1. A safety system ( 10) for vehicles comprising a controller fitted in use to a subject vehicle ( 16), sensor means ( 20) fitted to the subject vehicle in use and operable to sense a distance of separation and/or a relative velocity of a trailing vehicle ( 18) and operable to input data signals representative thereof to the controller, velocity sensing means ( 97) operable to sense the velocity of the subject vehicle relative to the ground and to input to the controller a velocity signal representative thereof, wherein the controller is operable to processes the received velocity signal and data signals to determine the existence of an unsafe condition, and the safety system further comprising warning means ( 10) controlled by the controller and operable to warn a driver of the trailing vehicle of the existence of the unsafe condition.

2. A safety system as claimed in claim 1 wherein the controller is operable to determine the existence of the unsafe condition by determining a safe distance corresponding to a safety envelope to the rear of the subject vehicle within which any incursion by the trailing vehicle constitutes the existence of an unsafe condition.

3. A safety system as claimed in claim 2 wherein the safe distance is determined to be substantially the safe stopping distance of a vehicle travelling at the velocity of the trailing vehicle.

4. A safety system as claimed in any of claims 1 and 2 wherein the safe stopping distance is determined to be proportional to the velocity of the subject vehicle.

5. A safety system as claimed in any of claims 2 to 4 wherein the controller is operable to determine an enhanced safe distance corresponding to an enlarged safety envelope and wherein the warning means is operable to indicate a first level of warning corresponding to incursion by the trailing vehicle into the enlarged safety envelope and a second level of warning which is more prominently presented to the driver than the first level of warning and corresponds to any incursion into the safety envelope.

6. A safety system as claimed in claim 5 comprising ground condition communication means operable to input to the controller a signal representative of the condition of the ground and or other driving conditions and wherein the controller is operable to determine the enhanced safe distance corresponding to the enlarged safety envelope according to the extent to which the ground condition communication means indicates adverse ground conditions likely to affect traction between the subject vehicle and the ground.

7. A safety system as claimed in any of claims 5 and 6 wherein the sensor means is operable to sense both the distance of separation and the relative velocity of the trailing vehicle and wherein the controller is operable to determine whether the relative velocity of the trailing vehicle relative to the subject vehicle when entering the enlarged safety envelope is greater than a threshold value of relative velocity and, if so, is operable to actuate the warning means to indicate the second level of warning.

8. A safety system as claimed in claim 7 wherein the ground condition communication means comprises means fitted to the subject vehicle for sensing at least one of rain, snow and ice and communicating a signal representative thereof to the controller.

9. A safety system as claimed in any preceding claim wherein the sensor means comprises a radar system transmitting and receiving radar pulses, from which received pulses information is derived sufficient to determine both the proximity and relative speed of the following vehicle.

10. A safety system as claimed in any of claims 5 to 9 wherein the warning means comprises a display ( 10) carried by the subject vehicle and positioned for viewing by the driver of the trailing vehicle.

11. A safety system as claimed in any of claims 5 to 10 further comprising communication means operable between the controller of the subject vehicle and a warning device fitted in use to the trailing vehicle, whereby the warning means is operable to indicate a warning to the driver of the trailing vehicle via the warning device.

12. A safety system as claimed in any preceding claim wherein the velocity sensing means comprises a sensor ( 97) mounted on the subject vehicle and responsive independently of elements of the subject vehicle's transmission train to movement of the ground relative to the subject vehicle.

13. A safety system as claimed in claim 12 wherein the velocity sensing means comprises a sensor constituted by a doppler shift radar device.

14. A safety system as claimed in any preceding claim wherein the controller is operable to actuate an impact absorbing device ( 100, 102, 104, 106) deployed internally or externally of the subject vehicle.

15. A safety system as claimed in claim 14 wherein the impact absorbing device comprises an air bag ( 100, 102, 104, 106) inflatable so as to be deployed externally of the subject vehicle and/or bumpers extensible by means of hydraulic rams operable to absorb energy upon impact.

16. A safety system for vehicles having means fitted in use to a subject vehicle for determining the magnitude of acceleration and deceleration of the subject vehicle and connected to a visible display mounted on the subject vehicle operable to indicate to an observer remote from the vehicle the magnitude of acceleration and deceleration.

17. A safety system as claimed in claim 16 wherein the visible display comprises an array of lights mounted on the side or sides of the subject vehicle.

18. A safety system as claimed in claim 17 wherein a first coloured array of lights represents acceleration and a second coloured array of lights represents deceleration, the number and/or intensity of actuated lights of a given colour representing the magnitude of the acceleration or deceleration respectively.

19. A safety system for vehicles comprising an anti-lock braking system fitted to a subject vehicle and operable to regulate operation of the subject vehicle's brakes in response to a signal generated by a speed sensor and representative of the speed of the subject vehicle relative to the ground, characterised in that the speed sensor is a radar system ( 97) operable to direct radiation towards the ground and to determine the speed by receiving and analysing radiation reflected from the ground.

20. A safety system as claimed in claim 19 wherein the radar system is operable to determine speed by measurement of a doppler shift in the frequency of the reflected radiation.

21. A safety system for vehicles comprising at least one impact energy absorbing device ( 100, 102, 104, 106) fitted to a subject vehicle, determining means responsible to output signals of the sensor means for determining whether the object is about to impact with the subject vehicle and actuating means responsive to the determining means to actuate the at least one impact energy absorbing device so as to be deployed at a position externally of the subject vehicle so as to be between the object and the subject vehicle.

22. A safety system as claimed in claim 21 wherein the impact energy absorbing device is an inflatable bag device comprising two or more inflatable bags ( 118, 120, 122) deployed when inflated so as to constitute successive layers including an inner bag ( 122) proximate the subject vehicle and an outer bag ( 118) distal to the subject vehicle.

23. A safety system as claimed in claim 22 comprising deflation means operable to deflate one or more of the bags in response to pressure within the respective bag exceeding a predetermined pressure level.

24. A safety system as claimed in claim 23 wherein the deflation means is operable to deflate bags in successive layers at pressure levels which decrease progressively from the inner bag to the outer bag whereby in use the outer bag is deflated prior to deflation of the inner bag in response to impact between the object and the subject vehicle.

25. A safety system as claimed in any of claims 23 and 24 wherein the deflation means comprises rupturable membranes forming parts of side walls of respective bags and arranged to rupture at respective predetermined pressure levels.

26. A safety system as claimed in any of claims 21 to 25 wherein the air bag device comprises three inflatable bags.

27. A safety system as claimed in any of claims 21 to 26 comprising a plurality of air bag devices ( 100, 102, 104, 106) located so as to be deployed at the front, rear, left side and right side respectively of the subject vehicle.

28. A safety system as claimed in claim 21 wherein the impact energy absorbing device comprises at least one bumper extensible by means of hydraulic ram means capable of absorbing energy when the bumper receives an impact.