This is what members over on the Prius Owners site said. I'm guessing Toyota stuck with it for the HL, maybe with some minor tweaks.

"It is for Traffic reporting or so to speak. If you have the full nav and Satellite package this lets you know traffic flow and that there is reporting available. If you go into your NAV Menu hard Key you can access info on it in there. Something in the menu to do with Traffic I believe."

 

Thanks. I'll try it next time. Maybe figure out what HD means.

 

Maybe? Was discussed on another forum. It also provides any colors on the map you see in regards to traffic flow if your in a region supported. No idea what those regions are though. Colors and flow update every 10-15 minutes.

"HD", Data Traffic, which is available on select model year vehicles, provides up-to-date traffic flow information on your vehicle's display screen.

 

HD= High definition for the radio when you near a hd station

 

That makes sense now. I live in the country and the HD indicator only comes on as I approach the city - where I know there's an HD station. Thanks