In this paper, we propose access control schemes for the contention resolution diversity slotted ALOHA (CRDSA) and the reservation-CRDSA (R-CRDSA) schemes to enhance throughput under high traffic loads. We also propose an estimation technique for the traffic load of CRDSA. In slotted ALOHA, many schemes have been proposed for the same purpose. Some of them adjust channel access probability according to the estimated traffic load. In this paper, these techniques are extended to CRDSA and R-CRDSA by considering the number of packet replicas and the status of the reservation channels. Simulation results demonstrate that the proposed scheme enhances throughput and channel efficiency under high traffic loads.