try routing with 
gosmore, traveling salesman, openrouteservice.org, cloudmade.com, .....
there is also a new routing plugin for josm.
wherever these create a scenic route you need to check the details. very common errors are wrong direction motorway segments or exits. broken connections are also very difficult to find. Use josm validator. It can find overlapping segments which are not connected. Mapsource isn't very good to find a detour. A Garmin device is usually better in finding a route.
In Mapsource itself insert more and more through points and the segment which is connected straight contains the error. 
The interesting behavior for my Etrex Vista is it can route a long route via multiple tiles even if there are some errors. But when you are driving near the error it fails, does recalculations all time or gives up. there must be some built in guessing code to bridge errors in the data




On Thu, Apr 9, 2009 at 2:38 PM, Clinton Gladstone <clinton.gladstone@googlemail.com> wrote:
You're right: The routing problems occur even with everything in one tile.

I don't know if this is caused by bad data or just general routing
weirdness. What would be the best way to analyze this?

Cheers.

On Thursday, April 9, 2009, Mark Burton <markb@ordern.com> wrote:
>
> Hi Clinton,
>
>> I have found a reproducible routing error, which occurs when I split
>> and compile the Italy extract from Geofabrik.de.
>>
>> - The error occurs when routing on the A1 from Modena to Parma. (I
>> have only tested this in MapSource so far.) If I route in the opposite
>> direction (South-East), the problem does not occur.
>>
>> - As soon the route crosses the tile boundary, the route starts
>> jumping back and forth over a very large area.
>
> Can you route successfully if the roads in question are contained in a
> single tile? I'm just wondering if it is a general routing issue or
> specifically an inter-tile routing issue.
>
> I only wonder because I have an example map that has broken routing so
> I know that there still exists problems with intra-tile routing.
>
> Cheers,
>
> Mark
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