In this paper we propose an alternative to the security protocols generally developed for VANETs (Vehicular Ad-hoc NETworks), which often rely on asymmetric keys and PKIs (Public Key Infrastructures). Instead, we propose a solution whose architecture is based on Physical Unclonable Functions (PUFs) inside the vehicles. We design a protocol with these PUF functions to securely spread a secret key by means of a Road Side Unit (RSU). This secret key will be used by the vehicles to warn neigboring vehicles of an emergency situation. The key is regularly changed by the RSU but its value evolves slowly over time in a random way. Thus a message using an old (but not too old) key can be accepted as valid. We define the whole protocol where a vehicle first identifies itself to the Road Side Unit for authentication. Then the RSU sends back a key which will be used to send alerts if an emergency situation occurs on the road. The RSU manages the evolution of this key by just randomly changing one bit at each change. Thus an alert can be accepted if the key used to issue an alert is "close" to the last key issued by the RSU. We investigate the advantages of this new scheme and evaluate the implications in terms of exchanges between the vehicles (and their PUFs) and the network connecting the RSUs.