Service continuity in heterogeneous wireless access technologies is a main issue. The challenge is to preserve continuous services while moving between these technologies. Making the decision to which network to switch (handoff) is one of the main key. In order to achieve the service continuity, we must rely on seamless vertical handoff techniques with a minimum processing-delay. In this paper we propose a vertical handoff decision scheme to enhance the service mobility using the simple additive weighting (SAW) method in a distributed manner, under heterogeneous environments. Our main goal is to reduce the overload and the processing-delay in the mobile terminal, by delegating the calculation of handoff metrics for network selection to the target visiting networks.