Using a OSM map, my Nuvi 510 handles roundabouts as ordinary streets with exits, i.e. the voice does not say “enter the roundabout and take the 2nd exit” - as it is happening in the original garmin map - but says “turn right” when entering the roundabout and then “proceed 50mt and turn right” etc . The same goes in the graphics: the icon on top-left does not display a roundabout.
I double-checked with potlatch and the roundabouts I went through are defined correctly (i.e. as roundabouts).
It is not the expected behaviour. mkgmap-generated maps that use Garmin type 0x0c for roundabouts should result in the Garmin unit recognising roundabouts as distinct and different from other junctions. I don’t know how the GFOSS guys generate their maps, however.
By the way, one of the roundabout I tested does not work as expected: http://www.openstreetmap.org/?lat=45.364391&lon=11.93057&zoom=18&layers=M: coming from west or south and going north, the way going east is ignored i.e. coming south and going notrh the voice says “enter the roundabout and take the 1st exit right”, while it should be the 2nd exit right (the path to follow on screen is correct). I checked with potlatch but the roundabout seems defined ok. Any clue?
I read in the past of a possible issue with roundabout incoming/outgoing streets if they are too close, but I am not able to find the doc again.