Emergence of approximate translation invariance in finite intervals as a speed selection mechanism for propagating fronts

We introduce a velocity selection criterion for fronts propagating into uns table and metastable states. We restrict these fronts to large finite inter vals in the comoving frame of reference and require that their centers be i nsensitive to the locations of the ends of the finite intervals, thus exhib iting effectively an approximate translation invariance. Only one monotonic front has this behavior, and its velocity is the one that is physically se lected. We present analytic results in the case of piecewise parabolic pote ntials and numerical results in other cases.