Most theoretical reflections on policy change are connected to the weak generalisation that policy network stability deeply affects the stability of the policy process and its outcomes as a whole. As a result we could expect policy inertia when inter-organisational relations are principally characterised by strong ties among consolidated and stable sets of actors. Changes of the policy paradigm could eventually intervene when exogenous factors alter the inner balances of the policy network. This analysis focused on transport policy in five nations shows that the paradigm shift from the nationalisation to the liberalisation of collective transports represented a shared governmental priority during the '90s. Nevertheless, changes of the policy paradigm happened only in those countries where transport policies were highly institutionalised while they were less evident or totally ineffective in cases of low policy institutionalisation. This finding strengthens the hypothesis which considers endogenous factors, rather than exogenous ones, as key variables for policy change and the institutionalisation of public policies as a crucial process to understand their variance.