Traffic speed and emissions

The Dutch Transport Research Centre, AVV, is preparing a brochure for local and regional government departments on how motorway traffic speed affects traffic flow, traffic safety, environmental impact and so on. CE was asked to contribute by reviewing the relationship between vehicle speed and environmental burden.

There are two types of traffic that cause unduly high emissions (NOx, PM10 and CO2): high-speed traffic and tailbacks.

For an array of technical reasons, vehicle air emissions rise significantly at speeds over 100-120 km/h, essentially because vehicles are optimised for operation at ‘normal’ speeds. Because of growing air resistance at higher speeds, there is also a sharp rise in CO2 emissions, contributing to climate change.

Tailbacks are the second category of traffic with a disproportionate environmental impact. In this ‘stop and go’ type of situation, with repeated revving up of engines, more fuel is burned, for the simple reason that it takes a lot of energy to get a vehicle moving. A second reason why pollutant emissions are higher in tailbacks is that the three-way catalytic converters installed in petrol-driven vehicles cannot keep up with the short bursts of engine firing, while in diesel vehicles local deficits of in-cylinder oxygen lead to a marked increase in PM10 emissions with each restart.

The main instrument of emissions reduction are currently the ‘Euro’ emission standards for new vehicles, progressively tightened every few years. To comply with Dutch legislation implementing European air quality standards, under certain circumstances there may be a need for additional local measures. One option here is the so-called ‘Overschie’ regime, but erecting roadside barriers, fitting buses with particle traps and introducing disincentives for ‘dirty’ vehicles are all means of improving local air quality.


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