An externally positive system has the property of giving a nonnegative output for any nonnegative input. By making the inter-vehicle spacing a nonnegative output, this system property is significant for collision avoidance in platooning. Yet, existing platooning results based on external positivity just apply to adaptive cruise control (ACC): as ACC uses on-board sensing only, these results do not apply when on-board sensing is integrated with inter-vehicle communication, as in cooperative adaptive cruise control (CACC). This letter provides an integrated external positivity design for CACC. When unreliable communication requires transitions between CACC and ACC, the design still guarantees graceful degradation in terms of collision avoidance and disturbance rejection. Such graceful transitions can be attained also in the presence of vehicle parameter uncertainty, via a suitable adaptive control design.