The Modern Street Examples Project contains a scene with a small city setup including street shapes ready to apply rules that generate detailed street geometry. The provided street rules contain many attributes to change the look of the street models, such as lamp density, lane dimensions, tree probability and many more. Download the example project to play around with the user attributes and include the rule file in your very own city.

For example, via the style attribute only, the appearance can be switched between American and European streets:

UPDATE May 19th: Screenshots of the updated example (available with the new CityEngine 2009) are shown below. Besides the improved street geometries (a.o. with round curbs and junctions), people and vehicles are featured in the rule set. These sample assets have been provided by www.lowpolygon3d.com - visit their page for more assets, resolution and information.