CLAIMS:

1 . A method of determining a reachable area for one or more vehicles traversing a navigable network in a region covered by an electronic map, the electronic map comprising a plurality of segments representing navigable elements of the navigable network, the segments being connected by a plurality of nodes, the method comprising, for the or each vehicle:

obtaining data indicative of a current location of the vehicle;

identifying at least one set of one or more constraints limiting the distance which may be travelled by the vehicle from a current location,

and, for the or each set of one or more constraints, using the identified set of one or more constraints, the obtained current location data and the electronic map data to determine data indicative of a polygon representing a reachable area as represented by the electronic map, wherein the perimeter of the polygon is indicative of a limit of an area which may be reached by the vehicle from the current location taking into account the set of one or more constraints, and generating data indicative of the polygon.

2. The method of claim 1 further comprising obtaining data indicative of one or more vehicle parameters relating to each one of the one or more vehicles, and using the obtained data to identify, for the or each vehicle, at least some of the set of one or more constraints limiting the distance which may be travelled by the vehicle from the current location.

3. The method of any preceding claim wherein the one or more constraints include one or more of; a vehicle energy related constraint, a time related constraint and a constraint relating to financial cost.

4. The method of claim 3 wherein the time related constraint is a remaining driving time for the vehicle.

5. The method of any preceding claim wherein the step of determining the data indicative of a polygon takes into account live conditions on the navigable network.

6. The method of any preceding claim wherein the step of determining the data indicative of the polygon takes into account historical data relating to the traversal of segments of the network, realtime information indicative of conditions on the navigable network and/or historical driving behaviour of the vehicle driver.

7. The method of any preceding claim wherein the method comprises determining the data indicative of the polygon based on a cost of traversing segments in the navigable network represented by the electronic map, wherein the method comprises determining a set of one or more applicable cost values in respect of each one of the segments of the electronic map to be considered, wherein each

applicable cost value is in respect of a respective one of the set of one or more constraints, and wherein each constraint defines an upper limit for the cost of a path through the navigable network represented by the electronic map in respect of the constraint.

8. The method of any preceding claim wherein the perimeter of the polygon is defined by a closed polyline, and the method comprises determining data indicative of a closed polyline representing a perimeter of the polygon.

9. The method of claim 8 wherein each point on the polyline is indicative of a furthest node which may be reached when traversing the navigable network represented by the electronic map without contravening the set of one or more constraints.

10. The method of claim 9 as dependent upon claim 7 wherein the method comprises using the cost values associated with segments to identify a set of furthest nodes representing junctions of the navigable network which may be reached from the current location by traversing a path comprising one or more segments of the electronic map without exceeding the upper limit defined by the applicable constraint for any one of the costs related to ones of the set of one or more constraints.

1 1. The method of any one of claims 1 to 8 wherein each point defining the polyline is a location along a path generated from the current location to a destination, wherein the location along the path is a location beyond which further travel is not possible without contravening one of the set of one or more constraints.

12. The method of claim 1 1 as dependent upon claim 7 wherein the location corresponds to a furthest location which may be reached from the current location along the path to the destination without exceeding the upper limit defined by the applicable constraint for any one of the costs related to ones of the set of one or more constraints.

13. The method of claim 12 comprising identifying a maximum range which may be travelled by the vehicle without contravening any one of the set of one or more constraints, generating a circular area centred on the current location of the vehicle and having the maximum range as radius, determining a set of points corresponding to locations at which the perimeter of the circular area intersects segments of the road network as represented by the electronic map, and using the set of points determined as the set of destinations used in determining the points defining the polyline.

14. The method of any preceding claim wherein the set of one or more constraints provides a first set of one or more constraints, each constraint being indicative of a maximum value of a respective parameter.

15. The method of claim 14 wherein the at least one set of one or more constraints comprises a plurality of sets of one or more constraints, the plurality of sets comprising a first set of one or more constraints, each constraint being indicative of a maximum value of a respective parameter, and one or more further set of one or more constraints limiting the distance which may be travelled by the vehicle based on the first set of one or more constraints, wherein each further set of one or more constraints is more limiting than the previous set of one or more constraints.

16. The method of claim 15 wherein a value of a respective parameter defined by one or more, or each constraint of the or each further set of one or more constraints corresponds to a predefined percentage of a value of the corresponding parameter defined in the first set of one or more constraints.

17. The method of any preceding claim comprising using the generated data to cause an indication of the or each polygon to be displayed superposed on a representation of the electronic map.

18. The method of any preceding claim, wherein the method is performed in respect of each one of a plurality of vehicles in a fleet of vehicles.

19. The method of claim 18 wherein the method comprises obtaining a polygon representing a reachable area for one or more corresponding sets of one or more constraints for each one of the plurality of vehicles, and, when the reachable areas defined by respective polygons obtained for different vehicles in respect of corresponding sets of one or more constraints overlap, determining data indicative of a polygon representing a single, combined reachable area corresponding to the combination of the reachable areas defined by the respective polygons for the different vehicles in respect of the given set of one or more constraints, and generating data indicative of the polygon.

20. The method of claim 19 further comprising causing an indication of the polygon representing the single, combined reachable area to be displayed on a representation of the electronic map.

21. The method of any preceding claim further comprising receiving an indication of a destination associated with a new driving assignment to be assigned to a vehicle of a fleet of vehicles, and performing the method in respect of a set of one or more vehicles available for the assignment.

22. The method of claim 21 wherein performance of the method is triggered by receipt of the indication of the destination.

23. The method of claim 21 or claim 22 further comprising determining a vehicle to be allocated the assignment based on the data indicative of the or each polygon indicative of a reachable area determined, and providing data indicative of the destination associated with the assignment to the vehicle.

24. A system for determining a reachable area for one or more vehicles traversing a navigable network in a region covered by an electronic map, the electronic map comprising a plurality of segments representing navigable elements of the navigable network, the segments being connected by a plurality of nodes, the method comprising, for the or each vehicle:

means for obtaining data indicative of a current location of the vehicle;

means for identifying at least one set of one or more constraints limiting the distance which may be travelled by the vehicle from a current location,

and, for the or each set of one or more constraints, means for using the identified set of one or more constraints, the obtained current location data and the electronic map data to determine data indicative of a polygon representing a reachable area as represented by the electronic map, wherein the perimeter of the polygon is indicative of a limit of an area which may be reached by the vehicle from the current location taking into account the set of one or more constraints, and means for generating data indicative of the polygon.

25. A computer program product comprising instructions which, when read by a machine, cause the machine to operate according to the method of any one of claims 1 to 23.

26. A computer readable medium having the computer program product of claim 25 stored therein.