## 7 Conclusions and Future Work  
In this work, we have developed and functionally validated an experimental vehicular on-board unit (OBU) based on an open-source software implementation of the ETSI C-ITS protocol stack. Moreover, we have used the OBU to experimentally evaluate the application-level latency for V2X information exchange within an anticipated cooperative collision avoidance (ACCA) service while demonstrating the effectiveness of interoperability in V2X communication between OBUs from different vehicle original equipment manufacturers (OEMs). Our experimental OBU can readily be integrated into any vehicle, and it can use as a driver-assistance system in collision avoidance use cases. The OBU offers the flexibility to inter-work between different V2X protocols to seamlessly communicate with heterogeneous edge-/cloud-hosted services provided by different vehicle OEMs and Mobile Network Operators (MNOs). As future work, a field trial of the ACCA service infrastructure will be carried out in the city of Barcelona using a deployment of LTE Small Cells at 3.5 GHz and with edge cloud servers located in the street cabinet to evaluate key performance metrics under real traffic conditions. Through this field trial, we plan to evaluate the performance of the OBU in terms of latency and throughput, integrate security for secure V2X communications, and extend in-vehicle capabilities with smart sensors for hazards detection. In addition, we plan to use a performance monitoring platform that is based on Elasticsearch and Kibana that will allow the evaluation of data dissemination performance, such as network-level and application-level reliability. Moreover, we plan to leverage existing hardware-in-the-loop (HIL) simulation platforms with our OBU in order to evaluate communication delays under high density and high mobility road traffic scenarios for different radio access technologies.  
R.S. ([PERSON]): methodology, software, validation, investigation, writing--original draft, visualization, data curation; F.V.-G.: methodology, investigation, writing--original draft preparation, writing--review and editing; R.C.: software, writing--original draft preparation; R.V.: resources, writing--review and editing; R.M.: supervision; R.S. ([PERSON]): resources, writing--original draft preparation, writing--review and editing; L.D.: resources, project administration; A.E.F.B.: project administration; X.V.: software, writing--review and editing; S.K.D.: software; J.H.: software, writing--review and editing; J.A.-Z.: project administration. All authors have read and agreed to the published version of the manuscript.  
This work is part of 5 GCroCo project that has received funding from the European Union H2020 Research and Innovation Programme under grant agreement No. 825050. It has also been partially funded by SPOT5G (TEC2017-87456-P), AUORRAS (RTI2018-099178-B-I00) and by Generalitat de Catalunya under Grant 2017 SGR 891.  
**Acknowledgments:** We would like to acknowledge the contributions of 5 GCroCo partners towards the development of the ACCA service. We thank the anonymous reviewers for their careful reading of our manuscript and their many insightful comments and suggestions.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.