Web-based mapping technologies have been utilized for traffic information systems. However, most such systems are for freeways and very few of them focus on arterials or urban streets. This paper presents a real-time Google-map-based Arterial Traffic Information (GATI) system for urban streets in the City of Bellevue, Washington State. Open source web tools and emerging web technologies, such as Ajax, are used in implementing the system to ensure its performance and minimize its cost. Convenient administrative functions are enabled through advanced database design and the Model-View-Controller (MVC) application. This GATI system, though presented and demonstrated by using Bellevue's data, is a general technology that can be applied to any arterial network.