In some instances, the digital map data 140 may be inaccurate. For example, the digital map data 140 may be out of date, or temporary speed limit restrictions may be in place due to road works for example. The control system 110 is thus configured to identify a traffic sign 300 in the received image data 125 and to determine a sign speed limit indicated by the traffic sign 300. The control system 110 may then output to the one or more further vehicle control systems 130 a sign speed limit signal 135 indicative of the sign speed limit i.e. the speed limit for the road segment conveyed or indicated by the road sign. In the embodiment illustrated in FIG. 3, the control system 110 may output a sign speed limit signal 135 indicative of a determined 30 km/h speed limit. Thus, the sign speed limit signal may override or supersede the map speed limit in a situation where the sign speed limit does not match the map speed limit. The further control systems 130 may then utilise the updated example 30 km/h speed limit, rather than the 40 km/h indicated by the digital map data.

In this way, a sign speed limit indicated by a traffic sign 300 detected by the sensor unit 120 may override the map speed limit. Vehicle control systems 130 such as a cruise control system 130 may then be able to account for recently changed speed limits and temporary speed limits which have not yet been reflected in the digital map data 140. Furthermore, the sign speed limit indicated by the traffic sign 300 may provide speed limit information where no map speed limit information is available, for example when the vehicle is on a road segment not present in the digital map data 140.

However, not every traffic sign detected by the sensor unit 120 is relevant to the vehicle 200. A traffic sign 300 may be defined as relevant to the vehicle 200 if it is associated with the primary road PR i.e. on which the vehicle is located.

FIGS. 4A and 4B illustrate two examples of a traffic sign 300 which is not relevant to the vehicle 200. The traffic sign 300 may be associated with another road proximal to the primary road PR, such that a road sign associated with the other road is visible to the sensor unit 120. For example, the traffic sign 300 may be associated with a side road SR. Any road aside from the primary road may be referred to henceforth as a side road SR. The side road SR may be any road on which the vehicle is not located. In FIG. 4A, the side road SR is a road branching from the primary road PR. The side road SR may also be a road intersecting the primary road or diverting from the primary road PR at a junction, such as a roundabout or rotary. In some examples, the side road SR may be a road that does not meet the primary road at all. In FIG. 4B, the side road SR is a separate road substantially parallel to the primary road.

In each example of FIGS. 4A and 4B, the traffic sign 300 may be detected by the control system 110 of the vehicle 200 travelling on the primary road PR. However, each illustrated traffic sign 300 is associated with a side road SR, not the primary road PR. Thus, if the control system 110 outputs a sign speed limit signal indicative of the 30 km/h speed limit, this may erroneously override the correct 40 km/h speed limit for the primary road PR indicated by the digital map data, causing the vehicle systems to adjust control of the vehicle 200 unnecessarily.

Embodiments of the control system 110 are thus designed to determine whether the traffic sign 300 is relevant to the vehicle 200 at a time at which the sign is detected by the sensor unit 120. By relevant to the vehicle 200, it is meant whether the traffic sign 300 is associated with the primary road PR. If the traffic sign 300 is determined to be not relevant to the vehicle 200, for example due to being associated with a side road SR, the control system 110 may suppress or refrain from generating the sign speed limit signal 135.

A method 500 of operation of the control system 110 according to an embodiment of the invention is illustrated in FIG. 5.

The method 500 comprises a block 510 for receiving image data indicative of an environment of the vehicle 200. The control system 110 may be configured to receive the image data 125 from the sensor unit 120, as discussed. The image data 125 may be indicative of the region in front of the vehicle 200 and may comprise a representation of one or more traffic signs 300 present in the image data.

The method 500 comprises a block 520 of identifying at least one traffic sign 300 in the image data 125. The traffic sign 300 may be identified by processing the image data 125 to search for predetermined shapes, colours and/or textures in the image matching known traffic sign criteria, as will be appreciated by the skilled reader. The block 520 further comprises determining a sign speed limit indicated by the traffic sign 300. The speed limit may be determined for example using any known image processing technique, such as using pictogram contours. For example, the block 520 may comprise identifying one traffic sign 300 in the image data 125 indicating a sign speed limit of 30 km/h.

The method 500 comprises a block 530 of determining a relevance of the sign speed limit to the vehicle 200. Determining a relevance may comprise classifying the speed limit as one of relevant or not relevant to the vehicle 200. The determined relevance may be indicative of a likelihood that the traffic sign is associated with the primary road PR. The speed limit may be classified as not relevant if it is determined to be likely that the traffic sign is applicable to a side road SR.

The sign speed limit may be determined as relevant or not relevant depending on a location of the traffic sign 300 with respect to a road feature in the environment. In different embodiments, the road feature may be one or more of: an edge of the primary road, an entrance stub to a side road or a further traffic sign, as will be explained. The location of the traffic sign with respect to the road feature may indicate a likelihood that the traffic sign is associated with a side road SR.

With reference to FIG. 6, according to some embodiments block 530 comprises identifying a road edge 600 of the primary road PR. The road edge 600 may be identified in the image data 125, for example by identifying changes in the contrast and/or colour of the image data 125 which may be indicative of a road edge 600. For example, a transition or boundary may be identified between a road surface which may be relatively smooth (for example defined by asphalt, concrete or other surfacing material) and an adjacent surface which may be relatively rough (for example composed of one or more of the following: grass, mud, gravel, sand and snow). In other examples, a road edge 600 may be identified in dependence on road markings identified on the primary road PR, or a physical delineation or boundary such as a kerb.

