This paper aims at devising a longitudinal vehicle controller to avoid collisions in a platoon of vehicles with explicit consideration of brake input limits. A new spacing policy is conceived for eliminating the dependence of the parameter of spacing on traveling velocity. It is shown that the proposed policy yields better traffic throughput and that the corresponding margin of traffic flow stability is extended in comparison to constant time-headway policy. An adaptive controller based on a semi-autonomous scheme is devised to bear the system uncertainties and to have more mobility to respond any individual emergency that may arise. The parameter design of the spacing policy is provided so as to account for any possible emergent stopping scenarios and for individual brake limitations.