3GPP LTE specified for 5G the support for Device-to-Device (D2D) communication in either supervised mode (controlled by the network) or unsupervised mode (independent from network). This article explores the potential of LTE D2D in a fully unsupervised mode for the broadcast of safety-of-life automotive messages. After an overview of the Proximity Service (ProSe) architecture and new D2D interfaces, it introduces a framework and the required mechanisms for unsupervised LTE D2D broadcast on the new SideLink (SL) interface, composed of (i) a multi-cell and panoperator resource reservation schema (ii) a distributed resource allocation mechanism (iii) decentralized channel congestion control for joint transmit power/scheduling optimization. The proposed scheme is first evaluated independently, then benchmarked against IEEE 802.11p. Complementary to IEEE 802.11p, unsupervised LTE D2D is an opportunity to provide redundancy for ultra-reliable broadcast of automotive safety-of-life messages.