The spectrum scarcity of VANETs (Vehicular Ad hoc Networks) can be alleviated by spectrum sharing technology. We present a framework of CCR-VANETs (Cellular-based Cognitive-radio Vehicular Ad hoc Networks). In CCR-VANETs, cellular network performs as primary network while VANET shares the downlink spectrum of cellular network. We consider a scalable urban grid scenario in which vehicles detect available spectrum holes and opportunistically access them according to a carrier-sensing multiple-access protocol. To restrict vehicles' interference to primary receivers, we set a square preservation region around each particular street block where an active base station is located. The number of street blocks in the preservation region is calculated with the practical assumption that vehicles only know the locations of primary transmitters. We analyze the aggregate interference power from primary and secondary networks, then derive the lower-bound of downlink capacity for the primary network and lower-bound of V2V (Vehicle-to-Vehicle) channel capacity for the secondary network respectively. The numerical results demonstrate the impacts of different network parameters on inter-networks interference level and network capacities.