By the end of 2021, 406 urban road tunnels had been built in 82 of China's 661 cities [2]. These urban tunnels have reduced travel distances and improved transportation efficiency, thereby alleviating urban traffic congestion. Curves are relatively dangerous sections of road networks [3]. In 2017, approximately 17.1% of road traffic accidents in China occurred on curves, and curved road accidents caused 22.6% of deaths and 18.4% of injuries [4]. When facing the safety challenges posed by curve risks on ground roads, horizontal curve signs and markings are used to enhance safety in curve driving [5,6]. Symbolic road signs can effectively improve drivers' estimations of the curvature of dangerous curves [7]. However, for urban underground roads, their limited building space restricts the development of curves, resulting in some curves having too small radii. This, and the dim and monotonous internal environments of subterranean roads, can cause drivers to experience velocity and curvature illusions. Moreover, the lateral walls of curves can obstruct the line of sight, restricting the visual field of the driver and further increasing the complexity and risk associated with driving. Meanwhile, as a semi-enclosed structure, the narrow and dark environment of tunnels can also cause tension and anxiety in drivers. Driving in a monotonous environment can lead to driver boredom and fatigue [8]. Furthermore, the current design standard for urban underground roads in China mainly refer to the Urban Underground Road Engineering Design Specification CJJ221-2015 [9]. However, this specification does not explicitly define the safety range for a curve radius, nor provide a definition for small-radius curves. In actual construction, due to the constraints of terrain and architectural space, the radius of underground road curves is often relatively small, resulting in the high safety risks in the design and construction of curves. Therefore, it is necessary to assess the risks of curves on urban underground roads. Based on the above discussion, in order to explore the operating characteristics and driving risks of vehicles on underground road curves, identify the locations with higher curve risks, and propose reference values for the geometric indicators of underground road curves. In this study, a real vehicle test was employed to acquire continuous-driving data, including the vehicle's operating speed, lateral acceleration, and longitudinal acceleration. The distribution characteristics of acceleration on curved urban underground roads were analyzed, and the safety and risk of driver behavior were evaluated based on the acceleration driving risk model (G-G diagram). Finally, the correlation between the geometric curvature parameters and the driving acceleration of the vehicle was explored. The findings are instructive for reducing the number of accidents on urban underground roads and for improving the driving safety of their curved sections. The main contributions of this study are as follows. 1. The acceleration cumulative frequency distribution characteristics of curves on urban underground roads and the comfort and stability of driving on each curve were obtained based on the collected continuous vehicle operation data (lateral and longitudinal accelerations). 2. The driving behavior risk of curves on urban underground roads was analyzed based on the acceleration data and driver risk model, and the areas with the highest driving risk on these curves were identified. Meanwhile, the safety improvement methods on risk curves were discussed. The findings provide valuable insights for traffic management authorities to develop safety improvement measures, and they serve as a reference for accident prevention efforts. * The correlations and sensitivities between the lateral acceleration, entering curve deceleration, exiting curve acceleration, and curve geometry parameters were obtained separately based on the acceleration and curve geometry parameter data.