We propose a fully-distributed approach to the on line estimation of vehicle traffic density. Our approach envisions vehicles communicating within a VANET and cooperating to collect density measurements through a uniform sampling of the road sections of interest. The proposed scheme does not require the presence of any network infrastructure, central controller or devices triggered by the passage of vehicles, and it is suitable for both highway and urban environments. Results derived through ns-2 simulations in realistic mobility scenarios show that our solution is very effective, providing accurate, on-line estimates of the traffic density with minimal protocol overhead.