Dynamic driving task (DDT) fallback turns to be an essential part in level 3 or higher driving automation systems, which is responsible to either perform the DDT or achieve a minimal risk condition after encountering automated driving system (ADS) failure. As a typical ADS failure, sensor failure can prevent ADS from performing on-road driving safely, and thus a minimal risk condition can be achieved when the failed vehicle stops away from the active lane. Therefore, this paper considers a level 4 ADS-dedicated vehicle encountering front sensor failure in highway traffic. The proposed fallback approach is designed to avoid potential collision with surrounding vehicles while bringing the vehicle to a stop on the designated parking zone, thus achieve a minimal risk condition. Safety constraint is proposed based on the assumption that the failed vehicle is remarkable by surrounding vehicles, and enforced using model predictive control. As a result, simulation is conducted in Carsim/Simulink environment.