The road curvature is an important characteristic, specially for systems tracking a target trajectory. However, in some cases, this value is not available and its dynamics is unknown and has to be estimated to be used by control. By considering vehicle lateral dynamics, the steering system and the Radar/Camera information, the standard Traffic Jam Pilot (TJP) system is presented. Current TJP solutions add the target road curvature in the state, which is typically estimated by a linear solution based on Kalman observer. A drawback of this method is that this class of observers is designed and tuned for several ranges of velocity, each range having its own observers gains. This feature strongly increases the amount of observer parameters to be tuned. The aim of this paper is to propose a new scheme of observation for TJP system, based on a nonlinear Sliding Mode observer, requiring only one set of gains. Such an observer solution is tested by simulation in different time-varying velocity conditions, curvature variation and types of roads. Another contribution, is the proposition of a recent automatically tuning procedure to optimize observer gains for any system with a large scale base. Finally, the road curvature estimation is used to discuss the performances obtained.