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"3di" means 3-Digit Interstate
Primary interstate highways in the United States have one- or two-digit numbers, such as I-5 and I-84. Three-digit interstate highways, or 3di's, branch off or loop with the primary routes. A 3di takes the number of its parent and adds a multiple of 100: I-405 loops with I-5, and I-384 veers off from I-84. For more information and a sample map, please see details.
While there are about 65 primary interstates, there are over 200 3di's , mostly in or around cities. Over 30 others, such as San Francisco's Embarcadero (I-480) or Boston's Inner Belt (I-695) have been killed, a couple have been decommissioned (converted to state or US highways), and a handful are proposed.
Because they typically are placed in high-population areas, 3di's encounter more controversy than their 2-digit counterparts. However, when the 1956 Interstate Highway Act was in trouble, an additional 1,000 miles of proposed roads, mostly 3di's in key cities, generated enough votes to pass it.
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Interstate 238 (Hayward, CA) gives many road geeks indigestion, as it breaks the rules: there has never been such thing as I-38.
The average 3di is about 20 miles long. Some, however, are longer than some 2-digit routes, like I-86 and I-97. Some are less than a mile long. For a list of some 3di's at the extremes, see Shortest and Longest 3di's.
Most primary interstates have at least one 3di (exceptions at left). Interstates 66 and 97 have had 3di's proposed or decommissioned, but don't have one now. Furthermore, some 3di's are not signposted: 110 and 345 in Texas, 444 in Oklahoma, and (depending on the phase of the moon) 124 in Tennessee.
Quebec's Autoroute system has a similar numbering convention: Autoroute 73 in Quebec City has a spur called Autoroute 573, for example.
Superlative and notable 3di's