I think I figured out a routing design flaw in my Rand device.

I have noticed in the last year or so that my Rand 730TD GPS likes to skip interstate exits then double back, adding time and miles to trips if I don't catch the routing flub.

It only seems to do this in interstate preferred mode. It happened to me again the other day, very close to final, so when I stopped, I looked at the intersection.

The problem seems to be that interstate exit and entry ramps, and bridges over interstates are not considered interstates.

This means that if you are Northbound on an interstate and need to exit West to a NON interstate, the GPS sees a longer interstate route and shorter non-interstate route (in some cases) if you go to the next exit, then double back. If you are Northbound, and exit East, there won't be a misroute.

Rand could dramatically reduce incidents of retarded routing by designating all interstate off and on ramps as interstates, as well as all bridges crossing interstates that are also connected to off or on ramps.

Yes, I sent this info to Rand, but I figure if everyone starts pestering them, it might get fixed sooner.

 

The map data looked good, at least the physical road layout.

It happened as I was westbound on I90 in Illinois, needing to go Southbound on IL-59.

Routing with interstate preferred skipped the exit, went to the next exit, then turned me around Eastbound on 90 back to the correct exit.

Routing with avoid small roads routed me to the correct exit.

I'm fairly certain the issue is what I described. Interstate ramps (and bridges connected to them that cross interstates) should be designated as interstates for more accurate routing.

This issue is not a rare issue. I just had not had time and desire to look at it closely before.

 

I know to watch for it. The paper atlas does still see some use, but it tends to be even more out of date for construction data than the GPS.