Monitoring for escape of genetically modified (GM) oilseed rape (Brassica napus) during transport can be performed by means of roadside evaluations in areas where cultivation of this GM crop is not conducted, such as in Japan. We performed a survey of oilseed rape plants growing along a 20-km section of Japan's Route 51, one of the main land transportation routes in central Japan for imports of GM oilseed rape from the Port of Kashima into Keiyo District. Oilseed rape plants were found each year, but the number of plants varied substantially during the three years of our study: 2162 plants in 2005, 4066 in 2006, and only 278 in 2007. The low number in 2007 was probably caused by roadwork. Herbicide-resistant individuals were detected in the three consecutive years (26, 8, and 5 individuals with glyphosate resistance), but glufosinate-resistant plants (9 individuals) were detected only in 2005. The roadside plants occurred mainly along the inbound lane from Kashima to Narita. These plants are likely to have their origin in seeds spilled during transportation of cargo from the port, since there are no potential natural seed source plants for B. napus near Route 51. This is the first detailed report on the transition and distribution of herbicide-resistant oilseed rape plants following loss and spillage along Japanese roads.