Google has added a street traffic button to the mobile version of its popular map site, the company announced in a blog on Wednesday.

Starting today, owners of smartphones—excluding the iPhone, which oddly doesn’t support the feature—will be able to see via a color-coded chart how bad traffic is in their area.

Following Google’s usual mapping color scheme, major “arterial”roads, such as highways or busy boulevards, will now be coded red when heavy with traffic, orange when moderate traffic, and green when no traffic.

Google has been providing traffic data for major highways through Google maps for years using road sensors and an armada of private cars, but it used a new approach for its mapping of major streets.

The search engine behemoth retrieves local traffic result from users of Google Maps mobile themselves who send information about how fast they are moving whenever they use the application and choose the “My Location”option.

“When we combine your speed with the speed of other phones on the road, across thousands of phones moving around a city at any given time,we can get a pretty good picture of live traffic conditions,” wrote Dave Barth, product manager for Google Maps, in a blog post.

Inherent to this tracking technology is the problem of privacy. Google  tried addressing the issue pre-emptively.

 “We only use anonymous speed and location information to calculate traffic conditions, and only do so when you have chosen to enable location services on your phone,” Mr.  Barth wrote.