as a former internetwork engineer specialized in network routing (before I switched to my present career in information security), i'm intrigued at the routing algorithm used by Garmin to compute the route for a certain destination.
with my recent experiments, i'm pretty sure the highway tag has more weight than the total distance against "lesser ranked" ways
my recent example is La Funeraria Paz in QC from Makati. Garmin wanted to route me to Buendia, quirino highway, nagtahan espana then quezon avenue
a few kilometers farther than the way via mandaluyong, san juan then sta mesa where araneta avenue is located.
i would love OSMers to revisit the way tagging convention to force garmin to show the quickest way. note from baguio to QC, it's still via macarthur highway and not via SCTEX-NLEX last time I checked, perhaps it's the link roads to the tollways should be upgraded so that garmin can easily route through it.
I think garmin looks for the nearest motorway-trunk-primary links then use the lesser ranked roads to route...just how much the weight considered is hard to know unless someone from garmin will publish the algorithm