Self-Organizing TDMA (S-TDMA) is a channel access technique based on a time-slotted channel and distributed slot reservation procedure. It is considered as a valid alternative to the Dedicated Short Range Communication (DSRC) standard for safety-critical (V2X) communications, because of its stable deterministic access opportunity and delay under various channel loads. This paper provides an analytical model of S-TDMA to compute the Slot Occupation Distribution and Packet Level Incoordination in closed-form expression. By means of the proposed model, we then provide an asymptotic evaluation of the impact of various configuration parameters on the communication performance of S-TDMA.