Hello,
-I 15: if you don't want to make a big detour or if you don't want to cycle on rough gravel roads in a semi-desert area, so you have to use this stretch on the I 15 from exit 93 (Overton) to exit 8 (Littlefield). But there is still a small portion of backroad between exit 112 (Riverside) and exit 122 (Mequite) which is called "Riverside road". If you cycle on I 15 this won't be a problem as it is not! prohibited for cyclists. Usually there is a wide paved shoulder and the car and truck traffic is not very busy out there. I even cycled the whole way from St. George to Las Vegas on the interstate because of lacking time. Only the scenic and narrow Virgin Canyon was not very pleasant due to the narrow (shoulderless) road. (cycled in June 2000)
- Zion NP: Yes, the Mount Carmel tunnel is prohibited for bicycles. If you go eastbound it will be uphill inside the tunnel. So be happy when at tunnel entrance a park ranger awaits you at the traffic lights. He will help you to stop or to ask any pick-up driver to give you a free lift through the tunnel. (btw. every 2nd car is a pick-up!) (cycled in June 2000)
- I 70: Yes, there is no frontage road between exit 180 and exit 173 (Ruby Ranch Road). But this is not a problem. This is a wide open road with shoulders and not too much traffic. From exit 173 into Green River there is still the old road. And this road is perfect for cycling very very lonely, narrow, mostly tarmac but sometimes bumpy. There are no services between Canyonlands Airport on highway 191 and Green River. Crescent Junction is a "Ghost Gas Station". If you continue from Green River to SLC, please remember there is again two miles to go on I 70 between exit 158 and exit 156. The following highway 6 is very busy with motorized traffic even you go through a very lonely area. There are no services between Green River and Wellington! (cycled in May 2005)
I hope this information will help you to make your travel plans.
Best regards
Frank
(please write the next time in German so will get a faster response)