A review of recent advancements and applications of 4D millimeter-wave radar in smart highways

Abstract Smart highways, as vital components of intelligent transportation systems (ITS), have driven a growing demand for sophisticated sensing technologies. Among these, 4D millimeter-wave (mmWave) radar has emerged as a cornerstone, thanks to its capacity for high-resolution, real-time monitoring of complex traffic scenes. Unlike traditional radar systems, 4D mmWave not only measures the distance, velocity, and azimuth of moving objects but also captures elevation information, yielding an enriched three-dimensional understanding of the road environment. Although substantial research has been conducted on 4D mmWave radar in onboard applications, a comprehensive review focusing on its potential in roadside scenarios, such as smart highways, is currently lacking. This paper addresses that gap by first reviewing the operating principles of 4D mmWave radar and then examining how it integrates with complementary sensors. We assess its performance in key roadside functions, including multi-target detection, vehicle trajectory tracking, and macroscopic traffic flow monitoring. Further, we discuss critical challenges—such as real-time data processing bottlenecks, system scalability across extended highway networks, and suppression of inter-radar interference. By synthesizing current research and pinpointing outstanding issues, this review offers a comprehensive overview of 4D mmWave radar’s expanding role in smart highway ITS and outlines promising directions for future investigation.