A speed limit of 80km/h with "strict enforcement" has been introduced in 2005 on zones of urban motorways in The Netherlands with the aim to improve air quality of NO2 and PM10 along these motorways. Strict enforcement means speed control by camera surveillance over the whole trajectory of 2-4km combined with licence plate recognition and automatic fining in case of exceeding the speed limit. Traffic data measured in Rotterdam and Amsterdam at the zones without and with speed management showed that traffic dynamics have been significantly reduced as a result of speed management with strict enforcement. Reduction of traffic dynamics results in more free-flowing traffic with relatively less NOx and exhaust PM10 emissions compared to congested traffic, i.e., stop-and-go traffic.The actual effect on NOx and PM10 emissions at these speed management zones was studied in the cities Rotterdam and Amsterdam. The study was performed in two different ways: firstly by measurements and by modelling the contribution to NOx and PM10 concentrations on both sides of the motorways, and secondly by estimating the change in traffic dynamics and the effect on emissions. From the results of both approaches in this study, it was concluded that in our case study in the Netherlands emission reduction by speed management is in the range of 5-30% for NOx and 5-25% for PM10. Actual emission reductions by speed management at a specific motorway mainly depend on the ratio of congested traffic prior and after implementation of speed management. The larger this ratio, the larger is the relative emission reduction. The impact on air quality of 80km/h for NOx and PM10 is largest on motorways with a high fraction of heavy-duty vehicles. Â© 2010 Elsevier B.V.