched off and they rely on the driver responding appropriately to the warning.
Other systems do not inform the driver of the current speed limit; they allow drivers to set a limit of their choice and warn when that limit is being exceeded. Used responsibly, such systems can also contribute to safer driving by helping drivers to control their speeds.