A vehicle maneuver detection application is proposed for driving assistant systems. The application can accurately and inexpensively detect and differentiate vehicle steering maneuvers by utilizing built-in sensors on smartphones or other portable computing device residing in a vehicle. By leveraging an effective bump detection algorithm and studying the nature of steering, the application is capable of differentiating various steering patterns, such as lane change, turn, and driving on curvy roads. Practicality of the application is demonstrates by two use cases: careless steering detection and fine-grained lane guidance. Thus, the application provides new functionalities without relying on cameras to provide a broader range of driving assistance.