Estimating PM-reductions from speed reduction policies

Abstract

Speed reduction measures have become an increasingly popular way to increase traffic safety especially in urban areas. Recently many cities have converted entire districts into 30 km/h zones. In many European countries the maximum speed of haulage trucks is under discussion or review sometimes in combination with a ban on overtaking. Reducing the maximum speed is perceived and promoted by policy makers as beneficial to the environment because of reduced fuel consumption and lower emissions. These claims however have not been scientifically validated. They stem from the popular believe that the widely used Copert-approach, which is scientifically valid for average trip speeds, can be used to assess the environmental impact of speed management policies at a local scale. It is obvious that speed reductions in urban areas or on highways may have very different effects on PM emissions. On the other hand the simplistic idea that speed reductions increase urban emissions and decrease emissions on highways is probably wrong. Although few experts would make this assumption explicitly, it is very frequently made implicitly by the way that traffic and emission models are integrated. Integrating macroscopic traffic models with emission functions based on average speed only is clearly unsatisfactory. In addition, the lack of such functions for the PM emissions of petrol fuelled cars is an important problem even with advanced models such as VeTESS. In this paper we study the problem of accurately estimating the effects of speed managements policies on exhaust emissions of PM. Emissions for specific types of vehicles were calculated with the microscopic VeTESS-tool using real-life driving cycles and compared with results obtained using Copert-like methodologies. Our results indicate that emissions of most pollutants should not be expected to rise or fall dramatically. Nevertheless the conclusion for emissions of PM could be different. The effects of specific speed reduction schemes on PM emissions from trucks are ambiguous, but VeTESS results indicate that the PM exhaust from diesel passenger cars shows a significant decrease in urban areas onverted to 30 km/h zones. Exposure of residents to one of the most toxic components of the urban air pollution mixture may therefore also decrease. Unfortunately linking microscopic emission and traffic models raises other concerns such as a lack of validation of the most prominent parameters: acceleration and gear change behaviour.