Vehicular traffic monitoring is a major enabler for a whole range of Intelligent Transportation System services. Real time, high spatial and temporal resolution vehicular traffic monitoring is becoming a reality thanks to the variety of communication platforms that are being deployed. Dedicated Short Range Communications (DSRC) and cellular communications like Long Term Evolution (LTE) are the major technologies. The former is specifically tailored for Vehicular Ad-hoc Network, the second one is pervasive. We propose a fully distributed Floating Car Data (FCD) collection protocol that exploits the heterogeneous network provided by DSRC and LTE. The proposed approach adapts automatically to the penetration degree of DSRC, achieving the maximum possible LTE offloading, given the VANET connectivity achieved via DSRC. Extensive simulations in real urban scenarios are used to evaluate the protocol performance and LTE offloading, as compared to baseline and literature approaches.