The fifth generation (5G) of cellular communications aims at encompassing a wide variety of types of communications, among which are vehicle-to-everything (V2X) communications. These will be characterized by the interchange of both low-latency messages for control and notification purposes, and delay-tolerant messages, devoted to infotainment services. In this paper, a mechanism for dynamic traffic steering over the Uu and PC5 interfaces of V2X communications is proposed to ensure low latency for critical messages, even in high load conditions. As a proof of concept, a simulation has been carried out using long-term evolution (LTE) macrocells and IEEE 802.11n access points to provide the Uu interface and an approximation to the PC5 interface, respectively. The proposed solution has proven to effectively steer data packets regarding their criticality label, providing a delay-optimized link for critical messages and a best-effort behavior for the non-critical (delay tolerant) messages.