An interesting thing I found whilst going about lately satiating my craving for information on the Mount Hood Freeway was a bit about the exits on the Banfield Freeway (I-84) that I'd never noticed before.
Exit numbers, as many are aware, in Oregon are keyed to the freeway mile. The exit from I-5 to Keizer is in freeway mile 260, so the Keizer exit is Exit 260.
On the Banfield eastbound, in the stretch that rounds Rocky Butte to the east, the 82nd Avenue exit is Exit 5; it's in freeway mile 5. The I-205 southbound/Salem exit is Exit 6, the Gateway exit is Exit 7, the I-205 northbound/Airport exit is Exit 8, and the 102nd Avenue offramp is Exit 9.
But the distance between exits 5 and 9 isn't between three and four miles...it's only about 1.2 miles. Strange, but explainable.
It developed that the stretch of Mount Hood freeway from I-5 to I-205 was to be added to the Interstate system as I-80N (or I-84 as we have today), and the stretch of Banfield between city center and I-205 was to be dropped from the Interstate system, simply signed as US 30. The shift of the route, with I-84 and 205 "duplexed" from what is now I-205 and Powell Blvd to the Gateway area, would account for the "lost" mileage. The mileage on I-84 east of I-205 is reckoned as though the Mount Hood freeway was actually built.
This, friends, is serendipity in action.