Cellular Vehicle-to-Everything (C-V2X) standard is introduced by the Third Generation Partnership Project (3GPP) in Release 14, where Mode 4 is defined for Vehicle-to-Vehicle (V2V) communications. To alleviate intense resource contention when vehicle density becomes high, Mode 4 is evolved to Mode 2 for New Radio-V2X (NR-V2X) in 3GPP Release 16. However, in Mode 2, periodic beacon messages can occupy the entire control channel, causing a decrease in Packet Delivery Rate (PDR), especially in the presence of high traffic density. To address this issue, in this chapter, an enhanced adaptive access mechanism for Mode 2, ADAM-M2, is proposed that improves the physical layer design of Mode 2. The ADAM-M2 introduces enhancement to the Semi-Persistent Scheduling (SPS) scheme and adaptively adjusts the Modulation and Coding Scheme (MCS) and sub-channelization scheme of the vehicular users based on the traffic density. Four types of packet delivery errors are analyzed, based on which the overall PDR is derived. Extensive simulation results show that the proposed mechanism surpasses the existing Mode 4 and Mode 2 under different traffic densities and sender-receiver distances, in terms of achieving higher PDR.