with reference to the drawings , fig1 illustrates a prier art arrangement , in which there is shown a lead vehicle a , a following vehicle b at a pre - set separation distance s 1 and an obstruction 60 , such as a road hump , in advance of the lead vehicle a . fig1 illustrates a steady state condition with vehicles a and b travelling at the same speed . in fig2 , the load vehicle a has slowed for the obstruction 60 , and in consequence the separation distance s of the vehicles reduces from s 1 to s 2 . in fig3 the lead vehicle a has passed the obstruction 60 and accelerated to the steady state speed . in consequence the separation distance s has increased from s 2 to s 3 . in fig4 the following vehicle b , reacting to the increased separation distance , accelerates to match the speed of the lead vehicle a , so as to achieve the desired separation distance s 1 . the consequence is that the following vehicle b momentarily accelerates towards the obstruction 60 , which is undesirable . fig5 is a schematic illustration of a vehicle 10 according to an embodiment of the present invention that is configured to overcome the problem described above with respect to the prior art . the vehicle 10 has a prime mover or motor 11 in the form of an internal combustion engine . the engine 11 is coupled to a transmission 12 by means of a coupling 13 . the coupling 13 is arranged to allow the transmission 12 progressively to reach a speed compatible with motor speed when the vehicle 10 is accelerated from rest . the coupling 13 is typically a friction clutch , torque converter or the like . an accelerator pedal 1 allows a driver to control an amount of torque developed by the motor 11 under the control of a powertrain controller 17 whilst a brake pedal 2 allows a driver to apply a braking system under the control of as brake controller 18 . the vehicle 10 has a vehicle control unit ( vcu ) 15 that is operable to implement a vehicle speed control function , which may be referred to as a speed control system . the speed control function is controlled by a user by means of input controls mounted to a steering wheel 171 of the vehicle 10 . the steering wheel 171 is shown in more detail in fig6 . the steering wheel 171 bears input controls allowing a user to control the speed control system . the input controls include a ‘ set - speed ’ control 173 , actuation of which sets the value of a parameter driver_set_speed to be substantially equal to the current vehicle speed . depression of a ‘+’ button 174 allows the set speed to be increased whilst depression of a ‘−’ button 175 allows the set speed to be decreased . in some embodiments , if the speed control function is not active when the ‘+’ button 174 is depressed , the speed control function is activated . the wheel 171 also has a pair of following distance control buttons 178 , 179 for setting a value of a parameter distance being the distance the driver desires the vehicle 100 to maintain behind a lead vehicle . the vcu 15 is operable to control the vehicle 10 to maintain a distance behind a lead vehicle that is substantially equal to distance_following . a first of the buttons 178 is operable to increase the value of the parameter distance_following whilst a second of the buttons 179 is operable to decrease the value of the parameter distance_following . the vehicle 10 has a radar module 5 mounted to a front thereof arranged to project a radar beam in a direction ahead of the vehicle . the module 5 is arranged to detect radiation reflected by a lead vehicle and to determine a distance of the lead vehicle from vehicle 10 ( being a ‘ host ’ vehicle ). the module 5 is provided with a signal indicative of a current speed of the host vehicle 10 . from this signal and data in respect of a variation in distance of the lead vehicle from the host vehicle 10 as a function of time , the module 5 is able to calculate a speed of the lead vehicle . other arrangements for determining distance from the lead vehicle and speed of the lead vehicle are also useful . when the speed control function is activated , the vcu 15 controls the speed of the vehicle 10 to be substantially equal to a driver set speed driver_set_speed . the driver may set the value of driver_set_speed by depressing the ‘ set speed ’ control 173 whilst the vehicle is travelling . when the vcu 15 detects that the ‘ set speed ’ control 173 has been pressed , the vcu 15 takes a snapshot of the current speed of the vehicle 10 and sets the value of driver_set_speed to correspond to the current speed . when the vehicle 10 is travelling along a road at a speed exceeding a minimum allowable set speed set_speed_min , the vcu is operable to allow the user to command the vcu 15 to maintain the current vehicle speed by depressing set speed control 173 . in the absence of traffic ahead of the vehicle 10 , the vcu 15 controls the speed of the vehicle 10 to maintain the set speed . if the