This study aims to model road traffic noise levels and estimate the human exposure at the 25 districts in the metropolitan Seoul, Republic of Korea. The SoundPLAN® Version 7.1 software package was used to model noise levels and simulated road traffic noise maps were created. The people exposed to daytime/nighttime road traffic noise were also estimated. The proportions of the population exposed to road traffic noise in major cities in the EU were also estimated and compared. Eight (8) districts show the exceeded rate (percentage of the exposed population exceeding the daytime standard) of 20% or more, and eleven (11) districts show 10%-20% and six (6) districts show less than 10%, which indicates considerable variation among districts. Two districts (Nowon-gu and Yangcheon-gu) show the highest exposure rate during the daytime (35.2%). For nighttime noise levels, fourteen (14) districts show the exceeded rate (percentage of exposed population exceeding the nighttime standard) over 30%. The average percentages of the exposed population exceeding the daytime/nighttime standards in Seoul and the EU were 16.6%/34.8% and 13.0%/16.1%, respectively. The results show that road traffic noise reduction measures should urgently be taken for the nighttime traffic noise in Seoul. When the grid noise map and the 3-D façade noise map were compared, the 3-D façade noise map was more accurate in estimating exposed population in citywide noise mapping.