How doe a SatNav know which is the fastest road?

So here’s a challenge for those of you who understand the way that Satellite Navigation Systems work things out.

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In the UK we have a system of road numbering which goes from Major to minor roads.  For example, there’s the A1, A2, A3, A4 etc.  all major trunk roads before the motorways appeared in the 1960’s (and if you notice these road numbers go clockwise around London from the 12 o’clock point.  Scotland follows the same road numbering system but the road numbers start with 8 or 9.

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Then there’s the roads with 2 numbers, A12, A13 etc.  Less important or major than the major roads

Then 3 numbers (the A120, A137 etc) smaller roads than the A1

and we have roads like the A1071.  Usually roads with 4 numbers used to be minor roads (called B roads).  some have been upgraded to A roads over the last few years.

Now onto the SatNav question.

I was driving down the A1 to get home.  The SatNav was programmed to get me home in the fastest time.  The machine kept insisting I went down the A14 (remember this road is supposed to be a more important road than the A120 if you follow the numbering system).  In reality the road from Stanstead airport to Braintree is a recently finished dual carriageway and just as fast to drive as the A14.

So my question is:  Are SatNavs programmed to follow this type of road system methodology, thinking that roads with 3 numbers are less “major” than roads with 2 numbers?  Do they know?  or do they just factor in the shorter distance regardless of whether it’s faster or not. (The A120 cuts 1 mile off my journey compared to going via the A14 so there’s not much in it really)

If you know do tell.  And stop hubby and I arguing over the navigation algorithm that SatNavs use whenever we get in the car…

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