A vehicle is moving on a rough road in  a straight line with uniform velocity. Then :                                                                                                                                                                                                                                                                           

a force must act on the vehicle 

Since the vehicle is moving with uniform velocity, thus its acceleration must be zero. The friction force exerted by the road on its tires equals the force provided by its engine. So, the acceleration of the vehicle is zero and thus the vehicle moves with uniform velocity.