The effectiveness of Cooperative Intelligent Transport Systems (C-ITS), both on energy and emissions savings as well as operational improvements, is an on-going research area. This research evaluates the effect of a pilot study on speed advice service, which was implemented along a major urban arterial corridor in the city of Thessaloniki, Greece. Real-time data pertaining to the operation of the C-ITS service were recorded and stored during the pilot operation period. The raw data were cleaned, filtered and then used for the estimation of efficiency indicators pertinent to the system's performance. Based on the real-world trajectories, a model-based approach was adopted, to quantify the fuel consumption and CO₂ emissions of the equipped vehicles. A rigorous statistical analysis of the cleaned data and the estimated indicators was conducted, to provide insights with respect to the operation and energy efficiency of the service, and drivers' compliance to the delivered speed advice. Results indicated that equipped vehicles compliant to speed advice emit 9% less CO₂ compared to the non-compliant ones. Additionally, driver's compliance to speed advice is strongly related to the type of message transmitted, the remaining time of the signal status, and finally the activation zone length.