Traffic prediction: what it can and cannot tell you

• Historical prediction: typical speeds for this road at this hour on this weekday, built from months of past measurements. Good for planning next Tuesday, blind to today.
• Live prediction: current measured speeds and reported incidents projected a short way forward. Good for the next hour, useless for next week.

A navigation app answers the question you asked: which road, starting now. It re-routes while you drive, which means it reacts to the queue you are already in rather than telling you in advance that today is a bad day to take this corridor at all. The prediction exists inside the app, but it never reaches you as advice before you turn the key.

• The cause, not just the delay. Twenty minutes from volume behaves nothing like twenty minutes from a full carriageway closure. One dissolves, the other does not.
• The direction of travel. Delay that is still building means the worst is ahead of you; delay that is easing means waiting is cheap.
• The alternatives, checked. A prediction that a road is bad is only actionable if something says whether the other road is better — including whether your vehicle may legally enter it.
• Weather and official warnings. Freezing rain, wind warnings and closures change the risk of a route, not just its duration.

For an individual trip, live feeds are strong for the next thirty to sixty minutes and get weaker fast after that. Historical patterns are the opposite: reliable in shape for a normal day, but silent about the crash, the roadworks or the storm that makes today abnormal. Serious prediction needs both, and it needs re-checking just before departure — an answer from thirty minutes ago is a guess.

That is what the pre-drive check on the home screen does. ExitMe.app reads live traffic, weather warnings and official incident feeds for your own route, builds real alternatives around the problem, checks them against your vehicle and emission-zone rules, and then commits to one call: leave now, wait, turn back, or take a safe stop. When leaving now is right, the verified route hands straight off to Google Maps.