"Over the Wilsonville-Hubbard Highway from its beginning at the Hubbard Interchange at Wilsonville on Pacific Highway, I-5, southerly to the end of the Wilsonville-Hubbard Highway at its junction with Pacific Highway East, OR99E, approximately two miles southeast of Aurora. (Est. 9/19/2002)"
~ ODOT, Descriptions of US and Oregon Routes, March 2007
OR-551 was Oregon's highest numbered state highway between 2002 and 2007 (the previous recordholder belonging to OR-245 in Eastern Oregon, and the current recordholder being OR-569 in Eugene), created in September 2002 when ODOT decided to give some of its unmarked state highways associated state route numbers. Usually, this system duplicates the highway and route numbers (such as Beaverton-Tualatin Highway #141 carrying OR-141), but in this case there already exists an OR-51 in Western Oregon the runs near Monmouth and Independence. Because of this, under ODOT's guidelines, the route number would have a "5" added to the beginning of it; hence, the Wilsonville-Hubbard Highway #51 became Oregon State Route 551.
While this designation is new, the highway has had a designated route number before. During construction of the Portland-Salem Freeway (I-5), the Wilsonville-Hubbard Highway temporarily carried US-99 from its junction with US-99E between Hubbard and Aurora to the completed portion of the freeway near Wilsonville. Once the freeway was completed to Salem, the designation was removed but the highway remained in the state highway system.
On a trip down I-5 in May 2005, I noticed that ODOT erected an OR-551 shield assembly before Exit 282A along I-5. This means that OR-551 is Oregon's highest numbered signed state highway ever. It currently is not signed on the route itself; however, its northern and southern termini have shields on I-5 South and OR-99E North respectively, and some intersecting streets post it above directional signs at traffic lights. However, this record will also be short-lived, as OR-569 is slated to be shielded in the summer of 2007.