In Oregon, you can ride on the interstate except where it is specifically prohibited. There will be signs at the on ramp that say motorized vehicles only, mostly in urban areas. There are some places where the interstate is the only road.

"12. SELECTED OREGON ADMINISTRATIVE RULES (OAR) THAT PERTAIN TO BICYCLISTS & PEDESTRIANS:

Prohibition of Non-Motorized Vehicles on Freeways

734-20-0045 (1) Non-motorized vehicles are prohibited upon the following segments of freeways within the State of Oregon:

(a) Portland area:

(A)The Columbia River Highway No. 2 (Banfield/I-84) from its intersection with I-5, MP 0.00, to 122nd Avenue, MP 10.25, east bound, and to Sandy Boulevard, MP 15.14, west bound;

(B) The Sunset Highway No. 47 easterly of the Jefferson Street Interchange, MP 73.35;

(C) Interstate 5 (Hwy. No. 1) from the Beaverton-Tigard Highway Interchange, MP 292.20, to the Delta Park Interchange, MP 306.70;

(D) Interstate 205 (Hwy No. 64) northerly of the Overcrossing of the Oswego Highway No. 3, MP 8.82;

(E) Interstate 405 (Hwy. No. 81) in its entirety; and

(F) Lower Columbia Highway No. 2W from its intersection with I-405, MPE0.00, to 23rd Street, MP 1.99.

(b) Medford area: Interstate 5 (Pacific Highway No. 1) from the Barnet Road Interchange, MP 27.58, to the Crater Lake Highway Interchange, MP 30.29 (in Medford).

(2) The closure of the above sections to nonmotorized vehicles shall become effective following the erection of adequate signing."

The above quote is from Swanson Thomas and Coon