However, I think enforcement has been relatively lax and needs improvement. It's mainly directed at the trucks and tourist buses. In part:
Doing Our Share for Cleaner Air: Idling Reduction: Chicago’s New Anti-Idling Law
City Council recently amended the City’s traffic code to limit standing (idling) of on-road diesel-powered vehicles within the City’s borders to a total of three (3) minutes within a sixty-minute period. The new idling limit will not only improve air quality, but will also result in fuel conservation. Reasons for the ordinance are:
Air Pollution
The City of Chicago is located in a US Environmental Protection Agency designated non-attainment area for particulate matter and ozone. Chicago is a hub for trucking, rail and marine transportation and relies on diesel-powered vehicles and equipment to provide critical services within the City. These diesel-powered vehicles perform crucial work in order to sustain our economy but are also a significant contributor to air pollution in the Chicago region. Unnecessary idling causes the release of a variety of pollutants, such as nitrogen oxides, carbon monoxide, carbon dioxide, particulate matter and other chemicals that form ground-level ozone and contribute to global warming. These pollutants aggravate respiratory ailments, cause lung damage, and lead to cancer. Read more about diesel exhaust health concerns from the US Environmental Protection Agency.
Wasted Fuel and Money
Idling wastes fuel and money. When idling, a typical diesel-powered truck burns almost a gallon of fuel per hour. Drivers that eliminate unnecessary idling can save significant dollars in fuel costs each year. For instance, by not idling 30 minutes a day for one year, a truck driver saves 125 gallons of diesel and therefore approximately $374 in fuel costs (assuming a diesel fuel price of $2.99 per gallon)—while also preventing at least 101 pounds of air pollution and 2,775 pounds of carbon dioxide from entering the atmosphere.
http://www.cityofchicago.org/city/en/depts/doe/supp_info/doing_our_share_forcleanerairidlingreduction.html