Vehicular safety communication promises to reduce accidents by assistance systems such as cross-traffic assistance. The information exchange is mostly foreseen to be handled via Dedicated Short Range Communication (DSRC). At intersections, DSRC reception is likely to be problematic due to Non-Line-Of-Sight reception conditions. Alternatively, the required information exchange could also be handled via cellular systems. While cellular systems provide potentially better coverage, they impose other performance constraints. This paper analyzes the suitability of UMTS and LTE for cross-traffic assistance as a worst case application in terms of load and latency demands. It investigates capacity and latency characteristics and discusses influence factors on performance as well as operational aspects. The focus is on the random access performance of the uplink channel. While cellular systems might have some advantages over DSRC, the study shows that UMTS will likely suffer from capacity limitations while LTE could perform reasonably well.