How do routefinders find their routes?

Satnav Computers now often decide the way

Many now rely on satnav or online routefinders for directions - instead of paper maps - but how do these gizmos find their way?

Once, you gazed at the map, remembered that cursed bridge on the old A30 near Bodmin where lorries got wedged, thought of a number, doubled it, gave up, guessed, set off at 4am, whatever.

Now, you consult an online routefinder.

Or maybe eight, as I did. I wouldn't usually, honest. It's like having eight argumentative backseat drivers, as you'll see.

So how far is it, anyway? And how long does it take? The occasion was another bank holiday, the journey was Hertfordshire to the South Devon coast near Kingsbridge, and here are the results:

Map and graph showing journey times

Is that a bigger spread than you'd expect? The distances are 246, 244, 243, 242, 221, 221, 220, and 220 miles. The durations vary by more than an hour. But the explanation is not that some are adjusted for traffic conditions.

Four routefinders recommend the M4 down to Devon, the other four the A303. When trying a few different times of travel on the TomTom - which is one of those that says it updates according to traffic conditions during the day - it varied by a maximum of 20 minutes from quickest to slowest in either direction.

I had expected more. In the event, we passed a life-sapping tailback on the M5 in the other direction that would add who-knows-how-much suppressed road rage.

So how do they do it? The one I spoke to was a bit cagey about this in case the competition would be listening. But the basics are that they first assume you drive legally, count the distance, make standard calculations for how much time a roundabout or junction adds, then check samples of these with real drives.

There's apparently no encyclopaedia of routes from every destination to every other. Rather, they look for the shortest way to the trunk road network and then refer to a database of main routes, before finding the shortest reasonable - ie not down a farm track - connection at the other end.

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Do you choose who values your house, audits your company or sets your exams because you think they will give you the most favourable verdict? Or the most accurate?”

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So we might expect variations to tend to be proportionately bigger where less of the route is on a trunk road.

But here's a question that moves the problem of measurement from one about miles to one about you. Do you choose the route finder that offers the quickest journey, the slowest, or one in the middle? Are you a travel optimist or pessimist?

If you think it possible that one of them knows a better route, it might make sense to choose the fastest. But how likely is that? Makes you wonder why they don't put a range of uncertainty around the journey time.

The distance is what it is, the journey will take what it takes. As they used to say about the timetable for certain continental rail networks - you leave when you leave and arrive when you arrive. This doesn't change just because someone thinks otherwise.

But I wonder if the hope that it might is common in many areas of life. For example, do you choose who values your house, audits your company or sets your exams because you think they will give you the most favourable verdict? Or the most accurate?

In other words, do we go for the one that we think will tell us what we want to hear - and stuff the facts?

Come to think of it, you do meet people who like to say they did the journey faster than they really did - yeah, my driving's that good. Or slower - see how I struggled to get here. See what you put me through?

It is what it is, but measurement is something else. Measurement, as we all know, is also about the measurer.


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  • rate this

    Comment number 186.

    The problem never lies with the satnav, but the people using it. Where else in life would you blindly follow what a computer tells you to do? People we need to realise that entering start and finish will never get you there the best way, you need to learn to manipulate the data we are given back, and tell it which roads we know are best and let it tell us the roads we don't.

  • rate this

    Comment number 185.

    The free SatNav that comes with my smartphone seems more accurate and easier to use than the systems supplied in the hire cars I use.

  • rate this

    Comment number 184.

    The GPS in my Lexus has the habit of directing me on to an off ramp of a highway and then immediately to the on ramp. How stupid is that?

  • rate this

    Comment number 183.

    I always refer to a route planner when I'm driving somewhere I haven't been before but I don't rely on them. I have a look at the suggested route, have a look at a map of the wider area and then use that as a basis to plan my own route. I rarely ever use the sat nav - it usually only comes out on the very rare occasion I get lost. It all means I can focus on the driving, not the directions.

  • rate this

    Comment number 182.

    You didn't mention Autoroute. You didn't say where in Hertfordshire so I guessed St Albans. Autoroute says 214 miles and time 209 mins. I have a laptop on a tray to the left of my gear shift and that is way way better than those stupid little dashboard things. You see the whole picture, like a whole map with your position on it, all possible scenic routes around you, how far along etc.


Comments 5 of 186



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