vcu 15 detects ( by means of radar module 5 ) the presence of a lead vehicle ahead of the vehicle 10 , the vcu 15 is operable to reduce the speed of the host vehicle 10 according to the speed of the lead vehicle in order to maintain a distance behind the lead vehicle that is no less than a prescribed distance . the prescribed distance may be set by a driver by means of ‘ following distance ’ control buttons 178 , 179 . the vcu 15 is configured wherein if the speed of the vehicle 10 is below a prescribed value and the vehicle 10 is following a lead vehicle , the vcu 15 monitors the speed of the lead vehicle and controls the speed of the host vehicle 10 such that it substantially matches that of the lead vehicle at a given geographical location when the host vehicle 10 reaches that geographical location . thus , if the lead vehicle slows down to drive over a speed bump or other obstruction , the host vehicle 10 slows down at substantially the same location to driver over the same obstruction . this feature has the advantage that acceleration of the vehicle 10 towards an obstruction such as a speed bump when following a lead vehicle is prevented . operation of a motor vehicle 10 according to an embodiment of the present invention is illustrated with respect to fig7 - 11 , fig7 - 9 correspond to fig1 - 3 . fig7 shows a lead vehicle a and a following vehicle b ′ ( being a vehicle 10 according to the embodiment of fig5 ) at a pre - set separation distance s 1 . the separation distance s 1 has been set by the user by means of the following distance control buttons 178 , 179 . the lead vehicle a is shown approaching an obstruction 60 , such as a road hump , in advance of the following vehicle b ′. fig7 illustrates a steady state condition with vehicles a and b ′ travelling at the same speed . in fig8 , the lead vehicle a is shown having slowed for the obstruction 60 , and in consequence the separation distance s 2 has reduced from s 1 . the separation distance s 2 has reduced from s 1 because the vcu 15 is arranged such that the separation distance s is a function of lead vehicle speed . the vcu 15 allows the separation distance to decrease as vehicle speed decreases , but increases the separation distance as vehicle speed increases , in order to maintain a safe following distance . in fig9 the lead vehicle a is shown having passed the obstruction 60 and accelerated to the steady state speed . in consequence the separation distance s 3 has increased from s 2 . as described above , the vcu 15 tracks the position of the lead vehicle a and the speed thereof at a given location and control the speed of the host vehicle 10 to match that of the lead vehicle b ′ at the same location . accordingly , the vcu 15 controls the speed of the vehicle 10 such that it approaches and traverses the obstruction 60 at substantially the same speed as the lead vehicle a , as shown in fig1 . in consequence , the user of the host vehicle 10 is not required to override the speed control function in order to maintain vehicle composure as the vehicle negotiates the obstruction 60 . as shown in fig1 , the separation distance s increases to s 4 , which is greater than s 3 , as the following vehicle 10 negotiates the obstruction 60 and the lead vehicle a accelerates away from the obstruction 60 . fig1 shows the following vehicle b ′ after passing the obstruction 60 , whilst accelerating to follow the lead vehicle a at the pre - set separation distance s 1 , or cruise control target speed ( driver_set_speed ) if the lead vehicle a is travelling faster than the cruise control target speed . by repeatedly determining the speed of the following vehicle b ′, the instant location ( actual or relative ) of the following vehicle b ′ and the separation distance of the lead vehicle a and the following vehicle b ′, the speed of the lead vehicle a and the location ( actual or relative ) of the lead vehicle a can be computed from the following vehicle b ′ is described above , the separation distance of the lead vehicle a and the following vehicle b ′ is determined by the following vehicle b ′ transmitting an electromagnetic wave , which is reflected by the lead vehicle a and received by the forming vehicle b ′. thus a command signal to the cruise control system of the following vehicle b ′ can give information concerning the speed which the following vehicle b ′ should observe at the instant location of the lead vehicle a . the following vehicle b ′ knows the instant relative location of the lead vehicle a , since the separation distance is measured , and accordingly the speed of the following vehicle b ′ can be adjusted by the time it has reached the instant location of the lead vehicle a , so that it substantially matches the speed of the lead vehicle a when it was at that location . embodiments of the present invention may permit the actual location of the lead and following vehicles a , b ′ to be determined