The principle of INRIX Traffic is simple: you download and install the phone app or tablet app and - when you tell it to - it anonymously tracks your movements, folding the speeds you are travelling into its database. In return, you can see, in real time, how traffic is flowing on your prospective route.
Using INRIX Traffic is simplicity itself. It's a simple map, with green, amber and red lines denoting how well traffic is running. You can compare traffic conditions, or ask INRIX to predict how vehicles will be flowing in a particular area at a specified time in the near future.
You can also manually correct INRIX, reporting the presence of police or an accident, or even humbly suggesting that INRIX has appended the wrong colour to a street on which you are travelling. More often you'll simply tell INRIX to follow you, and add to its data on traffic conditions.
And that's really about it. INRIX is a useful, although by no means indispensible, free tool. And the reason that it is free is because the makers of Traffic then resell its live-traffic data to the makers of high-end satnavs. There's nothing wrong with that, of course, but it does rather expose what is missing here.
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INRIX Traffic is not, for example, a route-planner or true satnav software - although it tracks where you are on a map. It simply offers a snapshot of live traffic conditions in your area, and anywhere to which you can scroll its pan-European map (it now covers France, Germany, Netherlands, Belgium and Luxembourg, as well as the UK, and there are more European countries coming soon). So you can choose to go a different way, but you'll need either a map-reading assistant or a satnav to do so.
There are lots useful data here. Information is available for virtually all major roads, but you're unlikely to find out if a milk float is blocking your cul de sac (unless you volunteer the information). Lots of user reviews of INRIX Traffic complain about crashes and bugs in the app. Running it on our Android 3.2 Samsung Galaxy Tab we had no such problems, although occasionally tiles of the maps wouldn't display. In almost all cases, changing the resolution of the map (a simple pinch) fixed the issue.