I'm planning a loop from Spokane, WA to Mullan, ID on 23cc road tires. I'll be picking up the Trail of the Coeur d'Alenes in Plummer on the way out, but need a more direct route back. The Trail of CdA's will take me back as far as Cataldo, and then Canyon Rd parallels I-90 to near exit 34. I can't pick up the North Idaho Centennial Trail until near exit 22. My question is: Is the stretch of I-90 between exits 34 and 22 in north Idaho legal for cyclists? Secondary to that question is: Assuming it is legal, is it safe or advisable?

Thanks for your help and advice!

I can offer this, on the Couer d' Fondo route they ride on I-90 for about a mile between CDA and 97.

https://www.isp.idaho.gov/patrol/faqs.html

May I ride my bicycle on the Interstate Highway in Idaho?
You may ride your bicycle on the interstate highway in Idaho. It is preferred, that if there is another route that you can travel, that you do so. Interstate speeds are dangerous to bicycle riders.

I've ridden a couple small sections of I-90 in Idaho, one between CDA and ID 97, and from the end of the Trail of the CDA at Mullan to Lookout Pass/MT state line.

Thanks for the link. Did you feel comfortable with the shoulder width?

For the sections I did, yes. Probably a 10 foot wide shoulder with rumble strips. Generally Interstate shoulders are pretty generous, you just have to watch out for debris.

IME the worst part about interstate highway riding is the noise and debris on the shoulder.

Yes, especially the little wire strands left behind when tire debris disintegrates. We got lots of flat tires from these wires while cycling on I-5.

True. Thankfully, I've never flatted when riding the numerous Interstates that were parts of my tours through the years. (Debris strewn bike lanes in my town, however, have given me plenty of flats over the years.) I would definitely recommend tires with decent puncture protection if you intend to do a bit of freeway riding.

For what its' worth, most of the shoulder riding I did on I-90 in Montana was pretty nice.