An Intelligent Transportation System, ITS, service station, comprising: a receiver configured to receive, from one or more sensors (E q) each of which is configured to perceive road users (U j) in the sensor's environment, information on a set of road users (U j) perceived by the sensors (E q), wherein said road user information includes, for each road user (U j) in the set, a respective geographical position (P j) determined by the sensors (E q); a controller connected to the receiver and configured to determine, on the basis of said geographical positions (P j), whether a subset (T Sk) of road users (U j) in said set meets a predetermined criterion of mutual proximity; and a transmitter connected to the controller and configured to transmit, when said subset (T Sk) comprises two or more road users (U j), a service message (M s) indicative of said subset (T Sk), wherein said determining comprises calculating, from said geographical positions (P j), distances (d j,j+1) between road users (U j, U j+1) in the set, and wherein said criterion comprises that the respective distance (d j,j+1) of a road user (U j) to each of a predetermined number of other road users (U j+1) in the set is smaller than a predetermined distance threshold (Td 1, Td 2), characterized in that said determining further comprises at least one of - calculating, from the road user information, heading differences (Δh j,j+1) between headings (h j, h j+1) of road users (U j, U j+1) in the set, wherein said criterion further comprises that the respective heading differences (Δh j,j+1) of the road user (U j) to each of the predetermined number of other road users (U j+1) in the set are smaller than a predetermined heading threshold (Th), and - calculating, from the road user information, speed differences (Δv j,j+1) between speeds (v j, v j+1) of road users (U j, U j+1) in the set, wherein said criterion further comprises that the respective speed differences (Δv j,j+1) of the road user (U j) to each of the predetermined number of other road users (U j+1) in the set are smaller than a predetermined speed threshold (Tv). a receiver configured to receive, from one or more sensors (E q) each of which is configured to perceive road users (U j) in the sensor's environment, information on a set of road users (U j) perceived by the sensors (E q), wherein said road user information includes, for each road user (U j) in the set, a respective geographical position (P j) determined by the sensors (E q); a controller connected to the receiver and configured to determine, on the basis of said geographical positions (P j), whether a subset (T Sk) of road users (U j) in said set meets a predetermined criterion of mutual proximity; and a transmitter connected to the controller and configured to transmit, when said subset (T Sk) comprises two or more road users (U j), a service message (M s) indicative of said subset (T Sk), wherein said determining comprises calculating, from said geographical positions (P j), distances (d j,j+1) between road users (U j, U j+1) in the set, and wherein said criterion comprises that the respective distance (d j,j+1) of a road user (U j) to each of a predetermined number of other road users (U j+1) in the set is smaller than a predetermined distance threshold (Td 1, Td 2), characterized in that said determining further comprises at least one of - calculating, from the road user information, heading differences (Δh j,j+1) between headings (h j, h j+1) of road users (U j, U j+1) in the set, wherein said criterion further comprises that the respective heading differences (Δh j,j+1) of the road user (U j) to each of the predetermined number of other road users (U j+1) in the set are smaller than a predetermined heading threshold (Th), and - calculating, from the road user information, speed differences (Δv j,j+1) between speeds (v j, v j+1) of road users (U j, U j+1) in the set, wherein said criterion further comprises that the respective speed differences (Δv j,j+1) of the road user (U j) to each of the predetermined number of other road users (U j+1) in the set are smaller than a predetermined speed threshold (Tv). - calculating, from the road user information, heading differences (Δh j,j+1) between headings (h j, h j+1) of road users (U j, U j+1) in the set, wherein said criterion further comprises that the respective heading differences (Δh j,j+1) of the road user (U j) to each of the predetermined number of other road users (U j+1) in the set are smaller than a predetermined heading threshold (Th), and - calculating, from the road user information, speed differences (Δv j,j+1) between speeds (v j, v j+1) of road users (U j, U j+1) in the set, wherein said criterion further comprises that the respective speed differences (Δv j,j+1) of the road user (U j) to each of the predetermined number of other road users (U j+1) in the set are smaller than a predetermined speed threshold (Tv).