The control system may then be configured to determine a distance 610 between the road edge 600 and the traffic sign 300. The distance may be a lateral distance 610, i.e. a distance substantially perpendicular to the road edge 600. The distance 610 may be determined from the image data 125 by determining a lateral distance 630 between the vehicle 200 and the road edge 600 and a lateral distance 620 between the vehicle 200 and the traffic sign 300. Each determined distance 620, 630 may be inferred from a size and/or location of the traffic sign 300 and road edge 600 in the image data 125, using known image processing techniques.

In block 530, the control system 110 may determine the speed limit to be not relevant to the vehicle if the determined lateral distance 610 between the road edge 600 and the traffic sign 300 meets a predetermined criterion. The predetermined criterion may be that the distance 610 is greater than or equal to a predetermined threshold. The predetermined threshold may be defined in dependence on where the vehicle 200 is located geographically, for example in dependence on which country the vehicle 200 is located within. Local regulations may define a standardised range of distances between a traffic sign 300 and a road edge 600. If the distance 610 is greater than an upper limit of the standardised range, it may be determined that the traffic sign 300 is not associated with the primary road PR because it is too distant from the road edge 600.

According to some embodiments, the road feature may comprise an entrance stub to a side road SR. With reference to FIG. 7, an entrance stub 700 may be a region at which the side road SR meets the primary road PR. The entrance stub 700 may be a turn-off, an intersection, a slip lane, or any other junction. In some embodiments, block 530 may comprise identifying the entrance stub 700 in the image data 125. For example, the entrance stub 700 may be identified in the image data 125 by identifying a gap or divergence in the road edge 600. Alternatively, or additionally, the control system 110 may identify the location of the entrance stub 700 with respect to the vehicle 200 in dependence on the digital map data 140. The digital map data 140 may indicate a location of a node at which a side road SR segment diverges from the segment of primary road PR on which the vehicle 200 is located.

In block 530, the control system 110 may determine a stub distance 710 from the traffic sign 300 to the entrance stub 700. A location of one or more boundaries 705, 706 defining the edge of the entrance stub 700 may be identified from the image data 125 or the digital map data 140. The stub distance 710 may then be determined as a distance between the traffic sign 300 and the closest boundary 705, 706 to the traffic sign. In the illustrated embodiment, the traffic sign 300 is positioned after the entrance stub 300 along the primary road PR with respect to the direction of travelled of the vehicle 200 and the closest boundary is the boundary 705. However, if the traffic sign 300 is positioned before the entrance stub 700, the closest boundary to the traffic sign 300 may be the boundary 706. According to some embodiments, the stub distance 710 may be defined as a distance from the traffic sign 300 to an alternative point on the entrance stub 700, for example a mid-point of the entrance stub 700.

The digital map data 140 may indicate a map speed limit associated with the side road SR. In block 540, the control system 110 may determine whether the map speed limit associated with the side road matches the sign speed limit. If the map speed limit associated with the side road matches the sign speed limit, this may indicate that the traffic sign 300 is associated with the side road SR, and thus is not relevant to the vehicle 200.

To more accurately ensure that the traffic sign 300 is associated with the side road SR, the control system 110 may be configured to determine whether the stub distance 710 is less than a predetermined stub distance threshold, that is whether the traffic sign 300 is located sufficiently close to the entrance stub 700. If the stub distance 710 is less than the stub distance threshold, and the map speed limit associated with the side road matches the sign speed limit, the control system 110 may determine that the traffic sign 300 is associated with the side road SR and thus is not relevant to the vehicle 200.

With reference to FIG. 8, according to some embodiments the control system 110 may be configured to determine that the traffic sign 300 is not relevant to the vehicle 200 in dependence on its location with respect to a further traffic sign 800. If a first traffic sign 300 and second traffic sign 800 are each identified by the control system 110, in block 530 the control system 110 may determine whether the first traffic sign 300 and the second traffic sign 800 span a side road SR.

In FIG. 8, the first traffic sign 300 and second traffic sign 800 are illustrated spanning the side road SR. By spanning the side road SR, it is meant that the first traffic sign 300 and the second traffic sign 800 are located on opposing sides of the side road SR and are generally aligned along an axis perpendicular to the side road SR. A pair of traffic signs 300, 800 as illustrated may be referred to as gated signs. Such gated signs may often be used to indicate a change in speed limit, and the provision of a traffic sign on each side of the road enables a driver to readily deduce to which road the traffic signs relate. By determining that the first and second traffic signs 300, 800 span a side road, it may be inferred that each of the first and second traffic signs 300, 800 are associated with the side road SR and thus are not relevant to the vehicle 200.

The first traffic sign 300 and the second traffic sign 800 are determined to span the side road SR if one or more spanning criteria are met. A first spanning criterion may be that the first traffic sign 300 and second traffic sign 800 are indicative of the same speed limit. A second spanning criterion may be that each of the first traffic sign 300 and second traffic sign 800 are each located to a same side of the vehicle 200. A third spanning criterion may be that a distance between the first traffic sign 300 and the second traffic sign 800 is below a threshold, i.e. that the traffic signs 300, 800 are sufficiently close together.

The methods described above for determining the relevance of a traffic sign 300 to the vehicle 200 may be performed in isolation or in combination. An example operation of the control system 110 during block 530 according to an embodiment is illustrated in FIG. 9. In the illustrated embodiment, the control system 110 performs each of the above-described methods in sequence to determine a relevance of the traffic sign 300 to the vehicle 200.

In step 531, it is determined whether the sign speed limit matches a map speed limit associated with a side road SR. If the sign speed limit matches the map speed limit, in step