Safety applications in vehicular networks involve real-time information included in periodical exchanged messages, called beacons. Denial of Service (DoS) attacks, especially jamming, may have high impact on these basic safety messages mainly because of the adequate condition of the attack at the lower layers. Due to the important role of these safety messages, securing beacons against jamming attacks is primordial. First, we investigate the impact of jamming attacks on broadcasting by introducing a new analytical model. We show that the degradation at certain levels in network performance is an indication of a jamming attack in the network; therefore, results from this analysis will allow us to determine network performance thresholds to distinguish normal operation and attacks. Second, we study the feasibility of the existing threshold-based methods to detect jamming in real-time applications. From our analysis, these methods are not suitable for safety applications. Hence, we propose a real-time Medium-Access-Control-based (MAC-based) detection method detection method to meet the requirements of safety applications in vehicular networks. Our detection method can more accurately distinguish the causes of failed transmissions, such as contention collisions, interferences, and jamming attacks. The jamming attacks are detected with low probability of false alarms. Finally, we evaluate our detection method with both analytical analysis and network simulations. Our detection method performs high detection probability in different studied scenarios while false alarms are almost avoided.