The difficulty of field surveillance of cars in service is investigated and various test procedures are discussed in this context.
Due to the high variability of the measured exhaust emission values, only cars with very high emissions, so-called outliers, can be identified in an emission test. This means that only vehicles with emissions far above the standards can be singled out. This also holds true for the idling test, where the variability of results is at its highest.
On the other hand, the decisive factor for the quality of our ambient air is the overall emission mean value.
Identifying and adjusting the outliers will, however, bring improvement in ambient air quality only if the number of such outliers is so high that they have a significant influence on the overall exhaust emission mean value of the vehicles on the road considered. There is a true dilemma in that it cannot be seen from single measurements whether or not the overall emission mean value has been changed at all and if so, whether or not the changed mean value exceeds the standard.
Relatively simple criteria and a straightforward evaluation of the whole story lead to the conclusion that an inspection of the car components influencing exhaust emissions represents an optimum in terms of cost and efficiency.