Illinois 78 is a long north-south route running from Jacksonville all the way to the Wisconsin border. It is one of the most prominent north-south state routes in Illinois. It spans over half the state from top to bottom.
Intersections from north to south:
IL78 begins at the Wisconsin border as Wisconsin Highway 78 crosses into Illinois
IL78 joins US20 west
Then it leaves US20 and goes south at Stockton
IL40 at Mt. Carroll
IL78 joins US30 east
Then it leaves US30 to go south again, at Morrison
IL92 joins it from the east
IL92 leaves to the west
I-80 near Annawan
US6 at Annawan
IL81 at its eastern end in Kewanee
IL78 joins US34 south at Kewanee
US34 leaves to head westerly again
IL17 joins IL78 south for seconds
Then IL17 leaves east
US150 joins IL78 south from the west
US150 leaves east*
IL8 joins IL78 south from the east to go through Elmwood
IL8 leaves west
IL78 joins IL116 west
IL78 leaves IL116 to go south again at Farmington
IL9 joins from the west at Canton
IL9 splits off to continue east
IL100 near Dunfermine
IL78 joins US24 west
IL78 leaves south onto a road that also carries IL97 (at the circle of multiplexes**); if you continue west on US24, you will also be following IL97 northbound
IL78 and IL97 join US136 east into Havana (forming a "tripleplex")
IL78 breaks off of US136/IL97 to go south at Havana
IL125 at Virginia
IL78 ends in the south at IL104 at Jacksonville.
It passes through these towns: Warren, Stockton, Mt. Carroll, Morrison, Prophetstown, Hooppole, Annawan, Kewaunee, Laura, Elmwood, Farmington, Norris, Canton, Dunfermine, Havana, Bath, Chandlerville, Virginia, Jacksonville. (wow, that's a lot, right?)
If you continue driving north from IL78 at the WI border, the number will still be 78!
*-There is an access road to I-74 where US150 and IL78 part.
**- The circle of multiplexes has a northwestern edge at Lewistown. Its eastern edge is IL78/IL97 (a multiplex); its northern edge is US24/IL97 (another multiplex); its western edge is US24/IL100 (yet another), and on the southern half of the western edge runs US136 added to US24/IL100, forming a "triple-plex"; and on the southern edge runs US136/IL100. (But for the eastern half of the southern edge, US136 isn't multiplexed anymore-- don't tell anyone.)