Feral populations of crop species along roadsides contribute to the uncertainty regarding the containment of genetically modified (GM) crops, as the feral populations could promote the persistence of transgenes outside of cultivated fields.  Roadside populations of several common crop species are known to occur far from arable fields, and the dispersal pathways that promote their recruitment in road verges are unclear.  Human-aided dispersal, in particular adhesive dispersal by vehicles, has been suggested as a possible vector, but this has not yet been proven experimentally.  We sampled the seed rain from vehicles inside two motorway tunnels in an urban environment to reveal the contribution of crop species to seeds unintentionally dispersed by traffic beyond agricultural production areas.  Three species of arable crops, wheat Triticum aestivum, rye Secale cereale and oilseed rape Brassica napus, were among the most frequent species deposited by vehicles inside the motorway tunnels.  Each of the three species was clearly more predominant in one direction of traffic.  While seeds of Triticum aestivum and Secale cereale were primarily transported into the city, Brassica napus was significantly more abundant in samples from lanes leading out of the city.  Seed sources in the local surroundings of the tunnels were virtually nonexistent, and the high magnitude of seed deposition combined with high seed weights suggests a dispersal mechanism different from other species in the sample, at least for Triticum aestivum and Secale cereale.  This provides evidence that spillage during transport is a major driver for long-distance dispersal of crops.  Our results suggest that seed dispersal by vehicles is the major driver in the recruitment of roadside populations of arable crops, providing a possible escape route for GM crops.  Risk management should thus aim at curbing transport losses of GM crops.