Many cities in Europe and elsewhere are introducing measures to improve air quality that affect traffic. Disregarding Singapore in the 1970's and Norwegian cities in the 1990's, many initiatives have been implemented from restrictions to certain license plates during rush hour in Bogota (Pico y Placa, 1998) to congestion charge schemes in London (2003), and other pricing initiatives in Stockholm and Milan. Many other cities have implemented permanent or temporary restrictions to traffic for air quality purposes, trying to reduce NOx and PMx.