This paper presents a detailed quantitative evalu- ation of standardised ETSI & 3GPP Decentralised Congestion Control (DCC) and packet dropping mechanisms for Cellular V2X (C-V2X) and New Radio (NR) V2X. Based on the identified shortcomings, an Access layer scheme, RRI DCC, is then pro- posed. RRI DCC explicitly accommodates the sidelink scheduling mechanism Sensing Based Semi-Persistent Scheduling (SB-SPS), eliminating incompatibilities between current standards and the scheduling mechanism, to avoid unnecessary and recurring collisions. Three variants are proposed; one is an evolution of the ETSI Reactive DCC mechanism, the second is based on the ETSI Adaptive DCC mechanism and the final aligns with the 3GPP approach based on channel occupancy ratio (CR). All approaches are compared with current ETSI and 3GPP standards and exhibit improved performance. An evaluation of the proposed RRI DCC mechanisms and existing DCC standards, to meet the Quality of Service (QoS) requirements of vehicular cooperative awareness applications is also conducted.