In event-driven wireless sensor networks nodes transmit information only if the monitored physical magnitude levels have triggered an alarm. In these networks, traffic exhibits a high spatial correlation, since it is likely that neighbor nodes detect and try to notify the same events. Thus, the probability of packet collision raises up, as well as notification delay, just as opposite as required. In this work we propose to use a p- persistent mechanism in the access control layer. The aim is reducing collisions and saving energy. We compute the optimal p for a coherent network deployment and describe the experimental implementation of our proposal. Theoretical computations predict a notable improvement, specially in terms of energy, and experiments reveal that our proposal achieves up to 67% of energy saving compared against a perfect (collision-free) mechanisms.