I think it's something to do with how fast/slow you're going, but some clarification would be nice.
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+1 Oh how I want to know the answer to this. My guess was it was showing the traffic-state of the roads left on your journey (green = clear, red = congested).– MattAug 18, 2011 at 10:32
1 Answer
Google Maps Navigation also provides Live Traffic Information along your route, updated every few minutes. A red/amber/green light in the corner shows if there are any traffic problems anticipated along your route. If there are, you can see where the holdups are and have the option to press to re-route which will find a route avoiding the traffic holdups.
Source: http://mobile.jack-frost.co.uk/google_maps_navigation_review_uk.php
Also: http://www.google.com/support/forum/p/android/thread?tid=4f7b34dd18f74b75&hl=en
(Via: Google Search For "google navigation amber green light")