Navman N20 - all return routes seem different to outward

[stevem999]

So, using my N20 nicely but have noted one little foible that I want to share. If I run a route from home, especially a long one, then the return journey from that place is almost invariably different, even though all the roads I travel down are two-way etc so no one-way streets to circumvent. It doesn't really bother me as the vast bulk of the journey will be the same. Just wondered why when I get to a car park, for example by turning left into a car park, then when I come to leave it tells me to turn left out of the car park and takes me round the block instead of turning right out of it!

Very odd, but very scenic!

[Gee-Pee]

Same experiences! Only occasionally though - one example was driving down an A road on a return journey - the outward trip being uneventful - and told to turn left at lights then immediately right down a residential street, turn right again and rejoin the A road. But, it's only a machine so everything it instructs me to do, I question.
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Gee-Pee
Lifetime member PGPSW - time rapidly decreasing

[robertn]

beer with me on the first bit - even if you hate techno-bable, you will find it useful information if you want to know why a particular route is chosen by the iCN

The iCN ( and virtually all routing devices on the planet) thinks of the map as a network of nodes and links, with each item having a 'cost' - a node (turn or intersection) has a cost as does a link (a section of road). Right turns and left turns have different costs.
Basically, to calculate a route, all the options are tried and the lowest cost is selected. (Changing preferences changes the relative "cost" of various items ). Theres other stuff goes on, such as optimisations etc, but this is the basis of it all.

Therefore a return trip is not the same as the original because of different right and left turns. It is likely the difference between the routes is minor. In the case of a major difference (in time particulalrly), it is possible a data error exisiting in teh map somewhere between the start and end points.

You may also not that the direction you are driving is significant in the route calculation - the iCN's appear to have a very high cost for a U turn, so will instruct you to drive a long way round the block rather than do a U turn.

[Gee-Pee]

So it's all about preferences, the device's and mine. Never knew when I studied econimics that it would come in useful in understanding the intracacies of the Navman. Thanks, Robert

BTW, thought your first three word were an offer...
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Gee-Pee
Lifetime member PGPSW - time rapidly decreasing

[Itzme]

Wow, I'm impressed by robertn's post. I learned alot! Thank you for that.

[stevem999]

Yeah, I'm impressed too. It was really informative although still pretty strange. I can see the concept but what I'm having difficulty with is that left and right turns have a different cost!

In my mind a left or right turn has no difference in length (well apart from a handful of yards anyway). However I can see a time difference. Say in the UK, where we might assume that a right turn on the whole would take longer than a left because we're often crossing traffic to do it. However in the States for example it will be the other way round. Is it taking account of a node/link physical length or is there a time aspect programmed into it as well?

Anyway, thanks for the information - at least I know neither me or my Navman are going mad when I swear I didn't come the way it's sending me home!

[zzf00l]

Wow I think I actually understood that Robert thank you.. However, can you explain why whenever I travel to relatives in Cullingworth mt icn530 takes a wrong turn off the Motorway and then tells me there are 'no driveable routes to my destination'. As I know the way to Cullingworth it is not a problem... and I actually find it quite amusing... I just wonder why.

Cheers

[robertn]

Lucky the iCN is wrong, other wise it would be a long walk :D
I suspect its a map error. report it to Navman, although it may be fixed in the new versions of the maps.

[MaFt]

as robert suggested, it could easily be that a right turn is more 'costly' then a left turn as you ae more likely to have to wait longer to pull out; whereas a left turn is generally quicker.

you might find that if you count the left/right turns each direction that there are more lefts than rights. i've never been sad enough to try this though!

MaFt

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