Hard shoulder running, in which the hard shoulder is temporarily opened to traffic during peak periods or under incident conditions, has already been implemented on some motorways abroad. To alleviate the recurrent severe congestion on Chinese expressways during major holidays and to improve the operational efficiency of high-volume segments, this study conducts an exploratory investigation of hard shoulder running on the Beijing–Shanghai Expressway. By combining theoretical analysis with real-time traffic-flow monitoring, detailed multi-lane traffic data were collected, and the characteristics of traffic flow with and without hard shoulder running were comparatively analyzed. The results indicate that, after the implementation of hard shoulder running, the traffic capacity is significantly enhanced: under high-density conditions, the flow can continue to increase without saturating, while the reduction in vehicle speed is considerably mitigated, thereby improving the overall smoothness of traffic operations. Hard shoulder running increases the cross-sectional capacity of the studied segment by approximately 20.82%, demonstrating a pronounced effect in relieving congestion.