→BIG DETOUR PREVENTION (BDP) Explained by Bill473
The "obvious" way to get to the destination is possible. Going
NB, take the exit and drive down to the bow-tie on US-36 W. Turn left and drive to the destination.
So why didn't Waze recommend that? Because it thought that would be a detour!
I-80 (E/W) and I-35 (N/S) run concurrently for a while near the center of Des Moines. Where I-35 N leaves I-80 E and heads north was connected by a single ramp segment. Everything was OK at that point because of #4 above ("detours" must be 2 or more segments). It was then cut to Seagull it. Now we have 3 segments and I-35 N was not added as an alternate to the new ramp segments. Within a few days there were numerous URs from commuters wondering why they were no longer being sent the normal way.
Although currently only the last of the three segments is checked for BDP (probably to make checking faster), PesachZ suggests that is likely to change. The best fix is to have I-35 N on all. Indeed, if that single ramp segment had I-35 N added earlier, even though not required to prevent a BDP error (since 2 ramp segments required for BDP here), when it was cut into 3 segments no BDP error would have occurred.
It's very easy to cause a BDP problem in a case like this without realizing it. Think "name discontinuity" any time you are working on ramps. As more Seagulling in being done, it's likely other such errors will occur. Again, the most dangerous areas are where two major highways diverge.