In 2013, International Agency for Research on Cancer (IARC) concluded that outdoor air pollution is carcinogenic to humans, with the particulate matter component of air pollution most closely associated with sufficient evidence of increased cancer incidence by exposure to particulate matter component of air pollution. Motor vehicles are one of a major emission sources of fine particle (PM2.5) in urban areas. A large number of epidemiological studies have reported a positive association of morbidity or mortality with distance from the roadside. We conducted this study to assess the association of PM2.5 concentrations measured at roadside hotspots with those at adjacent residential sites using real-time PM2.5 monitors. We conducted real-time PM2.5 measurements for rush hour periods (08:00∼10:00 and 18:00∼20:00) at 9 roadside air monitoring Hotspot sites in metropolitan Seoul over 3 weeks from October 1 to 21, 2013. Simultaneous measurements were conducted in residential sites within a 100 m radius from each roadside air monitoring site. A SidePak AM510 was used for the real-time PM2.5 measurements. Medians of roadside PM2.5 concentrations ranged from 9.8 μg/m3 to 38.3 μg/m3, while corresponding median values at adjacent residential sites ranged from 4.4 μg/m3 to 37.3 μg/m3. PM2.5 concentrations of residential sites were 0.97 times of hotspot roadside sites. Distributions of PM2.5 concentrations in roadside and residential areas were also very similar. Real-time PM2.5 concentrations at residential sites, (100 m adjacent), showed similar levels to those at roadside sites. Increasing the distance between roadside and residential sites, if needed, should be considered to protect urban resident populations from PM2.5 emitted by traffic related sources.