An optimized method is presented for the numerical evaluation of the sound field generated by an incoherent line source, which is commonly used to model road and rail traffic noise. Two different solutions for the numerical integration over the line source are distinguished, a point source solution and a line source solution. With proper segmentation of the line source, both solutions yield accurate results. Special attention is paid to receiver positions close to the (infinite) line through the (finite) line source. At these positions, conventional methods give numerical errors, which occur frequently in calculations of large-scale noise maps of cities, employing automatically generated geographical input data. The problems are avoided by using the optimized method presented here. The method is based on a combination of angular segmentation and linear segmentation of the line source and can be used to minimize the number of point-to-point calculations for noise mapping.