Method for determining a lateral position of a vehicle relative to the lanes of a roadway, comprising the steps of: - detecting roadway markings; - determining a starting position for the vehicle based on reception data of a GPS receiver; - wherein, when it is established that the detected roadway markings are not available in sufficient numbers and accuracy to ascertain a lateral position of the vehicle by comparing the detected roadway markings with roadway marking information from a lane geometry map for the starting position determined for the vehicle, an approximately lateral position is ascertained by taking into account information from the lane geometry map, wherein the following steps are carried out: - constructing an orthogonal line at the starting position orthogonal to a current vehicle movement; - determining intersections (FP, UP) between the orthogonal line and the lane centres of the lane geometry map for the starting position determined for the vehicle; - based on the intersections (FP, UP), ascertaining, by taking into account configured assumptions about possible driving manoeuvres which define which lanes are considered to be driveable, that only one of the intersections is located in a lane which is considered to be driveable; and - ascertaining the intersection with the lane which is considered to be driveable as an approximately lateral position of the vehicle; or - constructing an orthogonal line orthogonal to an averaged vehicle movement between a previous iteration of the position determination and the current vehicle movement and at a longitudinal position between a final position determined in the previous iteration and the current starting position; - determining intersections (SPL, SPM) between the orthogonal line and the lane markings and lane centres of the lane geometry map for the starting position determined for the vehicle; - based on the intersections (SPL, SPM), ascertaining, by taking into account configured assumptions about possible driving manoeuvres which define which lane markings are considered to be able to be driven over and which lanes are considered to be driveable that, proceeding from the final position determined in the previous iteration, it is not possible to change lanes; and - ascertaining a position in the lane identified by the starting position as an approximately lateral position of the vehicle. - detecting roadway markings; - determining a starting position for the vehicle based on reception data of a GPS receiver; - wherein, when it is established that the detected roadway markings are not available in sufficient numbers and accuracy to ascertain a lateral position of the vehicle by comparing the detected roadway markings with roadway marking information from a lane geometry map for the starting position determined for the vehicle, an approximately lateral position is ascertained by taking into account information from the lane geometry map, wherein the following steps are carried out: - constructing an orthogonal line at the starting position orthogonal to a current vehicle movement; - determining intersections (FP, UP) between the orthogonal line and the lane centres of the lane geometry map for the starting position determined for the vehicle; - based on the intersections (FP, UP), ascertaining, by taking into account configured assumptions about possible driving manoeuvres which define which lanes are considered to be driveable, that only one of the intersections is located in a lane which is considered to be driveable; and - ascertaining the intersection with the lane which is considered to be driveable as an approximately lateral position of the vehicle; or - constructing an orthogonal line orthogonal to an averaged vehicle movement between a previous iteration of the position determination and the current vehicle movement and at a longitudinal position between a final position determined in the previous iteration and the current starting position; - determining intersections (SPL, SPM) between the orthogonal line and the lane markings and lane centres of the lane geometry map for the starting position determined for the vehicle; - based on the intersections (SPL, SPM), ascertaining, by taking into account configured assumptions about possible driving manoeuvres which define which lane markings are considered to be able to be driven over and which lanes are considered to be driveable that, proceeding from the final position determined in the previous iteration, it is not possible to change lanes; and - ascertaining a position in the lane identified by the starting position as an approximately lateral position of the vehicle.