In highway jargon, multiplexing is using the same physical road for multiple route numbers, as mentioned in neil's WU at How the United States highway system works. It's relatively common in small amounts (2-3 route numbers), particularly for an interstate and the US route number that section road carried prior to its upgrade (e.g. Interstate 81 and US 11) or for a N/S interstate to share with an E/W interstate on a relatively diagonal stretch (e.g. Virginia's section of I-64/81).

I have spent a somewhat significant amount of time riding on two rather unusual multiplexed highways.

  • Interstate 81 (I seem to be using this a lot) runs nearly straight east-west with a very slight southwest lean through much of Southwest Virginia. Interstate 77 crosses 81 at Wytheville, VA, and is multiplexed with it for about nine miles. The strange part is that the two roads, both nominally north-south interstates, are labeled as running in opposite directions -- 77N is 81S, and vice versa. Surprisingly, very few people get lost on this stretch, due to the excellent signage.
  • The biggest stack of signs I've ever seen on one highway was in Greensboro, NC. One approximately 2-mile stretch of road carries the markers:
    • I-40 East
    • I-85 North
    • US 29 North
    • US 52 South
    • US 74 East
    • US 220 North