[OSM-talk] Roundabouts and routing

[Chris Hill]

On 08/09/11 19:48, Thomas Davie wrote:
> Hi,
>
> Today I experimented with using OSM maps on my Garmin sat-nav.  The 
> one thing that I noticed was that roundabouts do not work well. The 
> problem seems to be the slip roads entering the roundabout.  The 
> sat-nav recognises them as a roundabout in themself, and because of 
> that gives some fairly poor directions, for example:
>
> • Drive 10 miles then enter roundabout.
> (Drive 10 miles)
> • Enter roundabout.
> (Enter the slip road to the roundabout)
> • Enter roundabout and take the third exit.
>
> As you can see, because of the slip road's existence the GPS does not 
> announce which exit you're expecting to take until the last minute. 
>  This means that you can't get in the right lane early enough.
>
> Proposed solutions (all of which are horrible):
> 1) Don't tag sliproads onto roundabouts as junction=roundabout, 
> instead use some other tagging scheme.  Not greatly desirable because 
> it involves a *whole* lot of retagging.
> 2) Ask garmin to fix it (doesn't sound likely).
> 3) Write some fancy heuristics in the conversion from OSM to routable 
> garmin maps that figure out when you're looking at slip roads and when 
> it's an actual roundabout (sounds unreliable).
>
> Can anyone thing of a better solution to this?
>
I only tag the roundabout itself with the roundabout tag, that is the 
closed way which is often circular. The sliproads (technically called 
flares) are not part of the roundabout and, to me and supported in the 
wiki, they are not tagged as part of the roundabout. All of the 
approaches to roundabouts tagged like this seem to work with my Garmin 
well.

Maybe an example of what you find not working might help.

FWIW, Garmin do not support OSM's use of their devices so asking Garmin 
for help is not likely to help at all.

-- 
Cheers, Chris
user: chillly