In this work, we present an innovative approach for effective cross-network information dissemination, with applications to vehicular ad hoc networks (VANETs). The proposed approach, denoted as "Cross-Network Effective Traffic Alert Dissemination" (X-NETAD), leverages on the spontaneous formation of local WiFi (IEEE 802.11b) VANETs, with direct connections between neighboring vehicles, in order to disseminate, very quickly and inexpensively, traffic alerts received from the cellular network. The proposed communication architecture has been implemented on Android smartphones. The obtained experimental results show that an effective cross-network information dissemination service can entirely rely on smartphone-based communications. This paves the way to future Internet architectures, where vehicles will play a key role as information destinations and sources.