Impact on vehicle speeds and pollutant emissions of a fully automated section speed control scheme on the Naples urban motorway

Section speed enforcement systems are becoming established as cost-effective measures to reduce the incidence of speeding and thus improve road safety. It is also widely acknowledged that speed levels affect pollutant emissions. Therefore, significant co-benefits in environment and road safety are expected from the application of such systems. Yet few studies have investigated the environmental effects of speed limit reduction and even fewer those of tight speed limit enforcement. In this study an empirical analysis is carried out to investigate the impact of a new 80km/h speed limit and tight enforcement on an urban motorway in Italy. First a detailed analysis of individual-vehicle section speeds (i.e. section travel times) is performed. The study focuses on seven motorway sections and three reference periods, namely weekday peak hours, free-flow hours and the entire week. This allows the effect of road geometry and traffic conditions to be evaluated as well. Secondly, thorough calculation of pollutant emissions and fuel consumption is carried out. As it is based on individual vehicle speeds, the effect of changes in speed variance among drivers is also captured. This turned out to be relevant to the application. Basically, study outcomes suggest that the strategy can be a straightforward and effective policy to put eco-driving principles into practice, only in network stretches or in time periods in which congestion is not too heavy. Therefore, for such systems the opportunity for significant co-benefits in fuel economy and road safety appears to exist only under these strict conditions.