Using porous road surfaces can be an alternative to more traditional noise reduction measures. Two factors contribute to the fact that road noise can get lower than with a conventional dense road surface: the rolling noise emission decreases, and the sound-absorbing road surface can further reduce the level. In a traffic noise calculation with Nord2000, the influence of the road surface on the sound emission is specified using road surface corrections while the acoustic surface impedance of the road surface determines the ground attenuation. However, due to measurement technical reasons, porous road surface corrections will include some (undesirable) ground attenuation, leading to that the rolling noise emission might be underestimated. Another problem is that the impedance model in Nord2000 does not give correct results for porous road surfaces. These complications have been investigated with Nord2000 calculations using a program code extended to also include an impedance model capable of handling porous road surfaces. The results have been used to formulate official Swedish recommendations on how porous road surfaces can be handled in traffic noise investigations with Nord2000.