A computer-implemented method of determining whether a planned trajectory of a first model vehicle over a model of a road along which the first model vehicle and a second model vehicle are travelling, for use in autonomous control of a vehicle modelled by the first model vehicle, is invalid, the model of the road comprising a plurality of lanes defined by a plurality of lane boundaries, the method comprising: obtaining the planned trajectory, the planned trajectory comprising a respective first vehicle state of the first model vehicle defined relative to the model of the road for each of a plurality of time instants, each first vehicle state being defined by a respective lateral position of the first model vehicle, a respective lateral velocity of the first model vehicle and a respective lateral acceleration of the first model vehicle; obtaining a second vehicle state of the second model vehicle defined relative to the model of the road for each of the plurality of time instants, each second vehicle state being defined by a respective lateral position of the second model vehicle in the model of the road; determining, for each of the plurality of time instants, a respective lateral range extending from the second model vehicle, using a respective lateral position of the second model vehicle relative to a centre of a lane among the plurality of lanes in which the second model vehicle is located; and determining that the planned trajectory is invalid in a case where, for the first vehicle state and the second vehicle state at at least one time instant of the plurality of time instants, the following conditions of a first set of conditions are satisfied: the lateral position of the first model vehicle is within the lateral range of the second model vehicle at the time instant and within a lane boundary region extending along and comprising a lane boundary of the plurality of lane boundaries; and a direction of the lateral velocity of the first model vehicle is towards the second model vehicle and a magnitude of the lateral acceleration of the first model vehicle away from the second model vehicle is less than a predetermined lateral deceleration of the first model vehicle. obtaining the planned trajectory, the planned trajectory comprising a respective first vehicle state of the first model vehicle defined relative to the model of the road for each of a plurality of time instants, each first vehicle state being defined by a respective lateral position of the first model vehicle, a respective lateral velocity of the first model vehicle and a respective lateral acceleration of the first model vehicle; obtaining a second vehicle state of the second model vehicle defined relative to the model of the road for each of the plurality of time instants, each second vehicle state being defined by a respective lateral position of the second model vehicle in the model of the road; determining, for each of the plurality of time instants, a respective lateral range extending from the second model vehicle, using a respective lateral position of the second model vehicle relative to a centre of a lane among the plurality of lanes in which the second model vehicle is located; and determining that the planned trajectory is invalid in a case where, for the first vehicle state and the second vehicle state at at least one time instant of the plurality of time instants, the following conditions of a first set of conditions are satisfied: the lateral position of the first model vehicle is within the lateral range of the second model vehicle at the time instant and within a lane boundary region extending along and comprising a lane boundary of the plurality of lane boundaries; and a direction of the lateral velocity of the first model vehicle is towards the second model vehicle and a magnitude of the lateral acceleration of the first model vehicle away from the second model vehicle is less than a predetermined lateral deceleration of the first model vehicle. the lateral position of the first model vehicle is within the lateral range of the second model vehicle at the time instant and within a lane boundary region extending along and comprising a lane boundary of the plurality of lane boundaries; and a direction of the lateral velocity of the first model vehicle is towards the second model vehicle and a magnitude of the lateral acceleration of the first model vehicle away from the second model vehicle is less than a predetermined lateral deceleration of the first model vehicle.