In this paper, we present the overhead-free (OHF) spectrum sensing and reporting protocol which counters the deficiencies in typical cognitive radio vehicular ad-hoc networks (CR-VANETs) in high density networks. The OHF approach allows each vehicle to randomly sense only one channel to reduce the amount of sensing performed in existing approaches in which each vehicle senses all or a subset of the channels. More importantly, the OHF protocol significantly reduces the time needed to collect the sensing information, and hence, the effort of producing the radio environment map (REM), by having only one vehicle out of those inferred the occupancy of the channel by its primary owner reporting such an information. When OR fusion is applied, it is sufficient to have a single vehicle with a positive decision to report such information. Such a simplified and implicit aggregation is implemented via slotted contention that can be easily incorporated with the IEEE 802.11p standard. By properly choosing its contention parameters, the proposed overhead-free protocol achieves constant and low REM time, and higher CR-VANET throughput with lower outages to the primary networks compared to IEEE 802.11p based CR-VANETs.