Abstract: Different geometric design of roads has different requirements for lateral performance.Drivers affected by increased fatigue level will also show a decline in the ability to maintain in the lane.Current studies do not take interaction effects of geometric design and degrees of fatigue on lateral performance of drivers into consideration.This study aims to explore impacts of horizontal road design on lateral performance of fatigued drivers.In total, 41 middle-aged drivers in China were recruited to participate in a field driving test of 550 km.The self-reported fatigue level was recorded according to the Karolinska Sleepiness Scale (KSS);the standard deviation of lateral position (SDLP) was recorded for performance analysis, and the GPS data were recorded as location information.Driving performance under different fatigue levels and at different horizontal types and radii of road curves are analyzed.A multi-linear regression model is further proposed in this study to analysis the correlation among the SDLP, fatigue level and radius of circular curve.The results indicate that when participants are driving along a circular curve, their SDLP is greater than when they driving along a tangent or transition curve at the same fatigue level.The analysis on the impacts of curve radius on SDLP indicates that the driving performance of participants is worse when the circular radius is higher, especially when it is greater than 5 500 m.In conclusion, this study demonstrates the impacts of geometric design of horizontal curve types and curve radius on driving performance when drivers are influenced by different fatigue levels.In addition, if horizontal geometrical parameters can be obtained during driving, the accuracy of identifying fatigued driving based on driving performance can be improved.