Illinois Route 336 (also known as the Thomas A. Oakley Highway, after the executive of Quincy Newspapers) is a four-lane freeway/expressway combination that serves western Illinois. It is also used by Illinois DOT in FAP 315 to refer to a future project connecting the cities of Quincy and Peoria via underserved Macomb.

As of late 2008, the highway extends north from its starting point in Fowler (near Quincy) where U.S. Highway 24 and Interstate 172 intersect, to US 136 just west of Macomb. Illinois 336 is 80 miles (128.75 km) long.

For its entire length, Illinois Route 336 is a four-lane divided expressway without property access, but has many at-grade intersections with sideroads. The only interchange currently built on Illinois Route 336 is with US 136/Route 94/Hancock County Road 1500 in Carthage.

Illinois 336 overlaps Illinois Route 61 from south of Mendon to south of Loraine. These two roads serve the Adams County Fairgrounds. Further north, Illinois 336 overlaps Illinois Route 94 about 7 miles (11 km) past Loraine, and continues until Illinois 336 intersects with U.S. Route 136 in Carthage. (Illinois 94 continues north to near the Quad Cities.)

As of October 2006, an overpass has been completed just east of Carthage, as well as 5 miles (8 km) of four-lane expressway pavement. Illinois Route 336 currently bypasses Carthage to the south and east, now intersects with U.S. Route 136 at the overpass. In Summer 2007, construction continued east of Carthage toward Macomb. Major grading for bridges over the two branches of the LaMoine River as well as an overpass for the BNSF Railway and Illinois Route 61 was done and continued south of both Tennessee and Colchester.

By Spring 2009 the highway was completed between Carthage and Macomb. Illinois 61 has an at-grade T-intersection with the highway at the same location as the alignment of U.S. Route 136. The two highways are concurrent to the point west of BNSF Railway overpass, where U.S. Route 136 splits off to its former alignment through Tennessee and Colchester. Illinois 336 continues east and turns north to Macomb. There is an underpass under BNSF Railway and then two ramps which end in T-intersections with U.S. Route 136, which has been improved to four lanes within Macomb.

Another highway overpass of U.S. 136 north of the BNSF Railway underpass is required as the route turns north for an eventual bypass northwest of Macomb [WIU side of city]. At this point, Illinois 336 ends temporarily until further construction is completed.

North of Macomb along U.S. Highway 67, there will be a full interchange that will connect with the northern portion of the recently completed 336 west of Macomb. This segment was scheduled to be completed in 2008 or 2009, but visible work has not begun as of late 2009.

The Peoria-to-Macomb corridor being studied (DOT Job No. P94-025-00 URS Job No. 25364560; July 7, 2003) closely parallels U.S. 136 to Marietta, and then Illinois Route 95 to Cuba, along Hickory Road to Canton, north on Illinois Route 78 to Farmington, then turning east along Illinois Route 116 to the Peoria area. The existing interchange (Exit 3 on Interstate 474) at West Farmington Road would serve as the logical eastern terminus. There are four proposed routes within the corridor between these two points that are being considered.

In the mid 1990s, the road west from Springfield to south of Quincy, U.S. 36, was cosigned Interstate 72. As a result of this change, Illinois 336 from Fall Creek at the current junction with Interstate 72, north to Fowler, was renamed from I-72/Illinois 336 to simply Interstate 172. Interestingly, this spur serves no cities directly — it is effectively an eastern bypass around Quincy on its current alignment. Regardless, the upgrade of U.S. 36 to Interstate standards provided Quincy with a much needed regional expressway. Interstate 172 is a non-chargeable Interstate Highway, inasmuch it was built entirely with state funds designated as the original Illinois 336, until it received its Interstate 172 designation.

The completion of Interstate 72 left only one other area in the state without regional freeway access — the area between Galesburg and Quincy. Specifically, Macomb and Western Illinois University are currently an hour and a half from the nearest Interstate highways between the Illinois and Mississippi rivers in Illinois, Interstate 74 to the north and Interstate 72 to the south.

The Illinois Department of Transportation (IDOT) launched a series of studies to help facilitate access to west central Illinois, colloquially named Forgottonia for the lack of highways through the region. Currently in progress are upgrades (to a four-lane expressway) of U.S. Highway 67 from U.S. Highway 34 in Monmouth (near Galesburg) south to Macomb, and U.S. 67 from Macomb south to Alton, a suburb of St. Louis, Missouri. Also underway is a similar upgrade of Illinois 336 from Quincy to Macomb.

The Peoria-to-Macomb study involves an eastern link from Macomb to Peoria alongside existing U.S. Route 136. The corridor being studied runs from U.S. 67 east to Interstate 474, and has been studied since the 1950s by Illinois and Federal agencies as a potential link between Chicago and Kansas City, Missouri, the Chicago-Kansas City Expressway. As of May 2006, the corridor study had identified two alternatives for Illinois 336. The route would be a limited access freeway for a few miles outside of Macomb to Bardolph, and again for a few miles outside of Peoria, east of Hanna City. For either alternative, Illinois 336 would be constructed as an expressway between Hanna City and Bardolph. In Illinois, an expressway allows partial-access to the highway, with direct access to the expressway from private residences and fields, while retaining interchanges and frontage roads for businesses and arterial state routes.

The completion of Illinois 336 would also relieve congestion on U.S. Route 24, currently the only direct route between Peoria and Quincy. It would also allow Peoria to Quincy traffic to avoid the alternate route — Interstate 155 south to Interstate 55 through Springfield, to Interstate 72 west. This combination of highways is currently the fastest route between Peoria and Quincy. Illinois 336 would shave at least 50 miles (81 km) and up to an hour off the trip. Currently, both U.S. 24 and the Interstate 155/55/72 routes require upwards of 3.25 hours of travel.

This route would also provide a faster route from Peoria to reach cities such as Kansas City and Denver.

There have also been discussions on using future Illinois 336 for a Peoria-to-Chicago Highway but is still being debated.