1. A method to assist in driving a vehicle moving along an initial trajectory, in an environment containing at least one obstacle to be avoided, the method comprising the steps of: A/ checking, by a trajectory checking unit, during movement of the vehicle along the initial trajectory, in the environment containing the at least one obstacle to be avoided, the existence of at least one condition for modifying the initial trajectory of the vehicle; B/ determining: a) by a criteria determining unit, a criterion CR by which to avoid the at least one obstacle, wherein the criterion CR is determined according to the following expression: 
 CR=R 2 +a 1· D 2 +a 2· T 2 in which: R is an evaluation of a risk of collision with the at least one obstacle; D is an evaluation of a deviation relative to the initial trajectory; T is time needed to rejoin the initial trajectory; and a 1 and a 2 are weighting values; and b) by an avoidance trajectory determining unit, an avoidance trajectory according to a value E wherein: 
 E=∂CR/∂s∂q in which: s is a curvilinear abscissa of the vehicle moving along the initial trajectory; q represents a turn parameter of a control element that controls the vehicle (to follow said avoidance trajectory; and E represents a derivative of the criterion CR in relation both to said curvilinear abscissa s and to said turn parameter q; wherein said avoidance trajectory determining unit further determines values of the turn parameter q for which said derivative value E is zero, said turn parameter values being incorporated into the determination of the avoidance trajectory in which the vehicle reaches a planned destination while avoiding said at least one obstacle; and C/ assisting the driving of the vehicle by a driving assist device along the determined avoidance trajectory according to the determined values of the turn parameter in conjunction with the corresponding curvilinear abscissi.