Despite much promise, numerous recent studies have shown that 5G coverage remains sporadic and its performance is often suboptimal, leading to degraded QoE for high-throughput, low-latency applications. While two alternative approaches to multi-carrier access, link selection and link aggregation, have been proposed and shown to potentially enhance performance, their actual performance benefit in real-world 5G deployments remains unclear. In this paper, we conduct an extensive measurement campaign involving multiple cross-country driving trips spread out over 12 months and covering a total of 8k+ Km, while simultaneously measuring the performance of the three major US mobile operators, and explore the potential of multi-carrier access to improve throughput and latency. Our measurements show that there exists a substantial amount of diversity in cellular network performance across different operators at a given time and location in terms of both throughput and RTT. Our trace-driven analysis shows that multi-carrier access techniques have the potential to provide significant performance gains over the single-path throughput/RTT of the worst-performing operator.