A method of estimating a state of a target vehicle on a roadway, comprising: a. generating an estimate (C 0, C 1) of a curvature of the roadway; b. estimating an unconstrained state of the target vehicle and estimating an associated covariance thereof, wherein said unconstrained state is representative of a two-dimensional position, velocity and acceleration of the target vehicle; c. establishing at least one prospective constraint of the target vehicle, wherein at least one prospective constraint is responsive to said estimate (C 0, C 1) of the curvature of the roadway and said at least one prospective constraint is representative of a possible lane (38, 40) of the roadway in which the target vehicle is likely traveling; d. estimating at least one constrained state and associated covariance thereof of the target vehicle corresponding to said at least one prospective constraint of the target vehicle, wherein said at least one constrained state is representative of a two-dimensional position, velocity and acceleration of the target vehicle when traveling along a corresponding said possible lane (38, 40) of the roadway; e. determining a most likely state of the target vehicle to be the most likely state of said unconstrained state of the target vehicle and a most probable of said at least one constrained state of the target vehicle; and f. if said most probable of said at least one constrained state of the target vehicle is the determined most likely state, then fusing the unconstrained state and covariance thereof of the target vehicle with said most probable of said at least one constrained state and covariance thereof so as to generate a corresponding fused state estimate and an associated fused covariance thereof, and outputting, as the estimated state or covariance of the target vehicle, at least one of the fused state and the associated fused covariance thereof of the target vehicle; otherwise outputting, as the estimated state or covariance of the target vehicle, at least one of the unconstrained state and the associated unconstrained covariance thereof of the target vehicle. a. generating an estimate (C 0, C 1) of a curvature of the roadway; b. estimating an unconstrained state of the target vehicle and estimating an associated covariance thereof, wherein said unconstrained state is representative of a two-dimensional position, velocity and acceleration of the target vehicle; c. establishing at least one prospective constraint of the target vehicle, wherein at least one prospective constraint is responsive to said estimate (C 0, C 1) of the curvature of the roadway and said at least one prospective constraint is representative of a possible lane (38, 40) of the roadway in which the target vehicle is likely traveling; d. estimating at least one constrained state and associated covariance thereof of the target vehicle corresponding to said at least one prospective constraint of the target vehicle, wherein said at least one constrained state is representative of a two-dimensional position, velocity and acceleration of the target vehicle when traveling along a corresponding said possible lane (38, 40) of the roadway; e. determining a most likely state of the target vehicle to be the most likely state of said unconstrained state of the target vehicle and a most probable of said at least one constrained state of the target vehicle; and f. if said most probable of said at least one constrained state of the target vehicle is the determined most likely state, then fusing the unconstrained state and covariance thereof of the target vehicle with said most probable of said at least one constrained state and covariance thereof so as to generate a corresponding fused state estimate and an associated fused covariance thereof, and outputting, as the estimated state or covariance of the target vehicle, at least one of the fused state and the associated fused covariance thereof of the target vehicle; otherwise outputting, as the estimated state or covariance of the target vehicle, at least one of the unconstrained state and the associated unconstrained covariance thereof of the target vehicle.