## 5 Conclusions  
In this work, a first characterization of latency times in a bidirectional VLC link in a realistic outdoor scenario for V2V applications is presented. In particular, our system includes real motorbike LED head- and tail-lights as VLC sources, and experimental measurements have been conducted in an outdoor urban scenario. We characterized the communication performance of the system in terms of PER up to 27 m for a baud rate of 57 kBaud. Our results demonstrate an error-free bidirectional communication for distances up to 9 m, and a PER \(\leq\) 0.1 for distances up to 15 m.  
The analysis reported in this paper shows that our bidirectional V2V system is capable of establishing a VLC link with a minimum attainable bidirectional latency as low as 2.5 ms at 57 kBaud, yet granting latencies of the order of 5 ms for distances up to 10 m between vehicles. We highlight that this value is compatible with 5G URLL pillar for ITS, including road safety. We also present a mobility model aimed at comparing the braking distance attainable if two moving vehicles are equipped with VLC, 5G-URLL, LTE-V2X, and IEEE802.11p communication technologies, respectively. Our model, in combination with our data, shows that VLC technology attains a reduction of the braking safety distance in a platooning scenario by nearly one order of magnitude as compared to what, e.g, IEEE802.11p RF-based systems, could allow. Our results pave the way towards the integration of VLC technology as a reliable communication platform in future ITS services, opening for future works aiming to provide more insight into further relevant aspects, such as tests in real driving conditions, effects of atmospheric conditions on performances of bidirectional VLC systems, or effects of relative unbalance between the forward and backward channel quality on the bidirectional round-trip latency.  
Conceptualization, S.C., L.M. and J.C; methodology, S.C., L.M., M.S. and J.C.; software, S.C., M.M. and M.S.; validation, L.M. and J.C.; formal analysis, S.C., L.M. and J.C.; data curation, S.C., M.A.U., M.M. and M.S.; writing--original draft preparation, S.C.; writing--review and editing, L.M. and J.C.; visualization, S.C.; supervision, L.M. and J.C. All authors have read and agreed to the published version of the manuscript.  
Figure 5: Latency of a VLC bidirectional connection as a function of the speed of the first vehicle (v\({}_{1}\) in the \(x\)-axis) and the speed difference between vehicles (v\({}_{2}\) – v\({}_{1}\) in the \(y\)-axis). The red zone shows the speeds configuration so that a VLC connection cannot be established between the two vehicles. In the red zone, the latency is not defined since in that zone the two vehicles cannot support a stable connection.  
**Funding:** The work received financial support from the following projects: MiUR PON 2017 ARS01_00917 \"OK-INSAID\", CNR Progetti\(\acute{\text{e}}\)CNR \"FluoCom\", MiSE Centri di Competenza \"ARTES4.0\" and MiUR FOE Progetto Premiale 2015 \"OpenLab 2\", and MiSE PRato Industrial SMart Accelerator \"PRISMA\".  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** This work was performed as a collaboration between the VLC Joint Research Laboratory (VisiCoRe) of the Dept. of Information Engineering of the University of Florence, the VLC laboratory of CNR-INO and the European Laboratory for NonLinear Spectroscopy (LENS).The authors wish to thank the company ILES srl in Prato for providing head- and tail-lights, and in particular Eng. [PERSON] for valuable and constant support during the experimental campaign.  
**Conflicts of Interest:** The authors declare no conflict of interest.