In a former paper [Fluct. Noise Lett.13 (2014) 1450020] we introduced a vehicular communication system with unconditionally secure key exchange based on the Kirchhoff-LawJohnson-Noise (KLJN) key distribution scheme. In this paper, we address the secure KLJN key donation to vehicles. This KLJN key donation solution is performed lane-by-lane by using roadside key provider equipment embedded in the pavement. A method to compute the lifetime of the KLJN key is also given. This key lifetime depends on the car density and gives an upper limit of the lifetime of the KLJN key for vehicular communication networks.