by progressively determining the distance travelled by the following vehicle b ′ from a known location , typically by using the vehicle odometer or an analogue thereof . the vehicles a , b ′ may be assumed to be of similar capability and road conditions may be assumed to be substantially identical along the path of travel , so that the following vehicle b ′ can substantially mimic the speed changes of the lead vehicle a at the same geographic locations . embodiments of the present invention may be implemented in any suitable manner , but are typically incorporated in an autonomous cruise control module of a vehicle . the module may form part of a vehicle control unit 15 as in the embodiment of fig1 , or be in the form of a separate , discrete module . thus such a ( following ) vehicle b ′ provides a continual update of vehicle speed ( i . e . its own speed ), and the relative separation distance of the lead vehicle a . from these inputs the cruise control module can calculate the changing speed of the lead vehicle a rather than just monitoring the separation distance thereof . thus the cruise control module is continually updated with not only the instant separation distance but also the instant speed of the lead vehicle a . knowing this instant speed , the cruise control module determines that the speed of the following vehicle b ′ should be controlled so that after travelling the instant separation distance , the following vehicle b ′ will be at the speed of the lead vehicle a at that location . the cruise control module is also configured to determine that adjustments of speed of the following vehicle b ′ are within pre - set limits of acceleration and deceleration to ensure that comfort and safety of the vehicle occupants is assured ; typical limits are acceleration at up to 1 . 25 m / s 2 and deceleration at up to 2 . 5 m / s 2 . thus , if the lead vehicle a accelerates or decelerates at a rate exceeding the prescribed limits , the following vehicle b ′ does not try to match the rate assumed by the lead vehicle a , but caps the rate of acceleration or deceleration at the prescribed values . when the lead vehicle a decelerates at a rate exceeding the prescribed value , the following vehicle b ′ may be operable to commence deceleration early so as to ensure that a speed of the following vehicle b ′ does not exceed that of the lead vehicle a at a given position , whilst also avoiding deceleration at a rate exceeding the prescribed value . thus , the following vehicle b ′ anticipates the requirement to reduce speed and may reduce speed at a rate higher than the lead vehicle a at a given geographical location thereby preventing the rate of deceleration from exceeding the prescribed value . other arrangements are also useful , it is to be understood that in some embodiments the following vehicle b ′ may nevertheless be operable to decelerate at a rate exceeding the limit value in order to prevent collision when a collision risk exists . in the embodiment described above and illustrated in fig5 to 11 , matching of following vehicle speed to lead vehicle speed at the same geographical location is enabled automatically by the vcu 15 when vehicle speed is below a prescribed value . in the present embodiment , the prescribed value is substantially 60 kph although other values are also useful . thus , the speed control function may be optimised for urban driving , where obstacles may be encountered relatively frequently , and highway driving , where obstacles are encountered relatively rarely . when travelling at speeds exceeding 60 kph on a highway , variations in speed of a lead vehicle are typically due to density of moving traffic , rather than to stationary obstacles . accordingly , implementation of speed control to match lead vehicle speed at the same geographic location may be inconvenient to a user of the following vehicle . however , in an urban or off - road environment , speed control according to geographic location may be highly advantageous , such as when approaching an obstacle such as a road hump ( as described in detail above ), a pothole or any other suitable obstacle negotiation of which may involve a decrease in vehicle speed in order to avoid loss of vehicle composure . embodiments of the invention may be understood by reference to the following numbered paragraphs : 1 . a method of autonomous vehicle speed control whereby a following vehicle maintains a substantially pre - determined separation from a lead vehicle , the method comprising repeatedly : determining from the following vehicle the separation distance of the lead vehicle and following vehicle ; determining from the following vehicle , the speed of the lead vehicle ; determining from the following vehicle the relative location of the lead vehicle ; and controlling the speed of the following vehicle to substantially match the speed of the lead vehicle at the said location , whereby the method is automatically enabled upon operation of autonomous speed control at a speed of less than a prescribed value . 