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In this paper, we investigate data dissemination in delay tolerant networks (DTNs) via roadside wireless local area networks (RS-WLANs). The data dissemination service is destined to a group of nomadic nodes roaming in a large network region with a low node density. The local nodes within the coverage area of an RS-WLAN can provide packet caching and relaying capabilities. We consider a cooperative data dissemination approach where information packets are first pre-downloaded to the local nodes within the RS-WLAN before the visit of a pedestrian nomadic node, and then opportunistically scheduled to transmit to the nomadic node upon its arrival. In order to resolve the channel contention among multiple direct/relay links and exploit the predictable traffic characteristics as a result of packet pre-downloading, a double-loop receiver-initiated medium access control (DRMAC) scheme is proposed. The MAC scheme can achieve spatial and temporal diversity via the outer-loop and inner-loop MAC, respectively. A receiver initiated mechanism is used to reduce the signalling overhead, where the ACK message is used as an invitation of channel contention. An analytical model is established to evaluate the performance of the proposed MAC scheme. Numerical results demonstrate that the proposed MAC scheme can significantly improve the number of delivered packets from an RS-WLAN to a nomadic node as compared with the existing MAC schemes.