2 . a method according to paragraph 1 , whereby the method is automatically enabled upon operation of autonomous cruise control at a speed of less than a prescribed value being a first value . 3 . a method according to paragraph 1 whereby the method is automatically enabled upon operation of low speed progress control at a speed of less than a prescribed value being a second value . 4 . a method according to paragraph 2 whereby the method is automatically enabled upon operation of low speed progress control at a speed of less than a prescribed value being a second value , whereby the first and second values are substantially the same . 5 . a method according to paragraph 2 whereby the method is automatically enabled upon operation of low speed progress control at a speed of less than a prescribed value being a second value , whereby the first and second values are different . 6 . a method according to paragraph 1 , whereby the separation distance is determined by a wave transmitted by the following vehicle , reflected by the lead vehicle , and received by the following vehicle . 7 . a method according to paragraph 1 , and including transmitting and receiving from the following vehicle an electromagnetic wave . 8 . a method according to paragraph 1 , and including progressively determining the distance travelled by the following vehicle . 9 . a method according to paragraph 8 , whereby said distance is provided by a vehicle odometer system , or an analogue thereof . 10 . a method according to paragraph 1 , and including repeatedly determining the location of the following vehicle . 11 . a method according to paragraph 10 , whereby location is determined by reference to a gps system . 12 . a method according to paragraph 1 , and having a refresh rate of 10 hz or greater . 13 . a method according to paragraph 1 whereby the prescribed value of speed is in the range from 20 kph to 80 kph , optionally in the range from 40 kph to 70 kph , further optionally in the range from 65 kph to 65 kph , optionally around 60 kph . 14 . an autonomous speed control system adapted to permit a following vehicle to follow a lead vehicle at a substantially predetermined separation , said system being arranged to repeatedly determine the separation distance of said vehicles , the speed of the lead vehicle , and the relative location of the lead vehicle , and to control the speed of the following vehicle to match the speed of the lead vehicle at the said location when the system is operating and the speed of the following vehicle is less than a prescribed value . 15 . an autonomous speed control system according to paragraph 14 comprising a cruise control system , the cruise control system being operable to control the speed of the following vehicle to match the speed of the lead vehicle at the said location when the cruise control system is controlling vehicle speed and the speed of the following vehicle is less than a first value . 16 . an autonomous speed control system according to paragraph 14 comprising a low speed progress control system , the low speed control system being operable to control the speed of the following vehicle to match the speed of the lead vehicle at the said location when the low speed progress control system is controlling vehicle speed and the speed of the following vehicle is less than a second value . 17 . a system according to paragraph 14 , and implemented in a speed control module of a vehicle , said module having an input signal indicative of the instant location of said vehicle , and said module being adapted to track said signal . 18 . a system according to paragraph 17 , wherein said signal is indicative of the geographical location of said vehicle . 19 . a system according to paragraph 17 , wherein said signal is indicative of the location of said vehicle relative to a leading vehicle . throughout the description and claims of this specification , the words “ comprise ” and “ contain ” and variations of the words , for example “ comprising ” and “ comprises ”, means “ including but not limited to ”, and is not intended to ( and does not ) exclude other moieties , additives , components , integers or steps . throughout the description and claims of this specification , the singular encompasses the plural unless the context otherwise requires . in particular , where the indefinite article is used , the specification is to be understood as contemplating plurality as well as singularity , unless the context requires otherwise . features , integers , characteristics , compounds , chemical moieties or groups described in conjunction with a particular aspect , embodiment or example of the invention are to be understood to be applicable to any other aspect , embodiment or example described herein unless incompatible therewith .